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We believe that we are able, to a large extent, to explain the contradictory conclusions of various authors around the world on the hydroclimatic role of forests.
Firstly, forest impact on water balance depends on forest age and species composition, soil properties, and meteorological conditions. These factors must be taken into account for a comparison of the water balance of forested and treeless areas. Forest area percentage per se is not a very reliable predictor; for the same percentage of forest area the water-balance components and streamflow regime may be quite different depending on forest age and species, forest location within the watershed, and soil types. Open treeless areas and fields are also quite dissimilar.
The same conclusions may be drawn regarding the effect of forest cutting on streamflow. It depends on the time elapsed since the forest cutting, illustrated by the curves in figure 9.
Quantitative prediction of the hydroclimatic influences of forest requires further investigation by theoretical, field, and laboratory studies. For instance, the interaction of precipitation with various forest types has not been determined. Another important question is the relation between evapotranspiration from forests (transpiration in particular) and meteorological characteristics and the chemical composition of the lowest layer of the atmosphere. Our ignorance is most acute in view of the possible anthropogenic changes of climate due to the increase in carbon dioxide concentration and mounting air pollution from oxides of nitrogen and sulphur, which severely affect the state and development of forests, conifers in particular.
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Bratsev, S. A., and A. P. Bratsev. 1979. Izmenenie vodnykh resursov rek Komi ASSR pod vlianiem lesothoziaistvennoi deatelnosti chelovoka (The change of river-water resources in Komi ASSR subsequent to forest management). Trans. Komi Branch of the Academy of Sciences of the USSR, 42: 48-61.
Bulavko, A. G. 1971. Vodny balans rechnykh vodosborov (Water balance of river basins). Gidrometeoizdat, Leningrad, 304 pp.
Chertovskoy, V. G. 1978. Elovye lesa Evropeiskoi chasti SSSR (Spruce forests in the European USSR). Lesnaya promyshlennost, Moscow, 176 pp.
Fedorov, S. F. 1977. Issledovanie elementov vodnogo balansa v lesnoi zone Evropeiskoi territorii USSR (Study of the water-balance components in the forest zone of the European USSR). Gidrometeoizdat, Leningrad, 264 pp.
———. 1981. Opredelenie transpiratsii elovymi nasazhdeniami po koeffitsientu transpiratsionnoi aktivnosti (Evaluation of transpiration by spruce plantations with the help of transpiration rate coefficient). Trans. GGI, 279: 32-34.
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———. 1976. Rekomendatsii po raschetu isparenia s sushi (Recommendations on evaporation computation from land). Leningrad, 96 pp.
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———. 1982. Metody izuchenia gidrologicheskogo rezhima vodnyth ob'ektov (Methods for studying the hydrological regime of water bodies). Leningrad, 391 pp.
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Idzon, P. E, G. S. Pimenova, and O. P. Tsyganova. 1980. "Kolichestvennaya kharakteristika vodookhrannykh i vodoreguliruyushchikh svoistvesa (Quantitative estimates of waterconservation and runoff-controlling properties of forests)." Lesovedenie, 5: 3-12.
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Kaliuzhny, I. L., K. K. Paviova, and B. A. Popov. 1979. BiogeoJisicheskie uslovia vzaimodeistvia tverdykh osadkov s pologom khvoinogo lesa (Biogeophysical conditions for the interaction of solid precipitation with conifer-forest canopy). Vestnik LGU, 24, ser. Geologia-geografia, 4: 82-90.
Klintsov, A. P. 1973. Zashchitnaya rol lesov Sakhalina (Sakhalin forests as a protection factor). Yuzho-Sakhalinsk, 234 pp.
Konstantinov, A. R. 1968. Isparenie v prirode (Evaporation in nature). Gidrometeoizdat, Leningrad, 532 pp.
Krestovsky, O. I. 1969a. Issledovania stokobalansa i vodnogo balansa vodosborov (Investigation of runoff and water balance of watersheds). Trans. GGI, 176: 22-50.
———. 1969b. Vodny balans vodosborov VNIGL (Water balance of the Valdai/VNIGL watersheds). Trans. GGI, 165: 3-71.
