Coniiiieiiis oil Wrríer Birdgel Iiiuestipfiotis, especiull~v iiz tropical aiid siihtropical iiioi~ntciit~ regions

Comments on water budget investigations, especially in tropical and subtropical mountain regions

H. Flohn, (Univ. of Bonn)

SUMMARY: Three diíïerent techniqucs are used to evaluate the water budget of a given area: the hydrological balance, the surface energy budget and thc evaluation of the divergence of the atmospheric water vapor transport. Due to the deficiencies of the available observutions, relatively large systematic and random errors are unavoidable and necessitate the comparative rand critical) use of diíïerent techniques. In tropical and subtropical inountains local daytime circulations lead to a systematic under-

estimate of rainfall measured at most valley stations; evidence from runoff data show that the precipitation in the mountains can surpass these values by a factor of 5-20 and more (Troll-effect). In contrast to the daytime circulations, the effect of the reversed nighttime circulations on the precipitation pattern is much more infrequent; some examples are demonstrated.

COMMENTAIRES SUR DES RECHERCHES D U BUDGET DE L‘EAU SPECIALEMENT DANS LES REGIONS MONTAGNEUSES TROPICALES ET SUBTROPICALES

RESUMI! : Trois techniques différentes sont utilisées pour évaluer le bilan d’eau d’une aire donnée le bilan hydrologique, le bilan énergétique de surface et l’évolution de ia divergence du trans- port de la vapeur d’eau atmosphérique. D u fait de Ia déficience des observations disponibles, des erreurs relativement importantes sont inévitables et il en résulte ia nécessité d’application comparative et critique des diverses techniques. Dans les montagnes tropicales et subtropicales, des circulations locales du jour conduisent à

une sous-estimation des précipitations mesurées le plus souvent dans des stations de vallées : les données recueillies sur l’écoulement montrent que les précipitations dans les montagnes peuvent dépasser les valeurs mesurées d’un facteur de 5 à 20 et plus (Effet-Troll). En contraste avec ces circu- lations du jour l’effet de ia circulation inverse de nuit sur les précipitations est beaucoup moins fréquent : quelques exemples sont présentés.

COMENTARIOS SOBRE LAS INVESTIGACIONES RELATIVAS AL BALANCE HIDROLÓGJCO, ESPECIALMENTE EN LAS REGIONES TROPICALES Y SUBTROPICALES

RESUMEN: Para evaluar ei balance hidrológico de un área determinada se utilizan tres técnicas difcrcntcs: CI balance hidrológico, el balance energético de superficie y Ia evaluación de la divergencia del transporte de vapor de agua atmosférico. Debido a las deficiencias de las obser- vaciones disponibles, son inevitables crrores relativamente amplios de tipo sistemático y aleatorio y, por io tanto, es necessario el uso comparativo (y crítico) de técnicas diferentes.

En las zonas niontallosas tropicales y subtropicales las circulaciones locales durante el día cunduce a una subestimación sistemática de la precipitación medida en la mayoría de las ecta- cioiics dcl valle. Los datos de la escorrentía han puesto de manifiesto quelaprecipitación en las inontaiias puede sobrepasar estos valorcs en un factor comprendido entre 5 y 20, e incluso más (efecto Troll). En contraste con las circulaciones diurnas, el efecto de las circulaciones nocturnas inversas sobre Ia distribución de la precipitación es menos frecuente. Se demuestran algunos ejemplos.

MONTA ROSAS

251

H. Flolin

ï. ït is generally agreed that-in spite of the severe urgency caused by the ever increasing demand for water by a menacingly growing population-our quantitative knowledge of the water budget of most continental catchments is far from being perfect. In fact, measurements of some of the terms of the water budget are more erroneous than usually admitted. O n the other hand, the increasing demand enforces us to minimize all sources of error in evaluating the natural resources. In this situation it is intended to give a critical survey of existing long-term budget considerations from the view-point of climatol- ogical experience.

The basic budget equations can be written as follows: P = E+R+AWs Q = Tse,,+ Teti' Ts+ Thiol div (O q) dí = E- P+ A W,

Water Budget (Land) Energy Budget (Surface) Water Vapor Transport P precipitation; E evapotranspiration ; R runoff; Q net radiation (short-wave+ long-wave)

Ts heat flux into soil; TbiOl energy used for photosynthesis; U wind vector; 4 specific humidity of air; í height; A W, (A W,)

Vetij vertical transport of sensible (latent) heat inlo air:

storage of water in soil (in air, including cloud water content).

