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Forest impact on streamflow
The differences between annual runoff from forested and treeless areas is the main question of forest influences on the hydrological cycle. The reliability of forecasts of changes in water resources due to forest reclamation projects depends primarily on solving this. Investigations of forest influence on annual runoff have been conducted
Deep-Percolation Errors. The most important investigations are the long-term observations, conducted on experimental watersheds several hectares in area with different percentages of forests, by the Valdai branch of the State Hydrological Institute. The analyses of runoff from these watersheds made by several authors (Bochkov 1970; Krestovsky 1969b; Sokolov 1982; Fedorov 1977) for different time periods indicate that in all cases runoff from small forested watersheds is considerably lower (0.7 to 0.8 times) than from treeless or partially forested watersheds. The results were approximately the same when runoff from numerous small forested watersheds was compared to that of the non-forested watersheds in the Moscow and Kursk regions, in the Baltic Sea region, in the north-east, in the south of the Ukraine, and on Sakhalin. These results were reported by Subbotin (1966), Shpak (1968), Pobedinsky (1979), and Klintsov (1973). In the southern regions annual and spring snowmelt runoff from small forested valleys was in some years two to five times less than that from treeless watersheds. These data are explained by Soviet hydrologists not from the point of view that forested watersheds consume more water and yield less than partially forested and treeless areas but to the effect that on forested watersheds water readily infiltrates the more permeable forest soils, and part of the runoff reaches aquifers, thus avoiding the gauged stream channels. The ground-water component of runoff was reliably estimated for the first time for the completely forested Taiezhny watershed (drainage area 45 ha). It was, on average, 100 mm (Bochkov 1970; Sokolov 1982; Fedorov 1977) and, allowing for this, the total yield from the watershed appeared to be almost equal to runoff from small, less permeable non-forested watersheds and the discharge of the larger rivers of the region that drain the aquifers.
Thus the data from very small, leaky watersheds show only the effects of forest on overland flow, runoff control, and soil conservation. To solve the question of the change in total water yield from forested areas accurate estimates of the underground runoff component are required a difficult task, especially for forested watersheds.
Runoff Plots. Vodogretsky (1979) studied the influence of agrotechnical methods and forests on runoff using data from runoff plots and small watersheds in various natural zones. He always computed the magnitude of ground-water recharge relative to the depth of the ground-water table, surface slope, and soil type. He concluded that in the forest zone of the European USSR, for water-table depths less than 5 m, forests tend to slightly increase the annual water yield (1.0 to 2.5 mm for each 10% of forest area). In the foreststeppe zone with ground water deeper than 10 m, forests either have a negligible influence on annual water yield (where slopes are slight) or reduce it slightly (about 1 to 2 mm for each 10% of forested area).
Experimental Methods. Investigating forest influences on streamflow by applying experimental treatments, for example clear-cutting and reforestation, have not been widely carried out in the USSR. Such experimentation requires much time and money, whilst the results obtained are of rather limited use since the data collected on very small watersheds and for specific forest development phases cannot be regarded as representative for medium and large river basins, especially with regard to annual runoff. The experiments of this kind were undertaken mainly to study the ability of forests to control and prevent erosion.
At the Valdai research station various types of forests were planted in a small valley watershed and on runoff plots, and water-balance components have been studied there for some years (Sokolovsky 1968; Fedorov 1977). As the forests grew the coefficient of spring snowmelt runoff from these areas, 0.80-0.95 prior to afforestation, fell constantly and after 8 or 10 years was about zero in some years. This reduction was due to the transfer of some of the overland flow into ground-water runoff.
Experiments on the influence of forest cutting on runoff from small watersheds were conducted in oak forests (Voronezh district) and in conifer forests (Perm district, Bashkiria, the Southern Urals, and the western Sayan Mountains). Results (Lebedev 1982; Molchanov 1973; Pobedinsky 1979; Rakhmanov 1981) indicate that there is a sharp rise in runoff for some years after forest cutting and then, with the regrowth of the felled area, runoff values decrease. After forest cutting runoff and erosion increase, leading to greater sediment yields.
