the environmental tracer approach as a tool for hydrological...

Regionalization in Hydrology(Proceedings of the Ljubljana Symposium, April 1990). IAHS Publ. no. 191, 1990.

The environmental tracer approach as a tool for hydrological evaluation and regionalization of catchment systems

A. HERRMANN, B. FINKE, M. SCHÔNIGER Institut fur Géographie und Geoôkologie, Abt. far Physische Géographie und Hydrologie, Technische UniversitSt, Langer Kamp 19c, D-3300 Braunschweig, FR Germany

P. MALOSZEWSKI GSF-Institut fiir Hydrologie, Ingolstâ dter Landstr. 1, D-8042 Neuherberg, FR Germany.

W. SITCBDLER IAEA, PO Box 100, A-1400 Vienna, Austria

Abstract The contribution of environmental isotope tracer techniques (3H; 2H, 180) to new insights into turnover mechanisms of water in small hydrological catchment systems is discussed with respect to their regionalization. For this purpose, areal hydrological and hydraulic parameters with most important direct and indirect runoff proportions (d; i) besides mean transit times (rQ) and storage volumes of mobile water (V^) for distinct subsurface reservoirs, respectively, are being determined from isotopical hydrograph separation and mathematical flow model application to isotopical system functions by taking Lange Bramke basin (Harz Mountains) of fractured Palaeozic rock and 0.76 km2 as an example. Special attention is then drawn to differences between these modern and classical basin water balances. Accordingly, the latter considerably underestimate flood hydrograph generation by dominant groundwater supply, and groundwater recharge rates by up to two to three times. Finally, the concept for a hydrodynamic (numerical) catchment model is propagated for regionalization of main hydrological processes and parameters mentioned, by introducing the forest hydrological BROOK model as a very promising starting point.

L'approche des traceurs environnementaux comme outil pour évaluation hydrologique et la régionalisation des systèmes de bassins versants

Résumé On analyse la contribution des traceurs isotopes environnementaux (3H, 2H, 180) à de nouveaux aperçus sur les mécanismes de circulation de l'eau dans des systèmes de petits bassins hydrologiques, en vue de la régionalisation de leurs

45

A. Herrmann et al. 46

caractéristiques. A cet effet des paramètres hydrologiques et hydrauliqes concernant la répartition spatiale avec les proportions les plus importantes d'écoulement direct et indirect (d, i) sont déterminés en relation avec le temps moyen de transit (rQ) et les volumes, mis en réserve d'eau mobilisable (V^, à partir de l'application de la séparation isotopique sur les hydrogrammes et de l'application de modèles mathématiques de l'écoulement à des fonctions de systèmes isotopiques, en prenant comme exemple le bassin de Lange Bramke (Montagne du Harz) de roches paleozoïques fracturés. Il couvre 0.76 km2. On apporte une attention spéciale aux différences entre les bilans hydrologiques obtenus par cette approche moderne et par les moyens classiques. H en résulte que ces dernières méthodes sousestiment considérablement l'influence des apports dominant des eaux souterraines sur la genèse de l'hydrogramme de crue et les taux de recharge des eaux souterraines qui sont deux ou trois fois trop faibles. Finalement on présente le concept pour un modèle (numérique) de bassin hydrodynamique en vue de la régionalisation de la plupart des processus hydrologiques mentionnés et de leurs paramètres, en mettant en avant le modèle hydrologique forestier BROOK comme point de départ très prometteur.

INTRODUCTION

Environmental tracer techniques have contributed to remarkable new insights into runoff mechanisms and subsurface reservoir dynamics of small catchment basins during the last two decades. This is the conclusion from literature reviews by Stichler & Herrmann (1982, 1983) and Herrmann et al. (1989), and from our own field experiments. Accordingly, recent scientific progress in catchment hydrology was considerably due to refinements of experimental and modelling techniques in the fields of natural isotopic and artificial dye tracing, and to a certain but a lesser extent to the use of major ions and chemical compounds in natural waters. As to environmental hydrological tracers, the heavy hydrogen (2H, 3H) and oxygen isotopes (180; cf. Table 1) are still most important in this context, and comprehensively discussed in Moser & Rauert (1980), Fritz & Fontes (1980, 1986) or Gat & Gonfiantini (1981).