Since Teti = L.E (L = heat of condensation), we are tempted to use these three equations together to evaluate with a miniinuni error that quantity which apparently is most difficult to be exactly measured: the actual evapotranspiration E with its large time and horizontal fluctuations caused by the variation of the transpiration of vegetation and the soil moisture content in a inhomogeneous catchment area. However, the m.easure- ments of the remaining term, on a regional scale, are subject to many systematic errors, not to speak of many random. errors which may (or may not) vanish when horizontally integrated. Here it is intended to discuss some sources of error, especially in tropical and sublropkal mountain regions, which hitherto have been frequently underestimated or even neglected.

II. The hydrological balance of continental catchment areas is frequently considered to be most rcliable. As a niatter of fact, many data on precipitation (P) and i- ino off (R) are published or hidden in the files of governmental agencies. Howcvcr, the accuracy of many data is doubtful, and the systematic error of prccipilation measurements is greater than hitherto suspected. Recent studics in the USSR [23] have shown that in

252

Comments on Water Budget Inuestigoiions, especicrlly in tropical and suhtropicul inounfnin regions

areas with a great contribution of snow to the total amount of prccipitation, the annual rainfall is greatly underestimated by the existing network; reduction coefficients as high as 1.2 to 1.5 are applied to the measured annual amounts. The separate measurement of snowfall-as it is done in Canada with the aim of readjusting the conversion from. snow into ineltwater according to the best available knowledge-seems to be a reasonable solution. In many countries the difference between the usual raingauges at different heights

above the ground and protected by different wind-shields and the amount actually received at ground-level have been investigated. The main results are in short: Even under normal conditions precipitation measurements are systematically underestimated by a not negligible amount, usually by some 6-10 or 12 percent, perhaps with.exception of equatorial areas with very weak winds. It is generally assumed that the rainfall measured at individual rain-gauges is truly

representative for the area between the gauges-liaving in mind the fact, that even in densely populated countries (like Western Europe or Japan) the density of rain-gauges is in the order of 1:lOO km2 or slightly less. This means, that the measured quantity is about 2.10-” of the total amount; if we consider the enormous time-area variations of rainfall as indicated by radar, we need a sort of blind faith to believe in the represen- tativity of these data. In large areas this relation is IO-” to or even sinaller; in West Berlin, Hongkong or at the equatorial island of Sao Tomé, however, it climbs up to about 2.10-’. Since the average diameter of a Cb-cell is in the order of 3-5 km, many showers cannot be measured in a network of stations, where the average distance between two stations is much greater. In mountain areas this assumption of a random distribution is certainly invalid. The

variation of precipitation with height is little understood; we need sound physical models to simulate the interaction of many processes (horizontal and vertical motion, growth of droplets, entrainment and evaporation). In mid-latitude mountains, at the windward slopes, precipitation increases with height. This is only possible, if the exponential decrease of specific humidity with height is smaller than the increase of horizontal wind. In this case the total transport q.u increases with height and more water vapour is driven across a vertical area unit and condensed by ascending motion. in East Asia [3] and in the Alps [lo] an increase of q’u up to the 700 mb-level (- 3 O00 mj has been shown using either monthly averages or cases with strong rainfall. Above 3 O00 m very few reliable precipitation data exist in middle latitudes. ln th.e tropics both quantities (q and u) decrease with height, at least above the 900 nib-

level; liere we observe [22] a layer of maximum rainfall slightly above the condensation level, (fig. I) i.e. at an altitude between 1 O00 and 1 500m. Since the condensation level of maritime tropical air-masses lies frequently at 500-800 m, the small difference must be due to a balance between the orographically forced vertical component V, within the clouds and the fall velocity of droplets. A complete theory has to take into account not only V,, but also the frequent cccurrence of local thermal circulations with strong buoyancy and a relatively large entrainment area which are indicated by a cloudless ring around an isolated mountain covered by convective clouds as seen in many satellite pictures. Interesting models have been studied by Sarker [I61 simulating the orographic rains

at the western coast of the Indian peninsula. After extended experiences in the high mountains of South America, Africa and

Southern Asia C. Troll [21] indicated that the climatic effect of thermally induced local circulation is much greater than hitherto assumed. In many valleys with widths of 2-10 kms the daytime system of slope winds develops with such a regularity, that the pattern of convective showers along the ridges, together with a cloud-free strip along the bottom of the valley, is pictured in the vegetation (fig. 2). The reverse night-time circulation is much weaker due to the weaker heating differences and to the stability of