Network Studies. Since the 1960s the most widely adopted approach in the USSR to the study of forest impact on annual steamflow has been the statistical analysis of the numerous standard hydrometeorological observations from basins with different percentages of forest cover. These have been carried out for different regions and periods by V. V. Rakhmanov, A. P. Bochkov, A. A. Molchanov, P. F. Idzon, A. V. Lebedev, A. I. Subbotin, and S. F. Fedorov among others. The methods used were similar. In a given region river basins with reliable hydrometric data and different proportions of forest area (treeless to completely forested) were selected, and for the same long-term period relationships between the average runoff and the proportion of forest cover were determined. Other factors affecting streamflow (e.g. climate, topography, and soil types) were assumed to be the same or were incorporated in multiple regression equations.
V. V. Rakhmanov was the first to use this method in the European USSR. He began with correlations between streamflow and forest area percentage then included air temperature and later (for the upper Volga basin) other meteorological and basin characteristics. For all the regions studied his linear regressions indicate an increase of annual streamflow of 0.8 to 1.3 mm for each 1% increase of the forest area within a region. The difference in annual runoff between completely forested and treeless watersheds is 80 to 130 mm, other conditions being equal. Similar linear relationships with about the same or even greater changes in runoff were obtained by other investigators for different periods (Lebedev 1982; Sokolovsky 1968; Opritova 1978; as well as G. Pauliukevichus, L. M. Sidorkina, M. P. Shirokova, S. Kh. Budyka, and L. G. Onufrienko) for the Ukraine, Byelorussia, the Baltic Sea region, west and east Siberia, the Far East, and Primorie.
Bochkov (1954) analysed the average runoff for 92 river basins in the forest-steppe and steppe zones of the European USSR and developed curvilinear relationships between runoff and the forest area percentage and annual precipitation. Within the range of 10 to 40% forest area, an increase of forest area by 1% produces a 1.2 mm rise in average annual streamflow, whereas with forest areas of 40 to 70% the corresponding increase in streamflow is only 0.65 mm; with forest area exceeding 80%, an increase in forest area has no appreciable effect on streamflow. Bochkov suggested that the maximum effect on streamflow is caused by forest belts, thickets, and small forest areas of irregular shape. In such places water income substantially exceeds that of open areas or large forests, due mainly to snow blowing in from flat terrain but also to horizontal precipitation at the forest edges. He thought that evaporation by forest stands and by crops was generally equal and was doubtful about any increase of precipitation above forests due to their aerodynamic roughness (the maximum estimates are 3 to 5% of the annual precipitation).
Paired Observations. A positive forest impact on annual streamflow was also supported by A. A. Molchanov, using data from 88 pairs of river basins with similar physiography but different percentages of forest. Half the data indicated that streamflow of forested basins was greater than that of slightly forested watersheds. Thus Molchanov (1960, 1973) concluded that, on the whole, his findings supported the views of Rakhmanov, Bochkov, Sokolovsky, and A. D. Dubakh.
Paired watersheds were often used by Idzon and Pimenova (1975), Idzon, Pimenova, and Tsyganova (1980), and Idzon (1980). The 1980 papers summarize information from 433 pairs of watersheds in various zones of the European USSR and conclude that forests lead to an increase in the annual streamflow, but the size of the increase greatly depends on the region. In the northern and north-western moisture surplus zones an increase of forest area by 1% causes 0.8 mm increase in runoff; southwards in the same area the runoff increment decreases to 0.3 mm, whereas in the semi-arid zone it falls to 0.1 mm per 1% of forest area. The increase of the total annual streamflow is caused by an increase in low flow periods. In general Idzon agrees with Bochkov's curvilinear relationship between runoff and forest area percentage. Idzon, Pimenova, and Tsyganova (1980) suggested, in earlier comparisons of paired watersheds, that in some regions runoff from more densely forested basins was generally greater than that from partially forested drainage areas, whereas in other regions it was the reverse. Even within a given region both effects may be found. Such contradictions may result from the deficiencies of the method and the inadequate evaluation of physiographic factors, apart from forest cover, that influence streamflow.
Subbotin (1978) investigated spring-snowmelt induced flood runoff for rivers of central European USSR and arrived at quite different conclusions from the other authors. According to his findings an increase in forest area leads to a reduction in spring flood runoff of 0.7-0.8 mm per 1% of forest area. He did not examine base flow or annual streamflow. Fedorov (1977), conversely, using 29 carefully selected watersheds with forest areas ranging from 8 to 68% located in the Novgorod and Pskov districts (upper Dnieper and upper Volga basins), found that forests enhance annual, spring flood, and low-water flows. One percent of forest area increases the annual runoff by 1 mm on average, spring flood runoff by 0.8 mm, and low flow by 0.2 mm.
