thermal analysis of tiwanaku raised field systems in the lake
TRANSCRIPT
Journal of Archaeological Science 1989,16,233-263
Thermal Analysis of Tiwanaku Raised Field Systems in the Lake Titicaca Basin of Bolivia
Alan L. Kolata’ and Charles Ortloff’
(Received 9 June 1988, revised manuscript accepted 12 October 1988)
Raised field systems of agriculture in seasonally or perennially inundated landscapes have received increasing attention from scholars involved in the analysis of prehistoric agricultural intensification in the New World. This paper discusses the morphology and function of raised fields associated with the Tiwanaku civilization on the southern rim of Lake Titicaca in Bolivia. The thermal properties, and specifically, the heat storage capacity of raised fields in this high altitude environment are analysed by means of an ANSYS finite element computer model. The analysis concludes that enhanced heat storage capacity was an essential design element of raised field agriculture in the Andean altiplano, and that this thermal effect served to mitigate the chronic hazard of frost damage to maturing crops in this rigorous environment. An experimental verification of this conclusion based on the performance of reconstructed raised fields subjected to severe sub-freezing conditions is briefly described.
Keywords: AGRICULTURE, TECHNOLOGY, HYDROLOGY, RAISED FIELDS, SOUTH AMERICA, ANDES, BOLIVIA, COMPUTER MODELING.
Introduction The ability of irrigation agriculture to support large populations and underwrite the agrarian economy of state level society has long been recognized. Most historical and interpretive studies of agricultural reclamation have been concerned with the technologi- cal, sociological, and political implications of hydraulic agriculture in arid lands (Wittfogel, 1938, 1957). The interplay between social institutions, political organization, and agrarian economy based on artificial water distribution has been the subject of a large body of literature (Steward, 1955; Boserup, 1965; Sanders & Price, 1968; Fernea, 1970; Price, 1971; Sanders, 1972; Mitchell, 1973; Downing & Gibson, 1974 and many others).
However, the potential of intensive raised field agriculture to provide a similar econ- omic base and to sustain demographic growth is little known. In response to this lacuna, recent interest in the problem of agricultural reclamation in perennially or periodically inundated landscapes has expanded dramatically. The initial product of this emergent interest has been substantial new information on the morphology, functions and impli- cations of raised, drained field agric.ulture in various regions and culture areas of the New World (Denevan, 1980, 1982). A sampling of such studies reflects the geographical
“Department of Anthropology, The University of Chicago, U.S.A. *Central Engineering Laboratories, FMC Corporation, Santa Clara, California, U.S.A.
233 0305-4403/89/030233+31$03.00/0 0 1989 Academic Press Limited
234 A. L. KOLATA AND C. ORTLOFF
Figure 1. Location of the study area on the southeastern shore of Lake Titicaca in Bolivia.
breadth in the distribution of these paleohydraulic systems, as well as the relatively recent character of scholarly inquiry into them (MAYA-Siemens & Puleston, 1972; Turner, 1974, 1978, 1983; Matheny, 1976, 1978; Harrison, 1977, 1978; Puleston, 1977; Siemens, 1978, 1982, 1983a; Adams, 1980; Scarborough, 1983; VERACRUZ-Siemens, 19836; Wilkerson, 1983; VALLEY OF MEXICO-Armillas, 1971; Calnek, 1972; Parsons, 1976; Parsons et al., 1982; VENEZUELA-Zucchi, 1972; COLUMBIA-West, 1959; Parsons & Bowen, 1966; Broadbent, 1966, 1968; Eidt, 1984; ECUADOR-Parsons, 1969; Batchelor, 1980; Knapp, 1981; Denevan & Mathewson, 1983; Knapp & Ryder, 1983; PERU-Parsons & Denevan, 1967; Smith et al., 1968; Lennon, 1983; Erickson, 1985; BOLIVIA-Pflaker, 1963; Denevan, 1964,1966; Kolata, 1986).
In 1986 a binational, interdisciplinary research project directed by one of the authors (A.L.K.) initiated an intensive examination of raised field systems in the near hinterland of the pre-Hispanic city of Tiwanaku (Tiahuanaco) on the southern rim of Lake Titicaca in northwestern Bolivia (Figure 1). This research was centered on a 70 km2 zone referred to
TIWANAKU RAISED FIELD SYSTEMS 235
locally as the Pampa Koani (Figure 2). During the 1986 season, excavations in two hydrologically discrete raised field segments were undertaken to examine in greater detail the temporal context, as well as the methods of construction, modification and use of raised field systems within the Tiwanaku hinterland. These excavations, along with corre- lative investigations of local topography and groundwater, were also intended to form the empirical base line for a computer generated model of raised field function in the Andean altiplano. This paper reports the results of one aspect of that model: the thermal properties of raised field systems in the context of the specific hydrological and climatological conditions characteristic of the littoral and near-shore environment of Lake Titicaca.
Setting Lake Titicaca is located at 16” S, 69” W at 3810 m above sea level in the Andean altiplano. The bedrock of the Titicaca basin is mostly igneous (basalts and andesites) with some sedimentary rocks, mostly shales, sandstones, and isolated pockets of limestone (Newell, 1949). The altiplano and the lake were formed in the Miocene with the rise of the Andes (James, 1971), and attained present form in the Plio-Pleistocene (Lavenu, 1981).
Annual rainfall near Lake Titicaca averages 687 f 138 mm (Boulange & Aquize-Jaen, 1981: tables V 8z VI) which falls principally in a wet season between December-March. Annual mean air temperature at lake elevation is c. 9°C with an amplitude of c. 12°C (Boulange & Aquize-Jaen, 198 1). As is characteristic of tropical systems, diurnal tempera- ture variation is greater than seasonal variation. Mean temperature of surfZa1 waters is 1 l-15°C. Average incident radiation is high at 520 cal cmm2 day-‘, and has a subdued seasonality. Evaporative losses account for virtually all of the precipitation in the basin and only a small amount of water leaves Lake Titicaca through its single, sluggish outlet at the Rio Desaguadero, or by deep seepage (Winter, 1981).
Over the past 100,000 years the lake has been as much as 100 m above its current level (Servant & Fontes, 1978). Some sedimentological studies suggest a lake-level fluctuation of + 10 to - 50 m for the past 12,000 years (Wirrmann & Oliveira Almeida, 1987). In historical times, the lake level has fluctuated as much as 5 m within a 2 year period [Carmouze & Aquize-Jaen, 1981: figure 3(b)]. The last major fluctuation occurred between September 1985-April1986 when the level of the lake rose nearly 3 m, inundating many square kilometers of the littoral zone in Peru and Bolivia and destroying at least 11,000 ha of agricultural fields planted in potato and other crops.
The terrestrial vegetation of the altiplano is characterized by associations of compo- sites, grasses, legumes and solanaceous plants. Dominance of one group over another is primarily dependent upon local precipitation. Edaphic communities occur in areas of sandy, moist soils, or in zones possessing high groundwater (Cabrera, 1968). In general agricultural crops are segregated altitudinally (Bowman, 1916; Galdo Pagaza, 1981). Maize, barley and wheat are grown below 3350 m. Potatoes and quinoa (Chenopodium quinoa) flourish up to 40004200 m, while caiiiwa (Chenopodium pallidicaule) grows to 4300 m. Above 4300 m no agricultural crops are grown, and this high zone supporting natural vegetation such as bunch grasses is given over to pasturage for domestic animals, particularly llama and alpaca. Many indigenous crops of the altiplano are derived from the same families as natural vegetation.
Tiwanaku Raised Field Systems The excavation and analysis of raised fields in the near hinterland of Tiwanaku was initiated with a pilot programme spanning the period from 1979-82 conducted under the aegis of the Instituto National de Arqueologia de Bolivia (INAR). This pilot programme of research, which was directed by Alan Kolata and Oswald0 Rivera of INAR, focused on
Figu
re 2
. Map
of t
he P
ampa
Koa
ni ra
ised
field
syst
em st
udy
area
.
TIWANAKU RAISED FIELD SYSTEMS 237
Swole with standing water H Excavated section n
c 0 IO 30 M
Figure 3. Plan and section of raised field segment in the Lacaya sector. This field geometry is used as the basis for the finite element model of field function discussed in the text.
large scale raised field systems in the Pampa Koani zone, which Kolata (1987: 188) now designates as the northern component of the Tiwanaku sustaining area.