But as a matter of fact, benefit from the application of modern tracer techniques as a useful tool for hydrological system analysis is frequently ignored by traditional hydrologist. On the other hand, certain scepticism arises from the widespread (fundamental and regional; scientific and applied) experiences with the analytical tracer approach as far as applicability and reliability of some classical hydrological methods are concerned. One main task of this paper is, therefore, to inform about what is really the profit from environmental isotope tracer techniques on a small basin scale. For this purpose, some new and realistic hydrodynamic ideas are being developed from tracings of pathways of water in a small basin of fissured Palaeozoic rock (Herrmann et al., 1989) with respect of subsurface storage and transport of

47 The environmental tracer approach

Table 1 Main features of environmental isotopes for use as hydrological tracers (after Moser & Rauert, 1980)

isotope

*H tritium

2H deuterium

" O oxygen - 18

half-life

12.43 a

stable

stable

in water

as

'H3HO

'H2HO

H2 'B0

isotope ratio abundance in natural waters

3H/'H 0 - 1 0 "

!H/ 'H 90-170-10"6

, aO/ , 60 1880-2010 10"6

inter -nat.

standard

NBS-3H

V-SMOW

V-SMOW

unit

TU

6

5

measuring accuracy

1 -10% of measured

value

± 1%o

±0,15%o

•, , „ _ / "sample \ 62Hresp.61 80= ^ — -1 ) • 1000(Xo)

•• standard

R = aH/ 'Hresp. 'B0/ , 60

water. The paper tries, therefore, to make clear the hydrological isotope technique and its scientific and practical benefit for catchment hydrology.

To satisfy regional system hydrological demands as well, final discussion is sacrificed to the contribution of tracer hydrology to a modern, hydrodynamically-based regional hydrology to be developed. The idea proposed for use of new hydrodynamic insights for regionalization of hydrological facts on a deterministic basis and a basin scale relies on realistic water balance computations and the appropriate hydraulic basin parameters. It takes into account that permanently increasing environmental (ecological) problems strengthen not only the need for reliable quantitative hydrological data, but also for information about qualities of hydrological systems such as transit (residence) times, origin and pathways of water. Future realizations of the proposed concept for simple mathematical catchment model could even lead to new methodical considerations towards a modern comparative hydrology the latter being rather descriptive till now.

Finally, it should be mentioned that the isotope technique has above all been developed for the interpretation of water balances. As a consequence, isotope data cannot replace reliable hydrological data, but their interpretation has to rely on them, and at least on additional hydrogeological information about the studied system, too. Favourable conditions are being offered in several Central European research basins (IHP/OHP 1983) which have repeatedly served as pilot study areas in this field of research, e.g. Vernagtbach (Oetztal Alps; 11.4 km2, 2.640-3.628 m a.m.s.L, 82% glaciated; Moser et al, 1986), Lainbach (Bavarian Alps; 18.7 km2, 670-1.801 m, 90% forested; Herrmann & Stichler, 1980, Maloszewski et al, 1983) and Lange Bramke (Harz Mountains; 0.76 km2, 543-700 m, 90% forested; Herrmann et al, 1989).

-*) definitions:

1 TU = 13H-atomper1018 H-atoms or 0,12 Bq 1 . •„ . , . „

' „ ! m l HzO or 3,2 pCi /


INVESTIGATION METHODS AND MODELLING TECHNIQUES

Isotope hydrology

Environmental isotopes as hydrological tracers The environmental isotope 3H (tritium), 2H (deuterium) and % ) (oxygen-18) are constituents of the water molecule and, therefore, ideal (conservative) tracers in the water cycle. Table 1 comprises some details for the use of these isotopes as tracers. In this context, basic differences between the radionuclide 3H and the stable isotopes concern their origin and concentration changes in natural waters. These changes occur in the case of 3H by radioactive decay and a half-life of 12.43 years, whereas 2H and 1 80 are subject to temperature-dependent physical fractionation during phase change of water. This explains the large variation range for stable isotope concentrations in Table 1. Further details about physical fundamentals and measuring techniques can be found together with distributions in natural waters and selected hydrological applications, respectively, in Moser & Rauert (1980), Fritz & Fontes (1980), and especially for the stable isotopes in Gat & Gonfiantini (1981).