253

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5000 m

L O00

3000

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the air. Only in very wide rift valleys-such as Cauca Valley [6] [20] in Colombia-in the most humid parts of the tropics can the reverse night-time circulation develop nocturnal Cb-systems with rainfall along the bottom of the valleys (fig. 3). Aside from these few exceptional cases in the equatorial tropics the effect of the prevailing daytime circulation is that the rainfall data from stations which are usually located near the bottom. of the valleys greatly under-estimate the area-averaged precipitation. Exam.ples of this can be found not only in the Hindukush [7] but in many mountains in the Subtropics and outer Tropics. This is also true for the Rocky Mountains in Colorado or other isolated mountains in Utah, New Mexico or Arizona etc. Where precipitation data are scarce, runoff data may givemore reliable first-order estimates of the area precipitation (table i a). in all these moun tain areas the existing precipitation data are utterly nonrepresentative and misleading. The most striking example of this is the Karakorum Mountains: here only five climatological records of P are available (Leh 83 nim, Skardu J 62 mm, Gilgit 134 nim. Bunji 158 mm, Misgar 100 mm/a), all indicating desertic aridity, which is in striking contrast to the extraordinarily intense glaciation of the mountains themselves. Runoff data indicate (table 1) that the area-averaged annual precipitation is in the order of 90-180 cin. This is only possible when at altitudes of 3 000-5 O00 ni precipitation amounts to at least 200-300 cm. Based on measurements of the glacier motion, (glacio- logists !e.g. J7) have estimated the annual precipitation as high as 6-8 m/a; however, it seeins doubtful that one can extrapolate a few observations during suinmei' to the whole year. Evidence of similar iinderestiinatcs of rainfall exists at thc castcrn bordei. of the Bolivian Andcs ítublc 2, data aftcr [24]). Thc heat-biidgct dilierences between lakes and the surrounding land, originally

caused hy the terni T,,,,,< of equation II, produce during the daylight hours another type

254

255

H. Flohn

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256

Comments on Water Budget Innuesligatiom, especially iri tropical and sub fropicní moiriztain regions

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257

of thermally induced circulation with the subsiding, diverging branch just above the lakc. This circulation is frequently to be secn in satellite pictures. Therefore, we havc to expect relatively low precipitation at the borders and ïslands of the lake, and perhaps u ring of increased rainfall around the lakc. Under moist-unstable conditions the reversed night-time circulation can be well established as a regular feature at Lake Victoria [S, 81, provided that the horizontal scale allows the formation of an extended Cb-system abovc the lake. In this case suficient evidence could be found for a rainfall-maximum at the lake itself (fig. 4) and the prevailing nocturnal rains at islands and selected stations along the leeward coasts. Evidence of a similar feature at Lake Titicaca has been presented [12]. The diameter of a tropical Cb-system in the order of some 30 kms seems to be a minimum orographical condition for the full developinent of a nocliirnal system above lakes or wide valleys.

depends largely on the geological structure of the ararea; large errors can be caused if the groundwater flow djverges (or converges) at the boundaries of the catchment area, especially in karst areas.

III. An exact determination of all terins of the energy budget of the earth’s surface is only valid for one point; special investigations are needed to obtain representative area-averaged values for a whole area. A successful attempt of this sort has been made by E. Frankenberger [9] for Schleswig-Holstein based on the data collected during the IGY at an observatory near Hamburg. Realistic estimates of the net radiation Q can be obtained on the basis of the equation of radiation balance,

The storage term A

Q = (S+[f)(l -U,) - (E-G) .

FIGURE 3. Diirrilal trend uf rui/ifnll dung Currca Rift in perceiit of the duily ouern,qe; cinta ufier H. Trojer 1959)

Here the global radiation S+ H can be frequently interpolated froin existing nieasure- ments or estiinated from empirical relationships [l, ?]. Jt is mich inore difficult to obtain really reprcseiitative values of the surfacc al bedo CI, which varies both horizontally and seasonally with vegetation type and snow-cover. Regular flight nicasurenients such as made [i31 above westcrn North America yield trustworthy resdls. Thc long-wave radiation budget, E-G, can be evaluated from empirical relationsliips within i-easonable