During the pilot programme, test trenches were excavated across raised field segments in the Lakaya sector of the Koani zone in order to determine their chronological context and cultural affiliation, and to examine the technology of construction, modification and use of these massive structures. Diagnostic ceramics within the context of these agricul- tural structures were extremely rare. However, direct structural association between field segments and habitation mounds containing such diagnostics indicated a clear chrono- logical and cultural affiliation with the Tiwanaku IV-V period (c, AD 400-1200). The possibility of earlier Tiwanaku III (c. AD 10&400) utilization of habitation mounds and raised field segments was also raised by the initial work of the pilot programme, but in the absence of clear, unambiguous ceramic associations this conclusion remains tentative (Kolata, 1986: table 1).
Stratigraphic examination of the fields indicated that they were raised earth platforms formed by excavating soil from both sides of the projected field surface and mounding it in the center (Figure 3) This straightforward technique duplicates the process that has been reconstructed for raised field systems elsewhere in the Titicaca basin and in other areas of the ancient Americas. Repeated field surface modifications, and maintenance episodes in the adjoining swales, which averages 3-5 m in width, were recorded in these strata cuts. Together with the zonal settlement analysis undertaken simultaneously during the pilot project, this evidence implied that the Tiwanaku state designed, constructed, and managed a labour and technology intensive agricultural landscape capable of generating sustained surplus production of food crops (see Kolata, 1986 for an elaboration of this hypothesis).
A new phase of long-term interdisciplinary research examining the technology and organization of intensive agricultural production in the Tiwanaku state was inaugurated in 1986 near the important regional administrative site of Lukurmata. Aerial photographs indicated that a sector of agricultural fields was incorporated within the site boundaries of
238 A. L. KOLATA AND C. ORTLOFF
Figure 4. Aerial photograph of the site of Lukurmata. This photograph clearly shows the massive artificial canal and agricultural sector that cut access to the principal civic-ceremonial sector of the site. The canalized quebrada in the southern sector of the site can also be seen. (Photograph courtesy of the Institute Geografico Militar, La Paz, Bolivia, Series 54060, Sheet 20520).
Lukurmata (Figure 4). The fields themselves were built in a broad, crescent-shaped strip of land that lies between the civic-ceremonial district of the site (the acropolis of Lukurmata) and the steep hillslopes that constitute the site’s southern boundary (Figure 5). This location was particularly appropriate for exploiting drainage from a quebrada cutting into the southern hillslopes of the settlement. This quebrada was artificially canalized in the Tiwanaku IV period through construction of stone revetment walls. These hillslopes are sources of fresh spring water which flows as groundwater northward into a topographic low on which the raised fields of Lukurmata are constructed. The ground water ultimately discharges into Lake Titicaca itself.
The agricultural sector at Lukurmata is drained by an artificial canal that surrounds the acropolis escarpment of Lukurmata. Apart from its drainage function, this canal also acts as a moat cutting easy access to the civic-ceremonial architecture that surmounts this escarpment. Water within this canal flows in two directions: to the east and to the west around the peninsula-like acropolis sector of the site into Lake Titicaca. This ununsual bifurcated drainage results from local topographic conditions in combination with direc- tional flow from the canalized quebrada to the south of the canal. Flow from the quebrada
TIWANAKU RAISED FIELD SYSTEMS 239
Figure 5. Contour map of Lukurmata with various functional sectors of the site emphasized. The canalized queimdu cutting into the southern hillslopes of the site appears prominently in this map.
was directed into a trapezoidal-shaped aqueduct constructed of earth, cobble and gravel fill faced by stone retaining walls. The linked canalized quebrudu-aqueduct system discharged water into the surrounding canal in an easterly direction.
The strip of land dedicated to agricultural constructions encompassed approximately 65,000 m*, or 6.5 ha of the site, representing approximately 3% of the total estimated settlement surface. This relatively small agricultural sector could not have supplied the entire subsistence requirements of a settlement as large as Lukurmata. Clearly the bulk of the population at Lukurmata must have derived its sustenance from fields outside of the site boundaries. The massive field systems in the adjoining Pampa Koani zone are the logical candidates for the source of this non-local production.
240 A. L. KOLATA AND C. ORTLOFF
Results of Raised Field Excavations at Lukunnata The morphological and functional properties of high altitude raised field systems modeled in this paper are based on data from excavations in the fields at Lukurmata. A summary of the internal structural characteristics of Tiwanaku raised field systems is essential to understanding the variables relating to field function that have been factored into this model.
As indicated above, the raised field system preserved on the surface at Lukurmata consisted of two distint hydrological units demarcated by the breakpoint of bifurcation in the flow of water into the canal which defines the northernmost extension of the agricul- tural sector at the site. The surface fields and adjoining swales that constitute these two hydrological units are large curvilinear constructions of soil, gravel, and clay that arc around the base of the civic-ceremonial acropolis for a distance of approximately 800 m. The surface fields themselves were exceptionally large forming cultivation platforms that averaged over 10 m in width.
The adjacent swales related to these field surfaces were smaller, ranging from 3 to 5 m in width. Two test trenches (operations 1 and 2) that were cut into these surface field con- structions encountered distinct sets of stratigraphically separate (that is, vertically stacked) buried raised or ridged fields, representing repeated episodes of field construction confirming that the area had formed part of an ancient landscape of long term agricultural reclamation. As outlined below, these buried fields took several different forms.
Operation I: test trench 1: N2633-N2636 E2948 Buried raised field surfaces were encountered in this trench at a depth of 60 cm below the local datum (Figure 6: stratum 6). Here the soil changed from a dark brown fine sand to a dark grayish-brown loamy clay. This darker, more organic material was mounded to form several agricultural beds (Figure 6: stratum 6). These beds were oriented in the same east-west direction as their counterparts on the surface, although they are substantially smaller in wavelength, approaching 4.5 m when measured from adjacent centers-of-fields. The wavelength of the surface fields taken from the same center-of-field points of reference ranged between 12 and 15 m, or approximately three times that of the buried fields. The overall length of the buried field system was not determined, but as noted above, the surface field constructions both at Lukurmata and on the adjacent Pampa Koani run for several hundred meters.
No diagnostic artifacts were encountered directly within this set of buried fields. How- ever, diagnostic Tiwanaku IV and V ceramics were recovered from overlying soil contexts, indicating that these buried fields will date minimally to that period. They may, of course, relate to an earlier phase. The conspicuous absence of post-Tiwanaku and Inca ceramics within these strata, and within the agricultural sector of the site as a whole, strengthens this hypothetical chronology. Furthermore a minimal Tiwanaku IV-V date for these fields is entirely consistent with the similar relative dating of the adjacent Pampa Koani field systems (Kolata, 1986: 7545).
Two distinct buried field surfaces appeared in cross-section in the trench profile (Figure 6). As indicated in this profile, the buried fields were roughly 20 cm in height, from top of field surface to the base of the adjacent swale. However, this measurement may not reflect the original morphology of the fields when in production given the probability that processes of swale infilling and surface erosion were active throughout the life of the field system. The homogeneity of the soil matrix forming the agricultural beds indicated that they were the product of a single episode of field construction. The actual planting surface of the buried fields consisted of a loamy clay in which no microstratigraphy was apparent. The agricultural beds presented the appearance of a mixed matrix that was repeatedly disturbed, churned, and homogenized, perhaps through extensive use of a
Figu
re 6
. Stra
tigra
phic
prof
ile o
f Tiw
anak
u ra
ised
field
segm
ent a
t Luk
urm
ata:
N2
633
E294
8 to
N26
36 E
2948
. Ke
y: l
-10
yr 3
/3 d
ark
brow
n lo
amy
clay
with
som
e gr
avel
. St
icky
when
wet
, fria
ble
when
moi
st,
very
har
d wh
en d
ry.
Cont
ains
inf
requ
ent
carb
on
fragm
ents
(lar
ger
than
flec
ks).
This
is th
e m
odem
plow
zon
e; 2
-10
yr 4
/3 b
rown
loa
my
clay
with
som
e gra
vel a
nd fr
eque
nt w
hite
(sal
t) in
clusio
ns.
Stick
y wh
en w
et, f
riable
whe
n m
oist
, har
d wh
en d
ry. C
onta
ins
infre
quen
t ca
rbon
fra
gmen
ts;
3-10
yr
4/3
bro
wn s
andy
cla
y wi
th s
ome g
rave
l and
freq
uent
whi
te
(sal
t) in
clusio
ns.
Stick
y wh
en w
et, f
riable
whe
n m
oist
. Ap
pear
s qu
ite s
imila
r to
no.
2 a
bove
, alth
ough
re
tain
s m
oist
ure
in pr
ofile
. Co
ntai
ns o
ccas
iona
l ca
rbon
; 4-
10
yr 4
/4 d
ark
yello
wish
-bro
wn,
med
ium
san
d wi
th v
ery
little
gra
vel.