The tracing aptitude of 3H cannot be confirmed in Table 1 alone, for it originates from the nuclear bomb tests in the atmosphere before the moratorium in 1963. Since then, a distinct exponential depletion of two orders of 3H concentrations in precipitation is observed in the northern hemisphere. The 3H input development in Central Europe is actually characterized by less significant seasonal variations at an absolutely low level of 25-30 TU (Herrmann et al., 1986, 1989). The tracing quality of the stable isotopes dates from the seasonal isotope input variations which can be approximated by sinusoidal curves with the higher isotope concentrations during the warm season. The discussion of stable isotope inputs in the research basins mentioned by Stichler & Herrmann (1982) shows that their seasonal amplitude at the coast near the Harz Mountains is only half of that in the alpine area, thus causing specific hydrological interpretation problems.

Areal injection of the environmental isotope by rain and meltwater allows model treatments of the whole studied hydrological system such as separation of direct runoff in the sense of a bypass component and application of mathematical flow models described below. These techniques make use of isotopic input signals and their transformation to specific output functions, thus providing information about the storage properties of a drainage basin.

Hydrological models In order to make this information suitable for such treatment, the hydrologie system itself has to be modelled. The model concept must be adapted to processes as they could occur in reality, even if important simplifications are conceded.

Principal features of any hydrological system can be described by cybernetic models (Fig. 1):

Model 1 The system is treated as a whole where only the input and output water fluxes {Qm, QQUt) and isotope concentrations (Cin, C t) are known.


Model 2 A certain portion of the input (7) is treated as a bypass (direct) flow with a mean transit time (f0; cf. Table 3) up to hours or days, and going directly to the outlet of the system. The other portion (1 - 7) is considered as an indirect flow going through the subsurface reservoir.

Model 3 The subsurface reservoir is split into two boxes with different rQ. A certain portion of the outflow (B) of the upper (e.g. unsaturated) reservoir with a shorter tQ (e.g. weeks to months) supplies the lower (e.g. saturated) reservoir with a longer fQ (years). The mean low flow through the reservoir with the long rQ can be considered to represent the lowest discharge (baseflow) of the hydrological system studied.

Model 1

C i n( t) ;Q i n( t ] catchment

Cout^Qou.M

Model 2 y

(i-Yl

Model 3 Y

(i-Yl '

unsaturated

F saturated

(1-P) . .

Fig. 1 Cybernetic models of a hydrological system.

The hydrological system models in Fig. 1 are open for extension and hydrological definitions for single boxes. For instance, model 1 applies for whole drainage basins and confined aquifers as well, and model 3 can easily be extended by addition of a second reservoir with a long tQ, both boxes then for instance representing fissured rock and porous aquifers.

Hydrograph separation The proportions of direct (d = 1 - 7) and indirect components (i = 7) in model 2 can be calculated by applying the isotopic hydrograph separation technique where the direct runoff (Rd)_ is defined as having the isotope concentration of the actual input (Cjn) : Cd = Cin. The basic mixing formula is: CtRt= CdRd+ C^-with t = d + i = 1. The proportion of Rd is calculated using: d = RJRt = (Ct - Cl)/(Cd- Ct).

Because of the greater expediency of stable isotope measurements at a comparable accuracy level and of the worsening tracing aptitude of 3H due to the unfavourable atmospheric input development today most direct runoff separations of flood hydrographs are performed by using the stable isotopes. More detailed information about this technique is given in Stichler &


Herrmann (1982, 1983). For long-term separations (months, seasons, years) the required theoretical stable isotope concentrations of the indirect runoff component can indirectly be found from flow model application to 3H system functions as for instance demonstrated by Herrmann et al. (1986).

Mathematical flow models In the literature, several mathematical flow models are known which have been developed for the hydrological interpretation of environmental radioisotope data. Table 2 contains those models that are relevant in catchment hydrology, or being frequently applied.