258

Comments un Wuter Bitcigel Inoestigcifiut7s, e.rpecicil1.v in Iropicol ond sirhtropictrl inountnin regions

tolerances; here the surface temperature t,s as an approximation must be replaced by the air temperature ta. Due to the low horizontal variability of a, and t,s and the possibility of nieasuring t,s with adequate accuracy, the determination of Q at lakes is much more representative than at land areas. From the heat flux terms T, can be neglected in this tropics. In other climates, it contri-

butes not negligible amounts to the seasonal trend of the energy budget. This is especially true at lakes where T, largely controls the remarkable seasonal delay of evaporation. At land the energy needed to melt the snow-cover can be included in this term; during the melting period it is usually greater than all other terms. The biological term Tsio, uses not more than 0,5-1 percent of Q and can therefore be mostly neglected. The relation between T,,,,, and Teu- usually defined as the Bowen ratio T7e,,s/Teu-is

of fundamental importance not only above the ocean. Above continental territories, its horizontal (and to a lesser extent, time) variations are highly effective in producing local circulations, local and microclimates. From this view-point the net radiation is locally quasi-invariant and only subject to large-scale variations, while Tse,,, and, conse- quently, the Bowen ratio-with T,, - Q-T,,,,,-is largely controlled by man-made environmental changes, e. g. by agriculture, irrigation, engineering etc. The above- mentioned local variations of rainfall in subtropical and tropical mountains are produced by such local differences of TSens. In irrigated areas of arid climates as well as above coastal areas with upwelling water Tse,, can even be negative* and thus increase Teu.

O 35OE

1" N

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FIGURE 4. Ruinfull distribution at Luke Victoria

* In the oases of southern Tunisia Q is equivalent to a potential evapotranspiration of 120- I30 cm/a, while the actual evapotranspiration E varies between 90 and 380 cmla, according to the intensity of cultivation. This allows an estimate of the energy contribution by the down- ward flux of T,.,,, which may be greater than Q.

259

H. Flohn

in addition to the Bowen ratio another quantity is of basic importance in physical climatology: the relation between the net radiation Q and the energy needed to evaporate the total rainfall P-we may call it Budyko's ratio Q/LP [i]. Since this ratio is correlated with the runoff ratio RIP, it is also of basic importance for physical hydrology. According to Budyko the distribution of natural vegetation types-i.e. the natural climatic pattern-depends closely on QlLP.

IV. Since the first attempts of F. Möller [14] to determine with aerological data the divergence of the water-vapor transport, investigations have been started to obtain with this technique representative values of E- P above large areas. These investigations were successful only when applied to the largest-scale transports across latitudes circles or latitudinal averages of the zonal transport. Maps of the regional divergence of the water-vapour flux [18] demonstrate, that not even the sign of E- P could be found with sufficient reliability. Among the many sources of error only two will be mentioned:

1. The small-scale and meso-scale variability of atmospheric humidity.

2. The lack of homogeneity in the international aerological network. Even in an inter- national radio-sonde comparison the different radiosonde types showed systematic differences of the relative humidity up to J 5-20 percent.

Because of small and meso-scale variability of humidity a single aerological ascent cannot be taken as truly representing large-scale or synoptic-scale features. This necessi- tates a restriction to long-term averages. An attempt to evaluate daily values for a small area (24 O00 km2) in northern Germany [i91 resulted in varying correlation coefficients between the divergence of the vertically integrated water-vapour flux and estimates of E-P. These coefficients were only satisfactory in cases with high precipitable water and without large airmass differences. Only for selected groups of days-such as days without precipitation could realistic figures of E be obtained (table 3). Due to the small- and meso- scale variability of rainfall, satisfactory results can only be expected based on a sufficiently dense network of rainfall stations and a careful evaluation of area-averaged precipitation. rt is also necessary to take into account all other systematic and random errors of the divergence term [li] [I91 as well as those of P (cf. chapter I). Because of the lack of homogeneity of networks no realistic results of divergence

computations in areas (like Africa) with different types of radiosondes [4] are possible. But even in large areas with homogenous network (like the United States [i5]) regional studies do not yield absolutely convincing results; the errors are still in the same order of magnitude as with other methods. V. Summarizing the experiences of many investigators, il must be admitted that at present no technique is available to obtain truly representative values of the area-averaged actual evaporation E with an accuracy of better than, say, 10-15 percent, even under most favourable conditions. However, a similar lack of accuracy also occurs in most countries for the regional (and global) averages of P. Under such circumstances, global and/or regional studies on the water budget should be based, as far as possible, on more than one technique and should be accompanied by a careful critical evaluation of all basic assumptions in the use of empirical formulae. Of highest importance is a critical interpretation and correction (so far as possible) of the available measurements which are only too frequently hampered by significant systematic errors. Only after such precautions can a minimization of the inherent errors be achieved.