Not
stic
ky w
hen
wet,
not
friab
le wh
en m
oist
. Se
en at
nor
th e
nd o
f the
tre
nch
only,
ove
rlies
raise
d fie
lds
and
asso
ciate
d su
rface
s; 5-
10
yr 4
/4 d
ark
yello
wish
-bro
wn
loam
y cla
y, c
onta
ins
light
er c
oncr
etio
ns.
Very
stic
ky w
hen
wet,
very
friab
le wh
en m
oist
; 610
yr
4/2
dar
k gr
ayish
-bro
wn
loam
y cla
y wi
th v
ery
little
gra
vel.
Very
stic
ky w
hen
wet,
very
friab
le wh
en m
oist
. Con
tain
s ca
rbon
. Th
is is
an a
gricu
ltura
l so
il; 7-
10
yr 3
/3 d
ark
brow
n cla
y, w
ith n
o gr
avel
. Ve
ry s
ticky
whe
n we
t, ve
ry fr
iable
when
moi
st.
Cont
ains
fle
cks
of
carb
on;
8-10
yr
4/4
dar
k ye
llowi
sh-b
rown
fin
e sa
nd. N
ot s
ticky
whe
n we
t, no
t fri
able
when
moi
st.
Som
e gr
avel
; 9-
10
yr 4
/4 d
ark
yello
wish
-bro
wn
sand
y cla
y (v
ery
fine
sand
grain
s);
10-1
0 yr
3/2
ver
y da
rk g
rayis
h-br
own
loam
y cla
y. S
ticky
whe
n we
t, fri
able
when
moi
st. T
his
is a s
wale
soi
l, so
mew
hat
unifo
rm
but
som
e m
ottlin
g pr
esen
t.
242 A. L. KOLATA AND C. ORTLOFF
chaqui-t&la-like implement (the characteristic Andean digging stick) during episodes of field preparation and planting.
Microstratigraphy within the adjoining swales (Figure 6: strata 4G, 5 & 7) implies that this set of buried fields was in production for several years, and that periodic maintenance in the form of field resurfacing and partial reconstruction was necessary. A layer of sand and gravel overlies these fields indicating that an episode of flooding occurred soon after, or at the time of field abandonment (Figure 6: strata 8 & 8G). Thin bands of microstrati- graphy appeared clearly within this stratum in the form of water sorted sands and fine gravel with coarser material deposited at the bottom of the lens. This coarser material was more extensively deposited close to the edge of the adjacent swale, indicating that flooding within the channel with subsequent flow over adjoining field surfaces caused this characteristic pattern of water-borne sand and gravel deposition.
Abnormally high rainfall, possibly combined with an elevated lake level may explain this depositional event. Such stratigraphic evidence of ancient flooding events, if con- firmed by collateral data drawn from the project’s programme of lake coring and from future field excavations, may provide essential insights into understanding the processes of raised field burial and abandonment in the Lake Titicaca basin.
Operation 2: test trench 2: N2638 E2882 Two distinct types of buried fields were uncovered within this trench. The first type was similar to that encountered in test trench 1: non-complex, homogeneous mounds of dark, loamy clay forming agricultural beds (Figure 7: stratum 2). However, the soil forming these beds differed from that of the trench 1 context in colour and texture: this loamy clay was yellowish-brown in colour and contained a greater admixture of fine sand. We believe these differences relate to minor variations in the distribution of soils within the agricul- tural sector at Lukurmata, and that the general similarity in field morphology, context, and structure between these two sets of buried fields indicates contemporaneity. Moreover as indicated below, ceramic associations with this first type of buried field in test trench 2 supports a conclusion of contemporaneity with the set of buried fields in trench 1.
Type 1 buried$elds This first type of buried field was uncovered within the southern end of test trench 2: units N2635 E2882 and N2638 E2882 in the Lukurmata grid map. The buried field was first encountered at 80 cm below local datum. Several Tiwanaku IV sherds were found within the base of the field itself, as well as in overlying strata. In particular, one fragment of a decorated, banded vessel bearing the distinctive polished deep red and black colour scheme of the Tiwanaku IV period (c. AD 400-800) was recovered from the base of the field. These ceramic associations suggest that the field structure dates to this period, although an earlier dating cannot be discounted. No post-Tiwanaku or Inca sherds were encountered in this excavation or in the surrounding surface areas. Unfortunately, there was insufficient organic material in this field construction suitable for a radiocarbon assay.
As indicated above, this buried field was a simple mound of soil approximately 1545 cm wide oriented parallel to the modern drainage pattern in the same east-west direction as the surface raised fields. In profile, this field appeared roughly symmetrical in shape, but with a steeper declination along its southern border (Figure 7). At this point, several resurfacings of the original field occur in the profile (Figure 7: stratum 2). Here resurfacing of the raised field appears as arcs of loamy clay that curved over the southern border of the field contour. Each of these arcs was separated from the previous one by a stratum of yellowish-brown loamy clay identical to the field base (Figure 7: strata 2 & 3). The soil constituting the features that we interpret as sequential field resurfacing was
Figu
re 7
. Stra
tigra
phic
prof
ile o
f Tiw
anak
u ra
ised
field
segm
ent a
t Luk
urm
ata:
N2
638
E288
2. T
his
prot
ile i
llust
rate
s bu
ried
agric
ultur
al fe
atur
es
includ
ing
sequ
entia
l fie
ld re
surfa
cings
(st
ratu
m 2
). Ke
y: l
-10
yr 3
/3 d
ark
brow
n lo
amy
clay,
stic
ky w
hen
wet,
friab
le wh
en m
oist
; 2-1
0 yr
3/4
dar
k ye
llowi
sh-b
rown
lo
amy
clay,
stic
ky w
hen
wet,
friab
le wh
en m
oist
. Occ
asio
nal
carb
on f
leck
s; 2
’-Sam
e as
no. 2
with
mot
tling
ofno
. 3;
3-1
0 yr
4/6
dar
k ye
llowi
sh-b
rown
lo
amy
clay,
stic
ky w
hen
wet,
friab
le wh
en m
oist
; 4-
10
yr 4
/6 d
ark
yello
wish
-bro
wn
fine
sand
, not
stic
ky w
hen
wet,
not
friab
le wh
en m
oist
; 5-
10
yr 3
/6 d
ark
yello
wish
m
ediu
m s
and
with
gra
vel,
pebb
les a
nd o
ccas
iona
l sm
all c
obbl
es, n
ot s
ticky
whe
n we
t, no
t fri
able
when
moi
st;
6-10
yr
3/4
dar
k ye
llowi
sh-b
rown
(m
ottle
d) c
oars
e sa
nd w
ith l
arge
gra
vel,
pebb
les a
nd c
obbl
es, n
ot s
ticky
whe
n we
t, no
t fria
ble w
hen
moi
st; 7
-gra
vel;
8-10
yr
4/6
dar
k ye
llowi
sh-
brow
n sa
nd w
ith m
uch
grav
el,
pebb
les,
and
occa
siona
l co
bble
s, n
ot s
ticky
whe
n we
t, no
t fri
able
when
moi
st;
!J--S
ame
as no
. 2
with
gra
vel a
nd
occa
siona
l sm
all p
ebble
s, als
o wh
ite (
salt)
con
cret
ions
; 10
-10
yr 3
/3 d
ark
brow
n lo
amy
clay
with
gra
vel a
nd o
ccas
iona
l sm
all p
ebble
s, st
icky
whe
n we
t, fri
able
when
moi
st; 1
l-10
yr 4
/4 d
ark
yello
wish
-bro
wn
clay
with
mixt
ure
of s
and
and
grav
el, s
light
ly st
icky
whe
n we
t, fri
able
when
moi
st;
12-
10 yr
4/6
dar
k ye
llowi
sh-b
rown
sa
ndy
loam
with
poc
kets
of s
andy
cla
y, co
arse
gra
vel a
nd o
ccas
iona
l peb
bles,
stick
ines
s and
fria
bility
va
riable
; 13
- 10
yr 4
/2 d
ark
gray
ish-b
rown
cla
y, w
ith c
arbo
n fle
cks,
ver
y st
icky
whe
n we
t, ve
ry fr
iable
when
moi
st.
244 A. L. KOLATA AND C. OR-II-OFF
darker in cofour, bearing a larger organic content, as might be anticipated from its intended function as “rejuvenated”, organically-enriched planting surfaces.
At least five of these resurfacing features were identified in the profile, each capping the other. However, some of these arcs of organic soil are ephemeral, and difficult to distinguish in the highly complex stratigraphy that characterizes these buried agricultural constructions. Accordingly, we cannot state with certainty that this represents the entire sequence of resurfacing. Each of these features was approximately 5 cm thick, and each represents an eroded planting surface. Together they indicate that the field had multiple seasons of use, with regular periods of maintenance and resurfacing.