———. 1980. "Sostoianie vodnogo balansa i tendentsii izmenenia stoke v sviazi s promyshlennym ispolzovaniem lesov Volg-Kamskogo mezhdurechia (The state of water balance and trends in runoff changes due to industrial forest utilization in the Volga-Kama interfluvial area)." In The influence of man's activity on game anima/ populations and their habitat (in Russian), 1: 1317. VNIOZ Press, Kirov.
———. 1982. "Experimentalnye issledovania (Experimental investigations)." In Metody isuchenia gidrologicheskogo rezhima vodnykh ob'ektov, sect. 1.4, pp. 61 -79. Gidrometeoizdat, Leningrad.
———. 1983a. "Khoziaistvennaya deatelnost i vodnost taezhnykh rek ETS i Sibiri (Economic activities and water amounts in the taiga rivers of the European USSR and Siberia)." In Scientific Council of the Siberian Branch of the Academy of Sciences of the USSR on multipurpose cultivation of the taiga area: Thirteenth extended meeting. Publishing House of the Siberian Branch of the Academy of Sciences of the USSR (SO AN SSSR), Irkutsk.
———. 1983b. "Novy vzgliad na gidro-klimaticheskniu rol lesa (New vistas on the hydroclimatic role of forest)." In Chelovek i stihia '84, pp. 87-89. Gidrometeoizdat, Leningrad.
Krestovsky, O. I., A. N. Postnikov, and A. G. Sergeeva. 1979. Otsenka isparenia s lesa v ranniy vesenniy period (Estimation of evaporation from forests in early spring). Trans. GGI, 259: 75-86.
Krestovsky, O. I., and N. V. Sokolova. 1980. Vesenniy stok i poteri talykh vod v lesu i pole (Spring runoff and the loss of melt water in the forest and in the field). Trans. GGI, 265: 32-60.
Kuznetsova, L. P. 1957. Rol reliefa i lesov v raspredelenii kolichestva osadkov na ravnine (Topography and forest effects on the quantitative distribution of precipitation in a plain terrain). Trans. GGI, 72: 76-91.
Lebedev, A. V. 1982. Cidrologicheskaya rol gornykh lesov Sibiri (Hydrological role of Siberian mountain forests). Nauka, Novosibirsk, 183 pp.
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———. 1963b. "Summarnoe isparenie i transpiratsia v lesu i na bezlesnykh ploshchadiakh (Evapotranspiration and transpiration in forests and in treeless areas)." In Forest and water, pp. 55-76. Gos. geographgiz, Moscow.
———. 1970. "Tsykly atmosfernykh osadkov v otdelnykh prirodnykh zonakh v oldelnykh tipakh lesa (Precipitation cycles in different natural zones in some forest types)." In Reports of Soviet Scientists at the International Symposium on Forest Effect on the Environment, 1: 24-59. State Forestry Committee, Moscow.
———. 1973. Vlianie lesa na okruchaiushchuiu sredu (Forest effects on the environment). Nauka, Moscow, 359 pp.
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———. 1971. V/ianie lesov na vodnost rek v basscine Verkhnei Volgi (Forest influence on the amount of water in the upper Volga basin). Trans. of Hydrometeorological Centre of the USSR, 83, 179 pp.
———. 1975. Voderguliruiushchaya rol lesov (Water-controling role of forests). Trans. of Hydrometeorological Centre of the USSR, 153, 192 pp.
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———. 1972. Teplovoi balans rastitelnogo pokrova (Heat balance of the vegetation cover). Gidrometeoizdat, Leningrad, 210 pp.
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———. 1978. Struktura polovodia i territorialnye prognozy vesennego stoka rek v Nechernozemnoi zone Evropeiskoi territorii SSSR (Spring-flood structure and areal forecasts of spring snowmelt runoff in the non-chernozem zone of the European USSR). Gidrometeoizdat, Leningrad, 98 pp.
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———. 1979. Raspredelenie zapasov vody v snezhnom pokrove v severo-vostochnykh raionakh ETS (Distribution of water equivalent of snow pack in the north-east of the European USSR). Trans. GGI, 159: 58-67.