Thus the results of numerous investigations of forest impact on runoff testify to the positive forest effects on the annual and low-water flows, though the quantitative estimates of runoff increments vary over a considerable range. However, the paired catchment method has certain deficiencies. Firstly, it is very difficult to select watersheds similar in all respects except forest area. The selection is entirely subjective and the reliability of the results depends on the experience and conscientiousness of the researcher. The percentage of forest as a parameter is not sufficient as forests may affect streamflow differently depending on location of forest in a watershed, soil types, tree species, and age. The most important drawback of the approach is the absence of any explanation of the causes of these effects of forest on streamflow. Without understanding causes and effects one cannot predict the effects of the development of forest ecosystems and forest reclamation projects on water balances and water resources.
Microclimatological Methods. Recently comparison of water and heat balances of forested and treeless areas have often been used in the USSR to explain the effects of forests on streamflow (Zubenok 1976; Lebedev 1982; Gidromet. 1981, 1982; Rauner 1972; Fedorov 1977). These methods require reliable data on liquid and solid precipitation, as well as on interception, evaporation, and transpiration for forests (of different types and ages), crops, and grasslands in various hydrometeorological regions. When such data are available the effect of changing forest cover on runoff can be evaluated by comparing water-balance components of forest plots and surrounding treeless areas for a given basin or region.
Detailed water-balance data for the forest zone of the European USSR were used to investigate the correlation between water yield from field and forest areas during the spring snowmelt flood (Krestovsky and Sokolova 1980). The difference (D R) between snowmelt runoff from fields and forest areas was dependent on the region and the spring snowmelt flood factors of a given year (fig. 2). In all regions runoff from the forest was greater than that from fields in years of low accumulated snow-water content, whereas in years of very high water content the reverse occurred. The long-term average spring flood runoff from fields is usually 10 to 20 mm greater than that from forests in the north-eastern and central regions. In the north-western regions runoff values are similar; in the western and Baltic Sea regions runoff from fields is 10 to 20 mm less than runoff from forests. These variations are caused by differences in species composition and forest age in the different regions; different forest types in tercept, accumulate, and evaporate different amounts of winter precipitation. Contradictory estimates of the effect of forest on spring snowmelt runoff obtained from network data are thus explicable.
FIG. 2. Frequency of years (%) when snowmelt runoff (D R) from the field exceeds runoff from the forest in different areas in the forest zone of the European USSR. 1 = north-eastern and central regions; 2 = central part of the north-western region Novgorod district); 3 = western regions of the forest zone and Baltic Sea coast (Source: Krestovsky and Sokolova 1980)
Water-balance studies conducted at Valdai (Krestovsky 1969b; Fedorov 1977; Fedorov et al. 1981) have provided the reliable long-term mean annual values summarized in table 5.
It seems that in the Valdai Hills maximum runoff, about 20% greater than from treeless areas, is from pine forests (usually located on sandy soils), due to the somewhat greater precipitation and evaporation in the forest. For actual river basins of the region with about 75% forest area, annual runoff is usually 10% (30 mm) greater than for treeless areas (i.e. 1% of forest area results in 0.4 mm increase in runoff); this agrees well with the network data.
The research carried out in the USSR on the hydrological role of forests shows that they affect streamflow positively. Forests not only aid runoff control (i.e. through increased basetlow, reduced peak discharge, and water-quality improvement) but also increase the total annual water yield, on average for the entire country by 10 to 15% for completely forested watersheds relative to open treeless areas. This is due mainly to greater precipitation being accumulated in forests, evaporation being practically the same in forested and open areas.
TABLE 5. Annual water balance components in the Valdai region (long-term means)
(litres km-2 s-1)
|mm||% of runoff from field|
Note: forest age 80-90 years
These conclusions may be illustrated by the following examples. In densely populated agricultural areas of the USSR the percentage of forest area in drainage basins is not high (10 to 30%) and generally occupies small areas not suitable for agricultural purposes (sandy soils, rugged terrain, etc.). In such forests young and middle-aged trees (30 to 40 years old), capable of consuming maximum water amounts, predominate, and in summer these forests receive additional heat by advection from warm soils. However, the precipitation income (solid precipitation in particular) may be very high there due to the snow blown in from open areas and due to condensation. A third factor is the sandy soil, which has a high infiltration rate, low water-holding capacity, and restricts evaporation from the forest for some periods. Due to various combinations of the above factors, in different years these forests may have higher or lower yields of water compared to open areas. The long-term mean water yield from such forests is either similar or slightly higher than that from open areas.