It is difficult to estimate the interval between successive resurfacings of the field, but the relatively substantial soil deposition between these features suggests that this was not an annual event. However, distinct periods of field resurfacing with presumptive replacement of organic rich nutrients from adjacent wale contexts establishes quite clearly that these fields were maintained and in production for a substantial period of time.
Type 2 buriedjelh A second, strikingly different type of buried field was also encountered in test trench 2. This buried field construction is morphologically similar to the raised fields on the surface: both field systems are characterized by massive planting platforms with a wavelength that exceeds 10 m. However, the second type of buried field in test trench 2 possesses a remarkably sophisticated internal composite structure that was not found elsewhere in the agricultural sector at Lukurmata.
This second type of raised field was first encountered in the southern end of the trench in Units N2635 E2882 and N263g E2882. This buried field extended south and west, forming a more massive structure than any described above. Futher excavations to the west in Units N2635 E2878.5, N2635 E2877 and N2638 E2880.5 uncovered larger segments of this construction.
Excavation of the field revealed a complex, precisely layered sequence of cobbles, clay, sorted gravels, and top soil (Figure 8). The bottom stratum consisted of a packed, level layer of rounded cobbles which functioned as ballast, forming a massive, stable pediment or foundation for the field base (Figure 8). These cobbles were apparently transported to the construction site from the quebrudu that cuts through the southern hillslopes of Lukunnata. The function of the packed cobble pediment seems clear. Since the raised fields in the agricultural sector of Lukurmata were constructed in a topographic low, they would have required some form of stable foundation that provided superior drainage Properties to counteract the excessively marshy conditions of this area.
The architects of this field construction then laid down over the cobble pediment a layer of pure, dense clay of apparent lacustrine origin approximately 5-8 cm thick (Figure 8: stratum 13). The presumptive function of this clay stratum, which to our knowledge has never been described for any other raised field construction in the New World, is of special interest and importance for understanding the processes of agricultural reclamation along lake beds in the Titicaca basin. We currently hypothesize that this clay stratum served as an aquaclude which inhibited the percolation of slightly saline water from Lake Titicaca into the main body of the field structure. That is, the clay cap over the cobble pediment acted as an impermeable layer that was designed to mitigate the problem of the migration of salts upward into the crucial zone of root development within the field.
Alternatively (or perhaps simultaneously), this clay stratum may have served as a “micro-aquifer” maintaining a thin, evenly distributed lens of fresh water at a stable level into which the root.systems of the food crops could tap. The principal sources of fresh water in this vicinity lie in a series of permanent springs and seasonal streams originating in the mountain slopes to the south of Lukurmata. In effect, both seasonally flowing
Figu
re 8
. Stra
tigra
phic
prof
ile o
f Tiw
anak
u ra
ised
field
segm
ent a
t Luk
urm
ata:
N2
635
E288
2 to
N26
35 E
2877
. Thi
s pr
ofile
illu
stra
tes
a ser
ies o
fcom
plex
int
erna
l st
ruct
ural
fea
ture
s of
the
type
2 b
uried
fie
lds
disc
usse
d in
the
text
. Ke
y: l
-10
yr 3
/3 d
ark
brow
n lo
amy
clay,
stic
ky w
hen
wet,
friab
le wh
en m
oist
; 2-1
0 yr
3/4
dar
k ye
llowi
sh-b
rown
lo
amy
clay,
stic
ky w
hen
wet,
friab
le wh
en m
oist
. Occ
asio
nal c
arbo
n fle
cks;
2’-S
ame
as no
. 2 w
ith m
ottlin
g of
no.
3; 3
-10
yr 4
/6 d
ark
yello
wish
-bro
wn
loam
y cla
y, s
ticky
whe
n we
t, fri
able
when
moi
st; 4
-10
yr 4
/6 d
ark
yello
wish
-bro
wn
fine
sand
, not
stic
ky w
hen
wet,
not
friab
le wh
en m
oist
; 5-
10
yr 3
/6 d
ark
yello
wish
med
ium
san
d wi
th
grav
el, p
ebble
s an
d oc
casio
nal s
mal
l cob
bles
, not
stic
ky w
hen
wet,
not
friab
le wh
en m
oist
; 6-1
0 yr
3/4
dar
k ye
llowi
sh-b
rown
(m
ottle
d) c
oars
e san
d wi
th
large
gra
vel,
pebb
les a
nd c
obbl
es,
not
stic
ky w
hen
wet,
not
friab
le wh
en m
oist
; ‘I-
grav
el;
8-10
yr
4/6
dar
k ye
llowi
sh-b
rown
sa
nd w
ith m
uch
grav
el,
pebb
les, a
nd o
ccas
iona
l cob
bles
, not
stic
ky w
hen
wet,
not
friab
le wh
en m
oist
; !&-
-Sam
e as
no. 2
with
gra
vel a
nd o
ccas
iona
l sm
all p
ebble
s, als
o wh
ite (s
alt)
conc
retio
ns;
10-1
0 yr
3/3
dar
k br
own
loam
y cla
y wi
th g
rave
l and
occ
asio
nal
smal
l peb
bles,
stic
ky w
hen
wet,
friab
le wh
en m
oist
; 1 l
-10
yr 4
/4 d
ark
yello
wish
-bro
wn
clay
with
mixt
ure
of s
and a
nd g
rave
l, sli
ghtly
stic
ky w
hen
wet,
friab
le wh
en m
oist
; 12
-10
yr 4
/6 d
ark
yello
wish
-bro
wn
sand
y loa
m w
ith
pock
ets
ofsa
ndy
clay,
coa
rse g
rave
l and
occ
asio
nal p
ebble
s, st
ickin
ess a
nd fr
iabi
lity
varia
ble;
13-1
0 yr
4/2
dar
k gr
ayish
-bro
wn
clay,
with
car
bon
fleck
s,
very
stic
ky w
hen
wet,
very
friab
le wh
en m
oist
.
246 A. L. KOLATA AND C. ORTLOFF
surface water and permanent ground water percolating down from the heads of the mountain springs would have charged the micro-aquifer formed by the clay layer of the fields year round.
Surmounting the clay stratum of this buried field were two layers of sorted gravels and sands and a final layer of top soil that capped the entire field surface. The first layer of gravel immediately covering the clay stratum was characterized by a mix of medium to coarse grained sands with relatively large pebble inclusions (c. l&-1.5 cm diameter) (Figure 8: stratum 5). The second layer of gravel consisted of fine grained sands and small pebble inclusions ( < 0.5 cm diameter) with an admixture of sandy clay (Figure 8: stratum 12). These layers of carefully sorted sands and gravel, transported at the cost of substantial labour investment, clearly functioned to enhance drainage within the body of the field structure, thereby preventing oversaturation and subsequent crop damage.
The final stratum in this structure was the agricultural topsoil which originally covered the entire domed surface contour of the buried field (Figure 8: stratum 2). This topsoil consisted of composite layers of loamy clay of a dark gray colour reflecting a constitution higher in organic matter than the deeper, structural strata of the field. We may reasonably infer that the immediate source of this topsoil was the rich, organic mud deposits extracted from adjacent swales. As might be anticipated, this original topsoil showed evidence of erosion, particularly toward the field borders that sloped downward toward the adjacent swales. We assume that when this now buried field was in production, the layer of agricultural topsoil was considerably thicker than its current average of approximately 10 cm.
Evidence for agricultural resurfacing of this buried field was also detected in cross- section (Figure 8: strata 2 & 4). Multiple planting surfaces were documented in the western portion of the field, as well as along the eastern periphery of the structure as it slopes downward toward an adjacent swale. These soil layers were similar in colour and texture to the resurfacing observed in other contexts described above: dark, yellowish-brown loamy clays. These resurfacing layers were also similar in thickness, roughly 8 cm, and were separated by a loamy clay of a lighter colour. The exceptionally complex stratigraphy evident near the adjacent swale suggests that this was the source of material for the multiple resurfacings (Figure 8). Microstrata formed arc-like patterns near the swale, but the tortuous intercalation of soil deposits at this point prevented us from following these arcing patterns out over the adjoining field surface. However, the general implication of this stratigraphy is clear: soils from swales were periodically dug up and redistributed over field surfaces to provide a new, richly organic planting surface. Chemical analysis of soil samples drawn from different contexts in the excavated raised fields, such as from swales, agricultural topsoils, and field construction fill, supports the interpretation that these sediments represent anthrosols, or human altered soils (Eidt, 1977). In particular, phos- phate fractionations of selected soil samples from the Lukurmata fields exhibit high total phosphate values indicative of anthropogenic sediments (Table 1). According to Eidt (in litt. 24/04/87), these values are comparable to results of samples extracted from buried raised fields in Colombia that suggest a land use pattern of long-term mixed residential and associated agricultural activities. Such a conclusion is consonant with the archaeological evidence for the Tiwanaku IV-V phase settlement activities at Lukurmata.