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This Soviet research into the effects of land management on hydrology has kept practical objectives sharply in focus. The predictive model relating evaporation to the stage of forest succession by Shiklomanov and Krestovsky is of considerable interest and will be of considerable importance when its reliability has been determined. The workshop suggests that there is a strong case for international collaboration to see if the basic model holds in other parts of the world, especially in tropical and subtropical regions.
The model of the effect of stand age on evaporation and streamflow suggests one explanation for the conflicting results from studies comparing the different hydrological effects of deciduous and coniferous forests and between forests and fields. These investigations into the effects of land use on hydrology are part of the current trend that allows apparent anomalies to be understood as the finer details of the systems are revealed.
It was unfortunate that neither author was able to attend the workshop to discuss points raised and answer queries on the methods used. It would have been very relevant to learn of the techniques and their accuracy used for measuring such parameters as transpiration and deep percolation. The review omits much of this information so that the workshop can only consider the interpretation of the data whilst expressing the hope that critical tests of the methods have or will be made. However, there was concern about both the estimation of drainage and the corrections that have been applied to the measurements of precipitation, particularly as these appear to be of a magnitude comparable to the differences between water balances resulting from different land uses. While the need for such corrections is accepted, so too is the problem of assessing their actual values. Russian studies have shown that there is a global underestimation of precipitation, but it would be wrong to assume that this is directly related to local rain-gauge deficits. There is no substitute for local calibration of rain-gauges, made more difficult but even more necessary in the Soviet Union, where such a large proportion of precipitation occurs as snow.
It is clear that the effects of forests on precipitation at different scales must be distinguished. On a small scale, perhaps of a kilometre, the effects might be termed redistribution and are, by their nature, extremely hard to measure. As pointed out by the authors, these are essentially edge effects, and because of drifting are very pronounced with snow. Also at this scale it is necessary, in contrast to the section on Horizontal Precipitation, to differentiate between fog drip (mist trapping by vegetation) and occult precipitation (a condensation phenomenon). The latter probably represents little or no gain by the local water balance as it largely results from a closed, very smallscale cycle. At the mesoscale (say 20 to 30 km) it is difficult to conceive of a mechanism that would produce the reported precipitation increase of around 5%. At the regional/global scale (a few hundred kilometres and greater) recycling could account for precipitation increases by forests, perhaps by affecting convergence patterns.
The model of interception loss from forest and fields based upon the two parameters of leaf area and canopy saturation seems to have resulted in very large values compared with other work. A clear definition of these parameters and descriptions of the methods of measuring both leaf area and canopy saturation would be helpful. The latter divided by the leaf area gives a measure of the "equivalent film thickness" of the intercepted water; however, many of the data of Shiklomanov and Krestovsky suggest very considerable thicknesses. The model might also be improved by characterizing the rain climate in terms of season, storm size, duration, time of day, and interstorm interval. It would be of great value if hydrologists could include in their research results the type of precipitation, for example frontal, convectional, or even orographic. This distinction, together with the proportion of rain falling in each class for the locality, might be more important than species of tree for interception loss. In the tropics the recording of rainfall type may also be of great importance as it is suggested that the number of rain days is the most sensitive climatic parameter to land surface changes (see chapter 4). For general circulation models it might also be significant.
The values of evaporation from the forest floor gave rise to some comment due to their large size relative to the other components of evaporation. However, it is corroborated by UK research, which shows that in open forest stands the trunk space and the air above the canopy are at times strongly coupled by convective air exchange. A bracken understorey in eastern England contributed half of the total evaporation of the stand during a period of high atmospheric vapour-pressure deficit, when transpiration of the trees was reduced by stomata! control in response to this deficit. However, in UK conditions this would occur only for a short period in a year, while in the open woodlands of the USSR the conditions may be much more common. Conversely, this phenomenon is rare in dense forests in the UK and in Brazilian rain forest.
Although experimental investigations in hydrology are expensive and time consuming and are of limited areal application (see Experimental Methods section), they form the critical basis of the subject. Some of the conclusions reached by Russian workers are such that they should be tested objectively. While the extensive observational techniques and the correlative methods (sections Network Studies et seq.) indicate important relationships, these do need to be conclusively validated by scientific experiments before land-use policies are based on them.
Finally the workshop would like to thank the authors for their up-to-date review of Soviet hydrological research.