Table 2 Important mathematical flow models with weighting functions g(t), fitting (flow) parameters and fields of application (after Zuber, 1986, supplemented)

Model g(t) Parameters11 Application

exponential (EM) t«-'exp(-t/U) to porous aquifers

dispersive (DM) (4ntD/vxto ) - ' '2. to; D/vx porous aquifers •exp[-(l-t/to ) 2 vxto /4Dt]

ordinary dispersive (4ntD*/vxto ) - l ' 2 fissured rock (ODM) exp[-(l-t/to ) 2 vxto /4D* t] ti ; D* /vx aquif. {toil a)

unsat.soil zone (to jsmonths)

single fissure' 2' to ; D/vx; fissured rock dispersive (SFDM) a aquifers

(toil month)

'> for definitions see Table,3 l.\Ji. VJCJ,J.iiJ.UJ.Wii=> OCC i CU J. C . -J . 2 » 2 a / n ? u ( x ) [ t - u ( x ) ] - = / 2 p : p j - x 2 { l - [u (x) / t . B 2 f

• exp{ -{a2u2 ( x ) / t - u ( x ) ]j- dx ' w i t h u ( x ) = t o / [ 4 D x 2 / v x ] ; ©=1/2 (vxto / D t ) ' ' 2 ; a ( s e e T a b l e 3)

Flow models are specific basin response or exit age distribution functions g(t) related to tracer transport through a system. They apparently differ in the number and meaning of flow (fitting) parameters. The relation between the tracer input and output concentrations of a system [C-m(t); Cmt(t)] in the steady-state case (i.e. volumetric flow rate [Q] and volume of mobile water in the system [V ] are constant) is expressed by the following convolution integral:

CoutO QJf - t>) g(t<) cxpi-\t')àt'

For stable isotopes the decay constant is X = 0. Functions g(t) are ascertained by the response of the system to a pulse injection of tracer, and


theoretically found by solving one (dispersion) or two mass transport equations (dispersion, diffusion) according to natural conditions. The more difficult inverse problem is determination of flow parameters by solving the convolution integral, using the corresponding g(t) function. They are found by fitting the theoretical values of C t(f) as closely as possible to the measured values.

For the hydrological models in Fig. 1 it is above all assumed that the isotopic input function is largely transformed through dispersion in subsurface reservoirs. For this reason, the early one-parameter (tQ) exponential model (EM) introduced by Eriksson (1985) which is mathematical equivalent to the good-mixing model usually is only a rough approximation of reality whereas the two-parameter (fQ; D/vx) dispersive model (DM) yields more reliable solutions for tQ. The DM version in Table 2 which is suitable for environmental tracers was first proposed by Maloszewski & Zuber (1982). Application of convolution integrals to DM includes no integration over the recharge area. Thus, values for the dispersion constant (D/v) will result from space-extended injection, and they could be several times larger than those known from artificial point injections. It may be expected that the apparent values of D/v in environmental isotope studies will be roughly equal to the length (x) of recharge zones measured along the streamlines.

In the case of stable isotopes a simpler procedure can be applied for determination of tQ because Cin(t) can be approximated by a sine function with a period on one year (= 2n per 1 year):

Cin(t) = Ansin(wt)

Application of a convolution integral to a sine input function yields a sine output function with a specific amplitude and phase shift 4>:

CoutW = Bnsin(wt + <t>)

Therefore, fQ can be determined using either the amplitude damping or the phase shift. Since 0 is mathematically limited to three months and already amounts to 2.5 months for a mean transit time of only one year, tQ

calculations from it are rather inaccurate. Using g(t) for EM and DM rQ

can be easily calculated form the amplitude ration / = BJA :

for EM: tQ = w"1 \f2 - 1]*

for DM: tQ = w_1 [-In / / (D/vx)f2

DM is valid where the tracer transport is assumed subject to the physical process of dispersion only, ,e.g. in the case of porous aquifers. For double porosities, e.g. in the case of soils with macropores in or fissured rock aquifers with a porous matrix, tracer diffusion in the matrix has to be considered as a second relevant process. According the Maloszewski & Zuber (1985) environmental tracer data from such systems can be interpreted by applying the ordinary dispersive model (ODM) when tQ Ï 1 year. The fitting result are two new parameters: the mean transit time of tracer (tt) and a


purely mathematical parameter (D*/vx) describing the variance of transit time distributions of the tracer in the system as a result of dispersion in macropores, cracks and fissures, and of diffusion in the porous matrix.