A substantial quantity of ceramics were found incorporated within the various layers of structured fill in this buried field construction (approximately 250 m2 in a given level). The bulk of these sherds derived from undecorated utilitarian vessels. However, roughly 10% represented fragments from decorated ceramics, particularly keros, a high status drinking vessel characteristic of Tiwanaku occupations. Preliminary analysis of these sherds indi- cates that these decorated vessels had a red or orange-red slip, and were painted in Tiwanaku IV colour schemes and motifs. Of particular note was the modelled foot of a
Table
1.
Phos
phate
fra
ction
ation
va
lues
for
soil
samp
les
draw
n fro
m mu
ltiple
conte
xts
in bu
ried
raise
dfield
se
dimen
ts
Sam
ple
no.
Unit
Prof
ile
Dept
h be
low
datu
m (
cm)
Cont
ext
Fr. I
a
P in
ppm
%
P
Fr. I
b Fr
. II
Fr. I
II To
tal
Ia+I
b II
III
949
953
954
956
951
966
967
969
915
978
N263
3 Ea
st
E294
8 N2
636
East
E2
948
N263
6 Ea
st
E294
8 N2
636
East
E2
948
N263
6 Ea
st
E294
8 N2
635
East
E2
882
N263
5 Ea
st
E288
2 N2
635
East
E2
882
N263
5 So
uth
E287
8.5
N263
5 So
uth
E287
8.5
13
73
42
78
83
17
94
94
52
49
Agric
ultur
al So
il st
ratu
m 6
Ag
ricult
ural
soil
stra
tum
6
Abov
e fie
ld su
rface
st
ratu
m 2
Sw
ale
stra
tum
7
Swale
st
ratu
m 1
0 Ag
ricult
ural
soil
stra
tum
2
Agric
ultur
al so
il st
ratu
m 2
Ag
ricult
ural
soil
stra
tum
2
Agric
ultur
al so
il st
ratu
m 2
Ag
ricult
ural
soil
stra
tum
2
6.5
199.
6 12
2.2
689.
6 10
17.9
20
12
68
9.9
113.
0 94
.6
451.
2 66
8.7
18
14
68
2.6
69.1
14
2.9
355.
6 57
0.2
13
25
62
5.5
17.9
16
4.7
388.
0 63
6.1
13
26
61
8.1
84.7
11
5.1
391.
2 59
9.9
16
19
65
4.9
180.
1 12
4.6
459.
7 16
9.3
24
16
60
4.4
182.
3 18
8.6
482.
1 85
7.4
22
22
56
3.5
208.
6 16
2.8
461.
1 83
6.0
25
19
55
6.4
35.2
72
.5
321.
6 43
5.1
10
17
14
3.9
204.
4 25
1.9
605.
4 10
65.5
20
24
57
248 A. L. KOLATA AND C. ORTLOFF
classic Tiwanaku IV puma incensario, or incense burner. This material indicates that the fill of the field structure was scraped up from surfaces bearing cultural deposits and artificially transported to the construction site. Furthermore, the diagnostic ceramics suggest a chronological assocation with the Tiwanaku IV period, although a later Tiwanaku V association cannot be discounted until completion of the ceramic analysis from this context.
At some point in the history of agricultural reclamation at Lukurmata, the swales of this complex field structure were filled in and the old field surfaces covered over to provide the base for the overlying, surface raised field system (Figure 8: strata 8 & 10). No natural deposits, such as the fine, water-sorted sands overlying the buried field structures in test trench 1, were uncovered here. The lack of such natural water-borne, or aeolian deposits on the surface of the type 2 buried fields in trench 2 indicates that: (1) relatively little time elapsed between the cessation of production on this buried field and the period of recon- struction represented by development of the surface fields; (2) the episode of flooding that clearly affected the buried field system uncovered in trench 1 did not play a role in the history of this field system; and (3) therefore there is a high probability that the two types of buried fields uncovered in both test trenches represent temporally distinct phases of agricultural reclamation in the agricultural sector of Lukurmata.
Modeling Raised Field Function: Introduction and Analysis The representative field systems excavated within the Lukurmata and Lakaya areas reveal significant diversity in terms of ridge/swale geometry, orientation and stratigraphy. Despite this diversity, typical patterns can be discerned and analysed that relate field morphology and internal structure to specific function in the demanding environmental context of the Andean altiplano.
To gauge the types of analysis that need to be performed and to understand the work- ings and efficiencies of raised field systems, a number of factors need to be considered. These include: (I) sulinization of field growing zones by incursion of saline Lake Titicaca waters [(for cases for which the lake level (temporarily) exceeds the local (fresh) water table height or the water table recedes due to drought conditions leading to saline water intrusion)]; (2) drainage of (fresh water) saturated soils into raised field swales when the lake level is below the local water table height; (3) agricultural land area that depends upon the lake level (since the ground slope of lakeside land is low, a rise in lake level can reduce this area considerably); (4) heat storage capability of a raised field system; and (5) soilfertility (which may continually decrease due to nutrient leaching, monocultural cropping, and lack of replenishment of organic matter).
All of these parameters depend on the variability of Lake Titicaca water level. This level undergoes episodic and long-term fluctuations that are not currently well-understood, although potentially associated with global changes in climatic and precipitation patterns. The relation between these five parameters and lake level h,, measured from a given reference plane, is shown in Figure 9 in schematic form. In this figure ht,,,, -C/Z,,, where h,, and h,, represent maximum and minimum possible Lake Titicaca levels. The quantity h,, represents the extended drought period lake level while h,, represents the flood condition lake level. The “normal” lake level fluctuation range is defined to be h,, -C h < hN1, where h,, and hN1, represent the normal seasonal lake level variation.
Other lake level fluctuations may occur due to long or short term climate cycles involv- ing varying rainfall amounts into the altiplano drainage or watershed zone that channels groundwater and runoff into the lake. As,lake level increases, salinization, which is a function of (/z~-/z,)-~ (Figure 9) also increases. This reflects that fact that saturation by other than (fresh) ground water is increasing as h2-/z, decreases. The quantity h, is defined (Figure 9) to be the groundwater average height above a reference plane. Similarly,
TIWANAKU RAISED FIELD SYSTEMS 249
Lake Titicaco t Lake level level change
hm “f
hwi t G l Ridged fields hl
L&e Ridged fields
\\ Solinizqtion /
Drainage
(Stole) Heat storage, Dromage, Land area, Solinization
Figure 9. Survivability polygon for raised field agricultural systems.
drainage of groundwater varies with h-h,: that is, the higher the groundwater level with respect to lake level the greater the drainage capability of the raised field. This factor greatly affects the start and finish times for the growing season. The arrival of maximum groundwater flow to field systems near the lake edge occurs past the peak of the rainy season. This is due to the generally low flow speeds of ground water deposited at remote locations within the collection basin of the lake. The (local) low ground permeability constants and flow pressure gradients are then the cause of the continuous arrival of ground water to the lake edge, including significant groundwater flows in the dry season. Thus the swales can serve as channels to help drain ridges in dry (as well as wet) months provided h, > ht. The swales present alternate drainage paths for saturated soils in the ridges. Water may either flow into the swales and then to the lake (provided h,>h,) or through the aquifer to the lake. The residence time for water particles for the latter path is much greater than for the swale path, therefore the swale paths are better drainage routes to the lake.
The land area available for agriculture is likewise dependent upon the lake height as previously discussed. For cases in which the ridges protrude above the lake level (for near shoreline agriculture) and h, is approximately equal to h,, then artificial mounding and/or buried impervious clay layers, such as that found in the field system at Lukurmata, may be used to regulate the ridge surface to water table height differences to protect crop root systems from waterlogging.
The factor of field system heat storage capability depends upon the saturation state of the raised field soils. The heat storage from solar flux heat input plays the role of modulating diurnal ground temperature. As soil saturation increases as a function of groundwater height with respect to the field system surface, the heat storage capability of
250 A. L. KOLATA AND C. ORTLOFF
the soil expands. Expansion of the heat storage capacity enhances the survival of root crops during cold altiplano nights and determines an effective “end of the growing season” as such effects limit ground freezing and concomitant destruction of root crops.