As the percentage of forest area increases to 50-70% the intensity of forest utilization decreases and middle-aged and mature trees (50-70 years old) are dominant. In sparsely populated regions forest may occupy 80-90%, and mature and old trees (100-125 years old) are predominant with occasional newly felled areas and young stands aged about 20 years. These forests consume the least water in comparison to forests in densely populated regions and to open areas due to lower evaporation, even with equal precipitation.
Effects of forest reclamation projects on the water balance and water resources
The most important large-scale effects occur following forest reclamation projects such as clear-cutting and subsequent afforestation. Though forests in general tend to slightly increase annual streamflow, it does not follow that clear-cutting results in an immediate and inevitable reduction of the annual streamflow. Rather, clear-cutting causes sud den changes in the forest biogeocoenosis, the hydrologic effects of which last for many years.
The method of investigating these changes, developed by O. I. Krestovsky, is based on long-term experiments in the forest zone of the European USSR and on generalizations from the research work carried out by many Soviet scientists (see References).
The method incorporates measurements or estimates of all the water-balance components taken prior to and immediately following felling and during forest regrowth in the equation:
where P+Wsn, are precipitation and water equivalent of snow respectively, that is the water income, and
where Eforest is the total evapotranspiration; Etr is transpiration by trees; Es is evaporation from soil and ground vegetation; Ep is evaporation of precipitation intercepted by trees; R is total (overland and ground) runoff into rivers; D Vs and D Sgr are the changes in moisture storage in the unsaturated zone and ground water respectively, which are determined by the difference between the respective values at the start and end of the study period. All the components are given in mm depth of water. For a long-term period (over 10 years) one may assume that:
For the southern, central, and adjacent taiga subzones of the European USSR, long-term mean values are P » 600-800 mm, E » 400-500 mm, R » 200 - 350 mm y-1.
The method depends on the following three assumptions:
Since the third item is the main point of the given method and causes quantitative changes in water-balance components and water resources, we discuss it in detail in the following.
Evapotranspiration and Silvicultural Rotation
Molchanov (1960, 1961, 1963a, 1963b, 1970, 1973); Hilmi (1957); Fedorov (1977, 1981); Fedorov et al. (1981); Voronkov (1970); Voronkov et al. (1976); Bratsev (1982) showed that Eforest is closely related to the biological productivity of forests and especially to the annual increment of timber (m3 ha-1) and to the phytomass of the canopy (leaves, needles, t ha-1).
The phytomass provides for the development and growth of young and middle-aged trees and supports the life of old trees. The phytomass volume determines the water loss by transpiration and the amounts of light and precipitation penetrating through the canopy. These, in turn, determine the degree of development of ground vegetation, of epiphytes and brushwood. That means the phytomass volume is the factor on which all evapotranspiration components are dependent.
Figure 3 shows the relationship between the amounts of precipitation intercepted by the canopy and the phytomass in different phases of development. A similar dependence (fig. 4) was derived by O. I. Krestovsky for evapotranspiration from soil and ground vegetation.
Figure 5 (Rudakov 1979) shows the relationship between transpiration by pine stands and their phytomass for the forest, forest-steppe, and steppe zones of the European USSR and western Siberia. Rudakov used data on transpiration from pine stands of different ages and for years of different climatic conditions. The relationships show that the transpiration rate coefficients (Ktr) are independent of the tree age and locality: Ktr is the transpiration (m3) per ton of foliage per hectare of forest. For pure forest stands the mean Ktr for spruce is 85, for pine 200, for birch 500, and for aspen 450 m3 t-1 ha-1 (Krestovsky 1983a; Molchanov 1960; Rudakov 1979; Fedorov 1977, 1981; Hilmi 1957).