Even today many isotope hydrologists are not aware of the fact that they determine tt instead of f0 and, therefore, the whole (mobile plus immobile or stagnant) water volume (V) of a system instead of Vm which is the hydrological parameter being aimed at. The related problems are discussed in detail by Maloszewski et al. (1990) who show that tt > tQ: R = t(/t0 = V/Vm : 1 + (« /nowhere R is the retardation factor as a result of tracer diffusion in the matrix, n and n. are the matrix and fissure (macropore, crack) porosities. Thus, m order to calculate rQ from t( (fQ = ttIR~) which is R times greater than fQ at least both porosities n ;nf) must be known.

Finally, Table 2 contains the three-parameter (r0; D/vx; a) single fissure dispersion model (SFDM) which represents a simplified version of the four-parameter (?0; D/vx; a; I) parallel fissures dispersive model (PFDM) proposed by Maloszewski & Zuber (1985) for fissured rock aquifers. The high number of solutions for PFDM because of the four unknowns prevents from reasonable handling of the inverse problem. i.e. derivation of realistic parameter values from experimental tracer output concentrations. Its authors suggest using at least SFDM when r0 « 1 month, i.e. mainly for short distance experiments with artificial tracers. This would allow indirect local solutions for n„in the case when n , D and L are known (from nf= 2b/L with lb = (n Ja)(D)^ [for definitions see Table 3]) which in contrast to n is very difficult to determine, and needed for approximation of R (zl + [n ml).

Relevant hydrological and hydraulic parameters Table 3 compiles hydrological and hydraulic parameters (except \) as resulting from the application of environmental tracer techniques to hydrological systems. Hydrologically most relevant parameters are stressed, and orders of magnitude mentioned where of general interest. For hydrodynamic characterization and numerical modelling of hydrological catchment systems at least the water fluxes (Qin, QQUt), the direct and indirect runoff proportions (d; i) and the mean transit times of water through the system (f0) are needed.

Hydrology

All traditional hydrological investigation methods which are necessary for the realization of the hydrodynamic research concept to be discussed are comprised under this heading. In this context, it should be pointed out that the analytical tracer approach of hydrological systems cannot ignore the approved classical methods. This is mainly due for the assessment of areal water balances, i.e. relevant input and output water fluxes.

Quantitative evaluations of most dynamic components of basin turnover of water commonly make use of synthetic procedures where direct measurements are normally not practicable or too costly. For instance, synthetic hydrograph separation methods (IHO/OHP 1986: e.g. direct [surface] and indirect [subsurface, groundwater] runoff, respectively; low (dry weather)


Table 3 Notation of hydrological and hydraulic system parameters from application of tracer techniques with symbols and definitions

Symbol Explanation Definition Unit; Order

of Magnitude

a 2b C d Dp D/vx i X L n Tic i 1

nt' nP Q

R RP

t f to

tl

V Vm

-fissure aperture isotope concentration direct runoff proportion molecular diffusion dispersion coefficient

(nP/2b)/DP

indirect runoff proportion decay constant fissure width total porosity effective porosity fissure porosity matrix porosity volumetric flow rate through system runoff retardation factor

time variable transit time variable mean transit time of water mean transit time of tracer total volume of water volume of mobile water

V/V»; : tt /t. ; =l+(nP/nf )

Vc /Q:

V/Q;

Q' Q

•tt •to

; ti

to-

./RP

RP

h-1i* ; 0.01 mm, cm 5(%o); TU 1 m2 /s; 10- " - ;0.1-0.2

1 yr.- ' cm, m 1 1 1 1 here:m 3/yr.

here: 1/s - ; 1.5-2

here:yrs.-'

here:yrs.-!

here: m3

here: m3

discharge [recession flow] analyses) still play a predominant role in hydrology, water resources planning and watershed management whereas the appropriate results actually cannot satisfy system hydrological demands.

It should be mentioned that the reported water balance experiments rely on common network design practice for hydrological research basins (e.g. Toebes & Ouryvaev, 1970). Additional effort has to be attributed to manual and automatic sampling of input and output water fluxes (e.g. 100 ml of water for the 2H resp. 1 80, 400 ml for the 3H determination) of the whole basin (precipitation; snow cover and snow cover outflow; runoff) and relevant subsurface basin reservoirs (unsaturated soil zone; porous, fissured rock aquifers) in the case of Lange Bramke.