The degree to which these parameters are effective in determining the success of the agricultural system depends additionally on soil fertility. As soil fertility declines, we expect that crop toleration to salinization, to waterlogging, or to decreased heat storage capacity will similarly decline. As the range of toleration of crops to these conditions contracts, a new set of constraints affecting the type, sequence, and fallow periods of crops grown in given field segments will be established with a probable correlative reduction in plant productivity.
Based on the general effects of these five main parameters and their qualitative variation with lake level (Figure 9), a “normal operating range” of lakeside raised field agriculture can be determined for a given soil fertility level. If the normal height fluctuation of the lake is h,, <h <h,,, then a polygonal area A-B-C-D bounds normal operating ranges of the four parameters shown. Operation of the raised field system within A-B-C-D then insures stable agricultural production. If the lake level h, approaches a high (maximum) level (hEXI) then high salinization, low land area and low drainage considerations apply. Although high heat storage can result in field systems under these conditions, the water- logging of these fields lead to shallow root systems for any crop. This effect occurs as root systems do not penetrate the water table but rather spread out above it. Waterlogging of shallow root systems creates biological conditions unfavorable to crop maturation, and in these conditions will lead to subsequent decreases in crop yields. At low lake levels, (h&, low heat storage prevails while good drainage and low salinization accompanies agriculture on a large, exposed land area,
To a certain degree, salinization (or incursion of lake-borne concentrants), heat stor- age, and distance from roots to water table surface can be regulated by artificial mounding and elevation of planting surfaces. This control can be exercised if fill is available for transport or can be obtained from further swale dredging (or is simply built into the original ridge design based on anticipated lake level fluctuations). Observations of field plots in the Pajchiri area (Figure 2) clearly indicate that this strategy was actively pursued, perhaps for selective crop types. Note that in Figure 9, limitations may exist in the effective range of salinization, drainage and heat storage to permit effective crop yields depending upon soil fertility.
Further limits in land area may exist from topograhic constraints. The A-B-C-D “normal operating range” polygon may then effectively be replaced by a somewhat different polygon figure (&, A,., S,, HS,, A,, D,, H&, SW, D,,) to delimit more realistic operating limits for successful agricultural production. The M subscripts denote the maximum tolerable operating limit of a particular parameter while the M’ subscript denotes the minimum tolerable limit. As soil fertility decreases, S,, S,, HS,, HSw will contract inwards along the given salinization and heat storage curves respectively and produce a “distorted” normal operating zone for successful raised field agriculture. This “distorted” polygon may considerably limit crop types and growing season duration. More field system area may need to be created to maintain the same total yield from less productive field systems as soil fertility decreases. Outside of the “stability polygon”, agricultural production is severely compromised and continued production at high levels is jeopardized.
Heat Storage ia High Altitude Raised Field Systems The present paper examines only one of the important parameters delimiting successful operation of raised field systems: the heat storage capability. In order to analyse this
TIWANAKU RAISED FIELD SYSTEMS 251
p q density T = temperature c = specific heat t = time k = thermal conductivity K = thermal diffusivity = k/pc
Solution : x/2&i
T = (2T,,/&) I
ems2 ds q Toerf[X/2&?] J 0
T=O .X=0
- -- T=ToZO - -- -- -.-- -- -.---___ -- --
t-o
A Air 0.187 Rock 0*0188 Soil (41/l 0.0046 Soil (sandy, dry) O-0028 Soil (sandy, moist) 0.0033 Water O-0014
Figure 10. Heat conduction equation and solution with table of thermal diffusivity values for raised field systems.
effect on crop survival under cold seasonal temperature variations, consider first a model problem to illustrate the physical phenomena involved in the heat storage effect.
With reference to Figure 10, consider a one-dimensional unsteady heat conduction problem governed by the parabolic heat conduction equation. Variables and parameters are defined below the governing equation in this figure. The solution to the partial differ- ential equation can be written in analytic form in terms of the error function (erf) of argument (x/2(JKt)) where t is time, Kis the thermal diffusivity and x is the depth into a medium from its surface plane.
Suppose next that the initial-boundary value problem to be solved is one of a medium initially at a temperature To at t =0 for 0 <X-C co; at t > 0, the x =0 boundary is maintained at T= 0. The solution to the heat conduction problem then gives the ensuing T(x, t) temperature distribution in the medium for t 2 0,O <x < cc.
From standard tables of the error function (Abramowitz & Stegun, 1964), T= T,,/2 at x/2,/Kt = O-477; the temperature in the medium has therefore dropped to half its initial temperature at a depth x=x, from the x= 0 top plane at a time equal to t = K-‘(xl/2(0-477))2. The mechanism for the temperature drop is the continuous heat flow from the medium interior to the constant low temperature (T= 0) boundary at x = 0. From this solution, the time for a temperature reduction to T,,/2 at x = xi depends inver- sely on the thermal diffusivity, K. From the table of K values for different media given in Figure 10, it can be seen that if the medium of heat conduction is air then cooling at a depth of x, to T,,/2 takes place rapidly: that is, the thermal diffusivity for air is a large number so the time to cool to To/2 is a small value. If the medium for heat conduction is wet, sandy soil, the time to cool (at x,) is considerably larger. If the medium is water, then time to cool
252 A. L. KOLATA AND C. ORTLOFF
to T,,/2 is about 133 x more than if the medium is air. The thermal diffusivity thus regulates the heat storage and transfer capability of a given medium. The lower the thermal diffusivity value, the more heat storage effect occurs in a given medium.
Based upon these observations, given a soil medium (such as is found in raised field systems), if heat can be absorbed internally by means of daily solar flux input, then the possibility for heat retention during cold nights is high. In other terms, the time required for wet heated soil to cool is long due to its small thermal diffusivity value. The heat retention capability of soils then acts as a further protective enhancement to preserve root crops against cold (subfreezing) altiplano night temperatures that would ordinarily destroy crops by freezing. The presence of water in the swales of the raised field systems further enhances the heat storage capability of the system. Since the thermal diffusivity of water is low (Figure lo), it will retain its heat for long periods and thus serve as a heat source to the adjacent field ridges during cold nights. By means of these heat transfer mechanisms, an extension of the growing season may be achieved by (perhaps) a few weeks allowing for the possibility of double cropping of certain types of products or at least survival of the existing crops to further levels of maturity.
In order to study this heat retention effect in greater detail, the heat transfer model is increased in sophistication to more closely model actual Pampa Koani and Lukurmata raised field systems. To this end, a finite element model (Figure 13) of an excavated raised field profile (Figure 3) is constructed with matching geometry. Water is assumed to exist in the swales to a given height.
The finite element method is a numerical method (Zienkiewicz, 1971; Gallagher, 1975; Sagerlind, 1976; Swanson & DeSalvo, 1985) used to solve more complex transient temperature distribution problems in a multi-property medium. Effects of conduction, convection and radiation may also be included. In the model (Figure 13), a number of types of media are used. The raised field depth profile (considered as a two dimensional medium) is divided into saturated soils (below the height of the water in the swales) and soils above the swale water level which are in the phreatic and vadose zones: that is, these soils have a lesser water content than saturated soils. These phreatic and vadose region soils obtain their moisture through capillary action from subsurface groundwater. The air above the raised field system, as well as a stone layer in the internal structure of the field (Figure 13), are also included in the model. This construction has been found in typical excavated profiles. Heat input is through solar flux into the soil and water surfaces. Heat loss from the soil and water surfaces occurs by means of radiation and free convec- tive losses. Heat conduction occurs throughout the various soil types, the ambient air, and the water contained within the swales. The scale lengths used in Figure 3 are the same as those in the finite element model (Figure 13) for consistency.
Use of the ANSYS finite element code (Swanson & DeSalvo, 1985) is made to analyse the resulting heat flow problem to obtain transient soil temperature distributions. Time averaged solar flux values as a function of month in the Bolivian altiplano area of the present study are obtained from DulIie dz Beckman (1974: 34-7). Surface heat transfer coefficients for free convection as well as radiation and reradiation emissivities and related constants are estimated from DufIie & Beckman (1974), Kreith (1973), and Swanson & DeSalvo (1985). A listing of material property values is given in Tables 2 & 3 for the four material types (air, water, saturated soil, phreatic zone soil) used in the ANSYS model. The units are appropriate to the Swanson & DeSalvo (1985) input format. Physical constants are estimated from Kreith (1973). Because solar flux (BTU/s-area) varies on an hourly basis in a 24-h cycle an ANSYS cosine distribution-/PREP6/-model of the input flux (Figure 11) is made to represent the diurnal solar flux variation for use in the ANSYS input file (/PREP’I). Similarly, the air temperature changes continously on a 24-h cycle. The input temperature values (in degrees Rankine) are represented in Figure 12 and are
Table
2.