Figure 6 (after Bratsev 1982) shows the relationship between annual evapotranspiration from forests of different composition, age, and density and the mean annual increment for 5-8 years (D ). Evaporation estimates were obtained using the water-balance equation (3) for a large number of watersheds with drainage areas ranging from 1,000 to 6,000 km2 in the eastern central taiga subzone of the European USSR.
The phytomass and timber increments are not constant throughout the growth and development of a new forest (fig. 7). Therefore, the total evapotranspiration and its components undergo considerable changes with time (see fig. 1).
Krestovsky estimated from the analysis of experimental data on Ep and Es the longterm average values for forests of different density and composition and calculated mean weighted Ep and Es values for 10-year periods of forest development. Similarly, values of Etr were computed for 10-year development stages from the relationship Etr = Ktr M, where M' is the phytomass volume of tree crowns of different ages and composition. Eforest was then calculated by summing Ep, Es and Etr values for the 10-year periods. The estimates of Eforest were then adjusted to fit the heat and water balances of the experimental plots and watersheds and are shown in the first graph of figure 1.
Other studies may be collated by taking account of forest age and expressing Eforest relative to the evapotranspiration of a mature, 100-year-old forest (fig. 8). The evapotranspiration from a mature forest approximates the evaporation from fields and is comparable with regional evapotranspiration values on maps (Gidromet. 1980, 1981, 1982; Zubenok 1976).
The average time course of relative evaporation (figs. 8, 9) as a model of evapotranspiration from forests may be used to predict water-balance changes and runoff modifications in the southern and central taiga subzones of the European USSR and adjacent forest subzones. This model (curve 1 in fig. 8) really covers all types of forests and their development phases and indicates the average values of water consumption by all-aged stands (Krestovsky 1980, 1982, 1983a, 1983b).
Curve 2 in figure 8 (derived by Fedorov from long-term data from watersheds and experimental plots at Valdai) shows the changes in evapotranspiration by highly productive (class 1) spruce stands relative to changes of stand density (from 0.9-1.0 to 0.6-0.7) during development. Molchanov's generalizations (curves 3-10), from the findings of other scientists, refer mainly to high-density (0.8-1.0) pure forests located in various forest subzones and in the forest-steppe zone. Bratsev's data (curve 11) relate to mature and old (mainly productivity class IV) coniferous forests and young and middle-aged mixed stands of productivity classes III-IV.
It should be noted that forest development by natural regeneration with initial deciduous growth is a slower process than with pure stands, resulting in a smoother model curve of Eforest compared to pure stands.
The annual runoff values (overland plus ground-water components) are calculated by:
where P is the annual precipitation norm corrected for gauge errors. Runoff (Ri) computed for 10-year periods is expressed as a percentage of that from 100-year-old forest (R100) which is approximately equal to normal runoff in most regions of the southern taiga subzone of the European USSR (i.e. 250 mm or 8 litres km-2 s-1).
Runoff ratios Ri/R100 (fig. 9) are indicative of the total runoff changes throughout the life of new forest for a vast area. The model predicts changes in annual runoff (D R) due to felling and reforestation of medium and large river basins in the southern and central taiga subzones of the European USSR and also in the subzone of mixed forests. One only needs to know the age of forests in each part of the drainage basin or district. Then:
FIG. 3. Relationship between interception (Ei in % of total annual precipitation above forest) and the phytomass (M) in growing forests.
FIG. 4. Relationship between evapotranspiration from soil and ground vegetation and the phytomass of tree crowns. Figures at points indicate the forest age.
FIG. 5. Relationship between transpiration during the growing period (Err) and the weight of pine needles (M), 1. Ktr = 217 m3 t-1 ha-1; 2. Ktr = 205 m3 t-1 ha-1
FIG. 6. Relationship between annual evapotranspiration (Eforest) for watersheds in the north-east European USSR and the mean annual increment of timberD .
FIG. 7. Changes in the height and composition of forest stands (a) and phytomass (b) during natural regeneration of felled areas (1 = spruce; 2 = pine; 3 = birch, aspen).
