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5. The Influence of forests and forest reclamation practice on streamflow and water balance
Forest influences on precipitation
Evapotranspiration in forest and fields
Forest impact on streamflow
Effects of forest reclamation projects on the water balance and water resources
I. A. Shiklomanov and O. I. Krestovsky
This review presents results from research carried out between 1963 and 1983 on aspects of the hydrological role of forests. Forest effects on rainfall and snow storage are discussed. Evaporation is correlated with forest type, age, soils and subsoils, and the depth of the water-table. The ratios of evapotranspiration between forests and fields are given, as well as the influence of forests on annual river flow, spring floods, and low flows. A two-parameter model to simulate the effect of forest reclamation on regional runoff and water balance is given, together with an example of the estimated effect for the last 100 years. An observational rather than experimental method is used to study stream discharge (annual, spring, and low flows), indicating that, in the USSR, forests affect streamflow by increasing low flow, reducing peak flows, and increasing total water yield by 5 to 10%. The yield effect is associated with increased precipitation above the forest (4-6%) and reduced evapotranspiration from young and old forests as well as from forests on sands and swamps. The more extensive the forest area, the greater the effect of these factors. In the case of very small forest areas, the increase of precipitation and snow storage is relatively greater and water yield tends to increase. Taking account of the size of this effect, a predictive model is presented of the hydrological influence of a forest in relation to its age and composition over a complete rotation represented by the canopy phytomass. Evapotranspiration reaches a maximum when the forest is 40 to 60 years old, is approximately equal to that of a short vegetation at 100 years, and thereafter decreases somewhat. This model may explain some anomalous results of comparison between forests and fields. An example is given of its use to predict water yields over 70 years under alternative systems of felling and reforesting an extensive area. Finally, suggestions are made for future research.
The problem of forest influence on the hydrological cycle has been studied for a long time, yet many contradictory views still exist. The amount of research in this field has increased particularly since the 1950s due to the intensification of human activity, and the number of publications on the problem has been growing continuously. In the USSR alone, during the last 20 years a series of monographs on the hydroclimatic and natureconservation role of forests have been published. Among such publications one should mention the works by Molchanov (1960, 1973); Rakhmanov (1962, 1971, 1975, 1981); Fedorov (1977); Pobedinsky (1979); Protopopov (1975); Rauner (1972); Lebedev (1982); Sokolov (1982); Shpak (1968); Idzon (1980); Vodogretsky (1979); Melekhov (1980); and Nikolaenko (1980). Some of the meetings and conferences in the USSR on aspects of the problem have been the Fourth All-Union Hydrological Congress, in Leningrad, 1973; Water Protection and Conservation Roles of Forests, in Vladivostok, 1974; Water Protection and Conservation Roles of Mountain Forests, in Krasnoyarsk, 1975; and Forest and Environment Protection, in Tallin, 1976. International symposia were held on forest hydrology (in 1965 in Pennsylvania, USA), and on forest influence on the environment (in 1970 in Moscow), as were more specialized meetings, such as the Environmental Management of Agricultural Watersheds, in Smolenice, Czechoslovakia, 1979 (Golubev 1983) and the Extrapolation of Data from Representative and Experimental Basins, in Helsinki in 1980 (IAHS 1980).
It should be pointed out that many significant aspects of hydroclimatic forest impact are fairly well recognized and cause but little discussion. These are the favourable influence of forests on microclimate, on air and water purification processes, and on runoff and erosion control, all contributing to the well-known value of forests for conservation and recreation. However, specialists still disagree on the influence of forests and forest reclamation on total water resources and the water balance of regions and major river basins.
Hydrometeorologists and forest managers of the Soviet Union had (and still have, to some extent) substantially differing views on the above problems. However, following the integrated experimental research conducted during the past 20 years in various regions of the country the views of the majority of specialists have converged. Theoretical generalizations and experimental data have been used to estimate and forecast the effects of forest reclamation projects on the water balance, water resources, and the hydrological regime of river basins. Such knowledge will be of great practical value in the USSR for water-supply and irrigation projects in particular, and also to solve the problems of the inland seas, and for large-scale projects involving the multipurpose use and conservation of water resources. This last includes partial water transfers from the rivers in the north of the European USSR and Siberia to the south.
This paper presents the chief findings of the research carried out in the USSR, mainly during the last 20 years, on forest influence on components of the hydrological cycle (precipitation, evapotranspiration, and streamflow) and on predicting the effects of forest management on water balance, water resources, and the water cycle.
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