Mate
rial
cons
tantsf
orJin
ite
eleme
nt
mode
l: re
feren
ce
4 for
mat
LIST
IIA
TERI
ALS
1 TO
OD
Y 1
PROP
ERTY
- RL
L ww
Eafin
m DA
TA
TEnP
ERnn
JRt
D41
n e.
eeee
eE~e
e c1
.787
00E-
es
2381
.8
0.78
7eet
-e
s
PRO
PERT
Y TA
BLE
KXX
MAT
- 1
TEnP
ERAf
URE
DATA
e.
eeee
eE+e
e e.
356e
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254 A. L. KOLATA AND C. ORTLOFF
Table 3. Material and real consonants for finite element model: reference 4 formal
LIST tlATERIALS 10 TO OBY 1 PROPERTY- ML
PROPERTY TABLE KXX RAT- 10 TERPERATURE DATA
e.QeeeeE+ee B.leeeBE-82
PROPERTY TABLE KVY RAT- 1. TERPERATUKE DATA
e.eeeeeftee e.waeeE-e2
PROPERTY TABLE DENS RAT- 10 TERPERATLIRE DATA
e.eeeeeE+ee e.i19eeE-86
PROPLHTY TABLE C MAT- 18 TEflPERATlJRE DATA
e.eeeeeEtee 92.730 PREP7 -INP- RLIST.1.20.1
NM. POINTS. 2 TEtlPERATURE . DATA
2388.0 e.ieeeeE-02
Null. POINTS- 2 TEtlPERATURE DATA
2399.8 e.ieeeeE-e2
NUN. POINTS- 2 TEflPERATURE DATA
2368.8 e.lIQeeE-86
NM. POINTS- 2 TEMPERATURE DATA
2389.0 92.739
LIST REAL SETS 1 TO 18BY 1
REAL CONSTANT SET 1 ITEllS IT0 6 1. eeee e.eeeeeEte(r e.eeeeeEtee e.eeeeeEtee e.eeeeeEtee e.eeeeeE+ee
REAL COtISTAtiT SET 2 ITEMS IT0 6 l.eeeo e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee
REAL CONSTANT SET 3 ITEM IT0 6 1 .eeee e.eeeeeEtee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeEtee Q.eeeeeEiee
REAL CONSTANT SET 4 ITEMS 1TO 6 1 .eeee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeEtee
REAL CONSTANT SET S ITEM 1TO 6 15.880 e.eeeeeEt68 e.eeeeeEtee e.eeeeef+ee e.eeeeeE+ee e.eeeeeEtee
REAL CONSTANT is. eee
SET 6 ITEM IT0 6 1. eeee 8.48888 e.33ieeE-14 e.eeeeeEtee e.eeeeeE+ee
SET 7 ITEM I TO e.eeeeeEtee’ e.eeeeeE+ee6 e.eeeeeE+ee e.eeeeeftee e.eeeeeEtee
SET 8 ITEM IT0 6 e.eeeeeE*ee e.eeeeeE+ee e.eeeeef+ee e.eeeeef+ee e.eeeeeE+ee
REAL CONSTANT is.868
REAL CONSTANT i .eeee
REAL CONSTANT 1.8068
PREP? -1NP.
SET 18 ITEM 1TO 6 e.eeeeeE+ee e.eeeeeE+ee e.eeeo6f+ee e.eeeeef+ee e.eeeeeEtee
used as input to the heat transfer problem. These temperature values are estimated from observations taken on a 24-h basis in a typical altiplano winter month.
The finite element method for heat transfer problem solution starts with the Poisson Equation with temperature as the dependent variable. This equation, in matrix form, is:
where
[c] = specific heat matrix (including the appropriate mass effects), [k] = thermal conductivity matrix (including equivalent face convection terms), {T} = nodal temperature vector, {Q} = heat flow rate vector (including the applied heat flow, internal heat generation
and convection).
This equation is solved by an implicit direct integration scheme based on a modified Houbolt method (Swanson & DeSalvo, 1985) and uses a quadratic temperature function
TIWANAKU RAISED FIELD SYSTEMS 255
VOI.
0~008000
0.007200 -
0.006400 -
0-005600 -
0~004000 -
0~004000 -
0.003200 -
0.002400 -
0.001600 -
0*000800 -
0-005600 -
0~004000 -
0~004000 -
0.002400 -/
0.003200 -
0.001600 -
0*000800 -
Solar flux (BTWsec-in’)
0’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 14.80 19.60 114.4 119.2 124.02
2.40 7.20 12.0 16.8 21.6 Step
Figure Il. Graphic input for the solar heat flux diurnal cycle in the Pampa Koani zone.
VOl 525.00,
1
485.00 - Degrees ronkme
b Midnlght
/
475.001 I I I I I I I I I 0 14.80 19.60 114.4 119.2 124-O 2.40 7.20 12-o 16.8 21.6 Step
Figure 12. Diurnal air temperature variation in the Pampa Koani raised field zone.
T, = a+ bt + c?. This function is substituted into the governing equation to yield an equation in three (spatial) unknowns: a, b and c. If At is the step between iterations, a set of three equations may be defined at t, t - A t and t - 2 A t and solved simultaneously to give the integration equation
256 A. L. KOLATA AND C. ORTLOFF
Phreotic Zone
Water Stone Loyer
Soturoted Soil
Figure 13. ANSYS finite element model based on empirically determined raised field geometries.
which may be solved for T, since solutions at previous times are known. For the present problem, the deep ground water temperature is fixed at a given value on the lower bound- ary (Figure 13). A value of 50”F, determined by test measurements in the field, was utilized. The diurnal air temperature variations are prescribed as discussed previously.
Initially, typical raised field material constants are set (density, specific heat and con- ductivity) for the four material types (Table 3). Additional constants are set for convection and radiative heat transfer loss terms at the ridged field and swale water surfaces. Time dependent solar flux and air temperature values are input and the heat transfer problem solved for t > 0. Results are in the form of temperature profile contours within the raised field model at various times during the diurnal cycle.
Results of the Calculations Shown in Figures 14-15 are typical temperature profiles within the raised field system (Figure 3) on the Pampa Koani at times during the day and night. Contour letters in the plots can be identified with the RHS temperature values. At a time corresponding to noon-to-early-afternoon when solar flux and air temperature peaks, the soil temperature exhibits maxima on the soil surface (Figure 14). As solar flux and air temperature decrease toward late afternoon and early evening, surface soil temperate decreases while internal soil temperatures remain high due to the low thermal diffusivity of the soil and water compared to the adjacent air elements.
Toward evening and night (Figure 15) the temperature maximum is clearly internal, within the soil ridges, while the outer soil layers experience temperature minimums. The presence of groundwater at near uniform temperature above that of the ambient air temperature effectively serves to direct heat flows upward at night adding to the heat storage effect of the high thermal diffusivity soils.
During the day, the low groundwater temperature insures a thermal gradient thereby improving the input heat transfer. Although the groundwater temperature is low, it is still higher than ambient air temperature at night. This thermal differential leads to an upward heat flow. The presence of the swale water is also effective in maintaining higher than ambient air temperatures within the interior of the ridges. The zone of interest to root crops is that between soil surface and water height in the swales (one or two finite element
TIWANAKU RAISED FIELD SYSTEMS
H 1111111111111111111~ ------ 1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1 I I I I I IV,, r,-------- -------_------- ---
I I I I I I I I I I I I I I I I I I I I I I I I I I I I’,, .__--_------__-----------d-m -- II II I I I I I I I I I I I I I I I I I I I I I I I I I 1-l-l-l-l-l-~-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l- _____------------------ ----- ‘I-1 I I I I I III I I I I I I I I I I I I I I I I I I I --__------------------- ----- ‘I-1 I I I I I I I I I I I I I I I I I I I I I I I I I I I ‘l-l-l-l-l-l-l-l-l-l
---------------m-w -- I i I I I I I I I I I I I I I I I I I ----------------------------
i-1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I-I
-----------------------e--w- I I I I I I I I I I I I I I I I I I I I I I I I I I I
1-1-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l- ______-------- .I-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l I I I I I I I I I I I I I ---__-__-___-_--se-- j~l~l~l~l~l~l~l~l~l ,I *I ,I cI *I J J *I J ?I ?I J Cl pl *I -1 PI J ?I c
.-------------_--_-_------me-- ,wbmarn r:cLu WsIcn I- awL”~*‘
Figure 14. Raised field soil temperature profile: modeled in late morning. denotes maximum values; MN denotes minimum values.