260

Cotrimerits on Wuter Birdget hioestigaiions, especially in tropical and subtropical mountain regions

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REFER ENCES

I , BUDYKO, M. I. (1956): Tepluicioj ßar(crns Zenmoj Potverclinosri, Leningrad, 225 p. 2. BUDYKU, M.T. (1963): Atlas Teploiuugo Balnti.s.sa Zeninogo Schorci, Moskva. 3. FLOIIN, H. and OECKEL, H. (1956): Geopliys. Mag. (Tokyo) 27, 527-532. 4. FLOHN, H., HENNING, D. and Konm, H.C. (1956): Bunner Meteor. Aúh~indl., 6. 5. FLOHN, H. and FRAEDRICH, K. (1969): Meteor. Riiiidschuii, 19, 266-373. 6. FLOIIN, H. (1968): Wetter ritid Lehen 20, 181-191. 7. FLOHN, H. (1969): Erdkunde 23, 205-215. 8. FKAEDRICII, K. (1968): Arch. Meteor. Geophys. Biold., A 17, 157-166. 9. FRANKENHERGER, E. (1960): Ber. DI. Wetterdiemr, 73. 10. HAVLIK, D. (1969): Freibiirger Geogr. Hefte, 7. 11. HUTCHINCS, J.W. (1957): Qriart. Joiirn. Roy. Meteor. SOC., 83, 30-48. 12. KESSLER, A. and MONHEIM, F. (1968): Erdkritide, 22, 275-383. 13. KUNG, E.C., BRYSON, R.A. and LENSCHOW, D.H. (1964): Mo?ith/y Weather Review, 92,

14. MOLLER, F. and de DARY, E. (1951): Arch. Meteor. Geoph. Biold., A4, 142-155. 15. RASMUSSON, E. M. (1968): Monthly Weater Review, 96, 720-734. 16. SAKKER, R. P. (1966): Monthly Weather Review, 94, 555-572. 17. SCHNEIDER, H. J. Die Erde, Z. Ces. Erdk. Berlin. 100, 266-286. 18. STARR, V. P., PEIXOTO, J.P. and Coll.: Tellus, 10 (1958), 189-194; 37(1965), 463-475; Pageoplr.

19. STRUENING, J. O. (1970): Diss. Uniu., Bonn. 20. TROJER, H. (1959): Cenicafé, 10, 289-373. 21. TROLL, C. (1952): Bonner Geogr. Ahhandl., 9,124-182. 22. WErscHET, W. (1969): Die Erde, Z. Ces. Erdk., Berlin, 100, 287-306. 23. USSR Committee for the International Hydrological Decade (1967): Water Resources and

24. Secr. Gen. Est. Americanos (1969): Cuenca del Rio de la Plata. Investario de Datos Hidrolo-

25. WUNDT, W. (1953): Gewüsserkunde, Berlin, 320 p. 26. KELLEn, R. (1961): Gewü,rser und Wasserharishalt c h Festlandes, Berlin, 520 p.

543-564.

75 (1969), 300-331. I

Water Budget of the USSR Areu, Leningrad.

gicoc y Climatologicos, Washington D. C.

Vertically differentiated water balance in tropical high mountains - with special reference to the Sierra Nevada de Santa Marta/Colombia

Reimer Herrmann

SUMMARY: From the example of the northwestcrn Sierra Ncvada de Santa Marta/Colombia the vertical variation of thc water balance of tropical high mountains in lee of the trade wind can he sccn. The prccipitation increases according to height from sea level ( P = 350 mm) to a height of 1 660 m (P = 2 500 mm). Abovc this the precipitation measured in a raingage falls at less than P = I 800 mm in about 4 O00 m. By fog drip (P,.-220 nini) the height or maximuin prccipitation is assumed to bc raised to 2 300 rn. Thc runoff incrcaccs gradually from RO O min at sea levcl to ahout RO = 2 O00 mm at 2 200 in, then thc runoff decrcascs to RO = 1 400 inm at 3 600 m. The cvapotranspiratioii has its maximum A E = 1 260 mm at a height of I 300 ni. Both upwards

262