EXPERIMENTAL RESULTS

The following discussion of experimental results is focussed on the discrepancy between classical and modern approaches of direct flow and groundwater recharge rates from which new ideas are being developed for runoff formation and turnover mechanisms of subsurface water by taking the


Lange Bramke basin as an example. Since 1949, Lange Branke has belonged to the best studied basins of

the world where tracer studies started in 1980 (IHP/OHP, 1983). Principal tracer hydrological results are compiled in Fig. 2 where the subsurface reservoirs are: BS (unsaturated soil: residual weathering and allochthonic Pleistocene solifluidal materials with podsolic brown earth as the dominant soil type, on fissured and faulted rock), KS (fissured rock aquifer: folded and fractured lower Devonian sandstones, quartzites, and slates), and PS (porous aquifer: valley filling of sand, gravels, pebbles, and boulders). Water fluxes and storage volumes have been assessed by application of the mentioned techniques to the hydrological data as for instance demonstrated by Herrmann et al. (1986).

« 0 . 5 5 5

0.033

direct mnof

0.06:

PS 0.16

0.023

0.05 0.33 to 10 0.471 0.5 0.32nI!, l

0.532

Fig. 2 Model flow rates (1980-1986) and reservoir features of the Lange Bramke basin (Harz Mountains) (for symbols see Table 3; flow

•)6m3 rates and storage volumes in 10 m , mean residence times in years).

Basic findings such as d z 10% and tQ z 1.5 years for the groundwater system are not singular but fully compatible with literature (cf. Stichler & Herrmann, 1982) considering that /( instead of tQ was published in many cases. Moreover, Zuber et al. (1986) confirm that steady-state conditions for mathematical flow modelling can be assumed at least in the case of Lange Bramke, and Herrmann et al. (1987, 1989) confirm that interflow is in fact negligible. A main consequence from the extraordinary groundwater supply to runoff is a considerable average groundwater recharge rate of up to two or even three times of that assessed by using traditional hydrological methods.

Consequently, according to Table 4 which compares different water balance computation techniques, direct flow and recharge rates for active groundwater in the hydrodynamic sense are largely underestimated by traditional methods (here: mean values from different graphical hydrograph separation methods and low discharge/recession flow analyses, respectively [IHP/OHP, 1986]). This is apparently due to the inadequate runoff


formation concepts which are, therefore, to be re-examined. BROOK stands for a forest hydrological catchment model which was introduced by Fédérer & Lash (1978) for simulation of basin runoff and its major components, and needing daily precipitation and air temperature as the only input variables. Simulation results for years, seasons and single events are also very encouraging (Finke et al, 1989) thus predestining the modular model concept for regionalization purposes as discussed below.

Table 4 Water balances 1980-87 (mm year'1) for the Lange Bramke basin and different computation techniques (for explanation see text)

Component

Precipitation

Runoff direct direct delayed (interflow) indirect (groundwater flow)

Evapotranspiration Evaporation Transpiration

Groundwater outflow (net loss)

Groundwater recharge

Computa

Tracer

1 300

700 80 0

620

570 380 1 > 190» >

301 >

650

ition Techn

Traditional

1 300

700 \450

250

570 3801 > 190' )

301 >

280

ique

BROOK

1 300

710 80* >

430* > 200* >

580 255* > 325* >

20* >

220

1 > e s t i m a t e d 2 ' s i m u l a t e d

The respective turnover mechanisms for water which should be valid for cases without any clearly defined hydraulic bottom boundaries for unsaturated subsoils has earlier been discussed in detail by Herrmann et al. (1987). Accordingly, with the basin input groundwater potential surface is becoming steeper at lower slopes preferably by the combined effects of compression of the capillary fringe and the hydraulic barrier at the fissured rock/porous aquifer interface which accounts for hydraulic conductivities in the order of 10"5 to KT6.

Figure 3 is a typical example for frequent spontaneous groundwater table reactions upon quantitative inputs, thus allowing rapid peak discharge generation by exfiltrating groundwater, and direct runoff most frequently amounting to 1% and less of total actual input volume. Groundwater recharge is subsequently performed through limited but very efficient pathways of the macropore and fissure systems of the unsaturated zone thus maintaining the quantitative balance of the aquifer. The relatively small tQ

generally found for groundwater systems independently confirms such a turnover mechanisms. In similar cases considerable water quality problems


might result from such short-circuited storage systems. But as a matter of fact, none of the existing runoff formation and

simulation models consider these turnover mechanisms. Adequate hydraulically-based algorithms are, therefore, actually being developed for inclusion in a physically-based and modular hydrodynamic catchment model to be introduced now..