MX
heights from the ridge top surface). Observation of the temperatures below the surface then indicates the diurnal temperature variation of the soil containing the root crops.
Clearly, the heat storage effect is effective in keeping internal soil temperatures some- what warmer than air temperature, with the consequence of preventing, or ameliorating root damage during subfreezing altiplano nights. Although only a few degrees Rankine (T(“F) +460”F) separate internal from external temperatures during the night, this differ- ence is instrumental in keeping the internal ground temperature above the freezing limit at depths where root systems exist. As freezing temperatures are encountered within the interior of the raised fields, a certain amount of time is required to extract the latent heat of solidification from the soils (and root crops) before total freezing destroys the crop.
The presence of this auxiliary heat storage capability is therefore important to root crop survival. The freezing of plant parts at the soil surface level is likewise delayed by the heat absorption effect which helps to keep these temperatures slightly higher than ambient air temperature. It should also be noted that native aquatic macrophytes, such as Miriophillum-Elodea, Potamogeton, Ruppia, and Schoenoplectus totora, were most likely dredged from the swales between field platforms and used as green manure to replenish macronutrients (N,P,K) in the soil. The decomposition of this plant material, if mixed into the soil, may generate a further source of heat during the planting season.
The calculations presented here are for typical air temperatures, solar fluxes and soil material constant values as found in the Pampa Koani area for given field geometrical
258 A. L. KOLATA AND C. ORTLOFF
Figure 15. Raised field soil temperature profile: modeled in late evening. MX denotes maximum values; MN denotes minimum values.
patterns. A summary of the material and real properties in the Swanson & DeSalvo format are given in Tables 2 & 3. Although these field system patterns change locally on the pampa (and exhibit different orientations and thus different received flux values) with respect to the sun’s path, none the less, the results presented in this summary are typical of qualitative trends in heat storage capability of raised field systems. Soil temperature values may change in time with different soil types, ground cover solar flux, and air temperature levels, as well as with diurnal wind speed changes over the fields. However, the qualitative trends discussed here are expected to apply in a general sense.
Field measurements of temperature were made for air, swale water, and ground temperature at various times during the diurnal cycle to check the qualitative predictions of the theory. Generally, the heat storage effect was confirmed. The model assumed clear water absorption/reflection/transmittance radiation properties. However, the presence of darker organic material within the swales provided a further solar radiation trap that yielded higher swale water temperatures than predicted by the model. It was observed, for example, that actual swale water temperature can exceed ambient air temperature by as much as lO-15°F during peak solar flux times. The presence of this augmented thermal effect through higher (than predicted) swale water temperatures clearly enhances the heat storage capability of raised field agricultural systems. In combination with higher vadose zone ground temperatures, this effect provides an additional source of heat storage for raised field agricultural systems in high altitude environments.
TIWANAKU RAISED FIELD SYSTEMS 259
Conclusions The intensification of agricultural production through reclamation of seasonally or periodically inundated land in the Titicaca basin represented a prime economic strategy of the Tiwanaku state. The technology of land reclamation entailed construction of massive raised field systems in the near shore environment of Lake Titicaca. These raised fields, characterized by a complex internal structure not reported for such systems elsewhere in the ancient Americas, were linked to sources of fresh water by near-surface groundwater flow, and by spring fed, open-channel aqueduct and canalized quebrada delivery networks. ANSYS finite element modelling of the thermal properties of excavated Tiwanaku raised fields, utilizing empirically established field geometries, internal structure, and hydrological conditions, indicates that heat conservation was an essential design element of these agricultural features.
Although the general hypothesis of frost mitigation in high altitude (Brookfield 1961; Denevan, 1970; Wadell, 1972; Erickson, 1985) and in middle latitude (Riley & Freimuth, 1979; Riley er al., 1980) raised field systems has been suggested previously, the ANSYS finite element model presented in this paper represents the first formal consideration of specific heat storage pathways and potentials within these systems.
Graphic experimental confirmation of the heat conservation effects described here was obtained during the 1987-8 growing season in the Lakaya sector of the Pampa Koani. A 1.5 ha plot of land with well-preserved Tiwanaku raised fields was reconstructed by local Aymara communities between August and September, 1987 and planted in a variety of indigenous (principally potato) and introduced crops. No commerical fertilizers were applied to the experimentally rehabilitated fields, and cultivation and weeding proceeded in a traditional manner. On the nights of February 28-29, 1988, the Bolivian altiplano in the Pampa Koani region suffered a killing frost with temperatures in the Lakaya sector dropping to - 5°C in some areas. Substantial zones of potato and quinoa cultivation on plains and hillslopes along thesouthern rim of Lake Titicaca were severely damaged by this heavy frost. Many traditional potato fields within a few hundred meters of the experimental raised field plots experienced crop losses as high as 70-90%. In dramatic contrast, losses in the experimental raised field of Lakaya I were limited to superficial frost “burning” of leaves on potato plants. Barley, broad beans, quinoa, caiiiwa, onions, and lettuce were equally substantially unaffected. Only 10 experimentally placed maize plants were lost in this hard freeze. We attribute this remarkable differential in plant survivability to the heat storage benefits of the saturated raised fields.
Subsequently, several 100 sm2 test plots of potato in the reconstructed raised fields in Lakaya were harvested with an average yield of 33 metric tons/ha-’ (a figure that is consistent with similar yields from an experimental raised field in the Huatta district in Puno, Peru reported by Erickson, 1985). This elevated yield contrasts with an average potato harvest of 3-8 metric tons/ha obtained from traditional shallow furrow, dry farm- ing techniques practiced in the area (MACA, 1985). Although this tremendous differential between traditional and experimental raised field systems may be attributed in part to the fact that the experimental fields were reconstructed in areas not in intensive cultivation for over 800 years, and one would therefore expect higher than average production, the radically different impact of subfreezing air temperatures on these two forms of cultivation must play a significant role in explaining this yield differential.
At present, the empirical evidence supporting the heat conservation hypothesis can be characterized as anecdotal. Experiments explicitly directed toward testing the thermal properties of ridged fields in middle latitude, temperate geographical zones in the United States (specifically, central Illinois) by Riley et al. (1979, 1980) tend to confirm this hypothesis, even though the suggested pathways of frost mitigation in this experiment differ from those proposed here. Long-term experimental research in reconstructed raised
260 A. L. KOLATA AND C. ORTLOFF
field systems at high altitude will be required for confirmation of the heat storage capacity of these systems. We anticipate that current plans for continuing experimental crop production on the rehabilitated fields at Lakaya will contribute to verification and quantitative refinement of the model of the thermal properties of these systems presented here.
Nevertheless the broader historical implications of the model are evident. If the heat conservation effects in raised fields that we describe here are further verified, then archaeological perception of the subsistence base of the Tiwanaku civilization will be radically altered. We must consider the possibility that Tiwanaku agriculture in the circum-Titicaca basin routinely entailed a regime of double cropping. Demonstrating the viability of such a regime of intensification would, or course, have strong ramifications for reconstruction of the limits of surplus agricultural production in the heartland of the Tiwanaku state. This research will result in a new and more sophisticated understanding of the agricultural and demographic potentials of the Andean high plateau, which has long, and improperly, been regarded as a marginal environment for sustaining large and concentrated human populations.
Acknowledgements The results of the research discussed here are based on the project entitled “The Tech- nology and Organization of Agricultural Production in the Tiwanaku State”. This project is supported by research grants from the National Science Foundation (BNS 8607541) and the National Endowment for the Humanities (RO 21368-86), Alan L. Kolata, Principal Investigator. The United States Government has certain rights in this material. Additional funding was provided by grants from the Inter-American Foundation (BO-252,1987) and the Pittsburgh Foundation. We gratefully acknowledge support for computation from FMC Corporation, Santa Clara, CA. The research was authorized in Bolivia by the Instituto National de Arqueologia (INAR), La Paz, Dr Carlos Urquiso Sossa, Director. We are particularly indebted to Dr Carlos Ponce Sanginis, Director Emeritus of INAR and to Oswald0 Rivera Sundt (INAR), Co-Director of the project for facilitating research activites in Bolivia. Gray Graffam, Department of Anthropology, University of Toronto supervised the excavation and stratigraphic recording of raised fields reported here. Michael Binford, Graduate School of Design, Harvard University provided valuable data and references on the ecological setting of Lake Titicaca incorporated in this paper. Michael Moseley and Geoffrey Conrad offered valuable comments on a previous draft of this manuscript. Soil chemistry analyses were performed at the State Soils Laboratory of the University of Wisconsin-Milwaukee. A.L.K. wishes to acknowledge the regrettably brief, but productive collaboration of the late Robert Eidt in the analysis of the soil samples.
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