FIG. 8. Fluctuations of relative evapotranspiration during forest regrowth in the European USSR. (Evapotranspiration from forests 100 years old is taken as 100%.) 1 = Average values for deciduous, mixed, and coniferous stands of normal density during natural reforestation in the southern and central taiga subzones of the European USSR. 2 = Spruce forest in the southern taiga subzone. 37 = High-density stands in the broad-leaved forest subzone: 3-birch; 4 - mixed forest; 5 - oak and ash; 6 - spruce; 7 - pine. 8 = Spruce forest in the central taiga subzone. 9 = Mixed forests in the forest and forest-steppe zones of the European USSR. 10 = Mixed-forest stand in the forest and forest-steppe zone of the European USSR. 11 = Mixed-age forests in the central taiga subzone in the north-east European USSR. 12 = Cedar-fir stands in the western Sayan Mountains.
FIG. 9. Forest age versus 1: mean annual evapotranspiration; 2: annual runoff; 3: spring flood runoff, March to May; and 4: low water flow, June to February, all expressed in relation to values for 100-year-old forest.
where are the proportional runoffs in relation to age from figure 9, and A1, A2, ..., An, represent the amount of area of forests 10 years old (A1), 20 years old (A2), ..., to 140 years. Similar models of runoff changes were developed (fig. 9, curves 3 and 4) for spring flood (March-May) and low-flow (June-February).
The curves of spring flood runoff variations were based on long-term experiments in the north-west and north-east of the European USSR. These experiments included observations of water-balance components in forests differing in age, composition, stand density, soil type, and distance to ground-water table. Increased spring flood runoff, lasting 50 years after clear-cutting, is due to the dominance of young deciduous trees, which increase snow storage and reduce evapotranspiration in spring. Frequencies of differences in spring flood runoff from forested and treeless areas of the European USSR are shown in figure 2.
Low-water flow is computed as the difference between the absolute values of annual and spring flow (mm) and expressed as a percentage of low-water flow from 100-yearold forests (see fig. 9). In comparison to annual and spring flood runoff, low flow shows greater changes because the main differences in evaporation from felled areas and forest stands of various age occur during the warm season (June-September). Typically runoff rises sharply on newly felled areas, then sharply declines due to the ever increasing water consumption by young forests up to the age of 40-60 years, after which time a considerable increase of runoff starts to occur. In old forest, runoff is higher than normal.
During a 100-year period of forest regrowth on felled areas, mean annual runoff will be 10% less than the normal value (15% less than from old forest); low flow will be 25% less than the normal value (35% less than in old forest), and spring flood runoff will be 6% higher than the normal or old forest values. Consequently, clear-cutting of old forests and subsequent reforestation causes an overall reduction in water resources and makes the intra-annual runoff distribution more uneven.
These sharp changes in annual, low-water, and spring flood runoff due to clearcutting and reforestation apply to relatively small forested areas. Over vast areas, forests are cut gradually and in different parts of an area or a drainage basin and the maximum area felled in a year seldom reaches 5% and is usually 0.7-2% of the total forest area in a river basin or district.
Effects of Forest Management on Runoff
Figure 10 illustrates variations in annual, low-flow, and spring flood runoff from forests of Vologda district during 1880-1980 and forecasts water-yield changes through to 2050 based on the actual forest structure in 1978 and on four alternative felling intensities and types of future forest use. Figure 10 shows that with the start of intensive felling (1880-1900) streamflow increased. Then water yield decreased and for a long period (1940-1975) annual runoff was 8 to 10% less than the normal value. With the present forest structure, runoff is tending to increase and in 20 to 40 years will reach and even exceed slightly the normal runoff value for all four management alternatives.
FIG. 10. Total water yield tin % of the normal value) from forest areas into the rivers of the Vologda district resulting from actual forest cutting up to 1980 and planned cutting for subsequent 70 years under various forest cutting alternatives (1-4). Runoff: a = annual; b = low flow; c = spring flood.
Evaluating the effects of forest practices on runoff over 100 to 140 years for a number of large river basins in the forest zone of the European USSR showed that such changes were gradual and might be positive or negative. For slight changes in annual runoff (up to ± 4-8%), considerable variations were observed during a year: low flow was reduced by 14-17% and spring flood runoff increased by 8-12%. Bulavko (1971), using data on precipitation, forest area percentage, and streamflow of the Neman for a 150-year period, showed that during the past 100 years climate-induced changes of runoff characteristics were much greater than management effects, but were short-term and variable.
Bratsev and Bratsev (1979) and Bratsev (1982) predicted that annual streamflow of small- and medium-sized rivers in the north-east of the European USSR will be 10% less in the twenty-first century due to a predominance of young forests after cutting old forest areas.
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