WT (cml

July 198E

Fig. 3 Precipitation (P), runoff (R) and fissured rock groundwater table (WT) for a storm event from 1 to 4 July 1988, Lange Bramke.

HYDRODYNAMIC REGIONAOZAHON CONCEPT

The proposed basis of a deterministic regionalization concept is considering the fact that hydrodynamic (numerical) modelling of hydrological reality has a good chance for widespread application provided that the input variables are commonly available, and the (morphometric, vegetative, pedological, hydro-geological etc.) basin parameters simply being assessed. The latter should represent quantitative evaluations of areal means in order to allow easy regional transfer of the model and the hydrological information gained from its application.

For this purpose, a modular deterministic hydrodynamic simulation model for dominant runoff components (e.g. direct, delayed direct, indirect) on a daily basis is being developed in a first step on the basis of BROOK and similar system models for small forest ecosystems. A priority task consists, therefore, of developing adequate transfer functions for the subsurface storage systems. Model calibration and verification of water quantities should at first take into account appropriate results from environmental tracer studies which obviously represent good approximations of reality.

In this context, the literature review given by Stichler & Herrmann


(1982) and updated by Herrmann et al. (1989) should help to complete the model development according to regional variety of catchment systems. Our research activities are just being devoted to the hydrological analysis of small agro-ecosystems in the Quaternary lowland of Lower Saxonia, FR Germany. As a final result a numerical catchment storage and runoff simulation model might be expected which applies to basic types of small Central European catchment areas from high-alpine to coastal regions.

CONCLUSION

As demonstrated by the combination of classical and modern tracer hydrological investigation methods progressive insight into small basin turnover of water and, therefore, basin water balances of a new quality are possible. With the simulation of discharge quantities for single runoff components from small, autochthonic hydrological systems a big step forward towards areal dissemination, i.e. regionalization of improved hydrodynamic knowledge can be done, thus even indirectly increasing the benefit from the costly isotope technique. Furthermore, the application of hydrodynamic basin simulation models should contribute to more reliable interpretation of basin output concentrations even of reactive solutes provided that respective idea (e.g. algorithms) about reactions during water transport in the system is available.

REFERENCES

Eriksson, E. (1985) The possible use of tritium for estimating groundwater storage. Tellus 10, 472-478.

Fédérer, C. & Lash, A. (1978) A Hydrologie Simulation Model for Eastern Forests. Water Resour. Res. Center Res. Report no. 19, Univ. of New Hampshire, Durham, New Hampshire, USA.

Finke, B., Herrmann, A. & Schôniger, M. (1989) Simulation von Wasserfliissen in der Langen Bramke (Oberharz) mit dem forsthydrologischen Wasserhaushaltsmodell BROOK (Simulation of water fluxes in the Lange Bramke basin with the forest hydrological water balance model BROOK). DVWK-Mitteilung 17, 343-349, Bonn.

Fritz, P. & Fontes, J. C. (eds) (1980) Handbook of Environmental Isotope Geochemistry, vol. 1: The Terrestrial Environment, A. Elsevier.

Gat, J. R. & Gonfiantini, R. (eds) (1981) Stable Isotope Hydrology. Deuterium and Oxygen-18 in the Water Cycle. IAEA Tech. Reports Ser. no. 210, Vienna.

Herrmann, A. & Stichler, W. (1980) Groundwater-runoff-relationships. Catena!, 251-263. Herrmann, A., Koll, J., Maloszewski, P., Rauert, W. & Stichler, W. (1986) Water balance

studies in a small catchment area of paleozoic rock using environmental isotope tracer techniques. In: Conjunctive Water Use (Proc. Budapest Symp., July 1986), 111-124. IAHS Publ. no. 156.

Herrmann, A., Koll, J., Schôniger, M. & Stichler, W. (1987) A runoff formation concept to model water pathways in forested basins. In: Forest Hydrology and Watershed Management (Proc. Vancouver Symp., August 1987), 519-529. IAHS Publ. no. 167.

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