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Evapotranspiration in forest and fields

Evaluation

Evapotranspiration in the forest (E_forest) is a complicated physical process. It includes transpiration by the green mass of the tree crowns (E_tr), evaporation from the soil and by ground vegetation (E_s), and evaporation of intercepted precipitation (E_p). The techniques of evaluating each kind of evaporation are quite different. E_s values are usually evaluated by direct observation of evaporation from various types of forest soils and ground vegetation using weighed evaporimeters and lysimeters. E_p values are derived from precipitation measured by the gauges installed under different types and densities of forest canopy. Transpiration (E_tr) is most difficult to measure; it can be determined with the help of big-drainage or weighing lysimeters, by water-use coefficients (consumptive use), by the heat balance of plants, and by the rapid weighing method using freshly cut branches. Very often E_tr is estimated by the water-balance method using meteorological evapotranspiration values or measurements of evaporation.

Most often evaporation from forest (E_forest) is determined by computing the water balance of forest areas, small forested watersheds, or relatively large river basins. Using this method the value for evaporation is very dependent on the accuracy of the measurements of other water-balance components, such as the correction factors for precipitation gauges and the accurate evaluation of water volumes infiltrating the aeration zone and the aquifers (for small areas and small watersheds). Many investigators (Bochkov 1970; Bulavko 1971; Vodogretsky 1979; Lebedev 1982; Rakhmanov 1971) have obtained average values of evaporation from forests in different regions of the country using long-term water balances of completely forested middlesize and large watersheds. Provided reliable precipitation and runoff data are available, one obtains an averaged evaporation value for all types and ages of forests. Sometimes E_forest is computed by the heat-balance or turbulent-diffusion methods. These two methods are labour consuming in the collection of data, especially for high stands, and provide estimates of relatively low accuracy.

Evaporation from fields (E_field) is usually determined with the help of lysimeters, of various types and the values obtained are corrected by comparison with water and heat balance estimates from agricultural fields and watersheds.

In view of the different methods used, and the corrections made, estimates of evaporation from forests obtained by different authors at different times often show poor agreement. Some studies of evaporation from forest do not give comparative data for adjacent fields, so there is no possibility of comparing evaporation values and hence obtaining reliable conclusions about the forest influence.

Very many estimates of evaporation from small forested watersheds, or from forest areas, do not take account of water percolating into deep ground-water aquifers. These include the studies by G. N. Vysotsky; V. I. Rutkovsky; I. S. Vasiliev; N. A. Voronkov; A. A. Rode; A. I. Mikhovich; I. S. Shpak et al. According to their estimates evapotranspiration from the forest in all regions of the country — from the north moisture-excess areas to the southern arid regions—greatly exceeds evapotranspiration (by 50-150 mm) from the treeless areas. Their findings and research results are critically reviewed in the works of Rauner (1966, 1972), Konstantinov (1968), and Rakhmanov (1971, 1981). One doubts such a conclusion as it is unsupported by critical experiments (Molchanov 1960, 1961, 1963a,b, 1970, 1973; Fedorov 1970).

Forest Evapotranspiration

Evaporation from forest is often estimated by the heat-balance method (Krestovsky 1983a; Kuzuetsova 1957; Fedorov 1977). Using this approach Rauner (1966) calculated the evaporation from forests of the European USSR and plotted isoline maps. According to Rauner the average evaporation values for forests in the north are 400 to 450 mm, in the Leningrad and Vologda districts 500 mm, in the upper reaches of the Volga and the Dnieper 525 to 550 mm, in the Moscow-Voronezh region 575 mm, and in Byelorussia, the north of the Ukraine and Kursk region, 576 to 600 mm. It has been suggested (Zubenok 1976; Rakhmanov 1981; Gidromet. 1976; Fedorov 1977) that Rauner overestimated forest evaporation by approximately 5 to 15%; his data do not agree with the experimental data on evaporation from forests and with the latest maps of evaporation from land obtained from the water-balance equation and by other computational methods (Bulavko 1971; Gidromet. 1976, 1980).

To study heat and moisture exchange processes in forests and to calculate evapotranspiration by tall spruce forest, a special installation was built in 1958 at the Valdai station. It consists of three masts, each 45 metres high, and a system of cables supporting instruments. The installation is equipped with telemetering devices for collecting all main meteorological data at 10 levels (from 1 m to 47 m), radiation, and precipitation (Fedorov 1977). The average annual value of evaporation from forest computed at Valdai by the heat-balance method is about 480 mm, lower than Rauner's evaporation norms of 540 mm for this region.

Recently a programme of experimental investigations has been carried out in different regions of the forest zone of the European USSR. The studies have been partially reported by Krestovsky (1980, 1982, 1983a, b), Fedorov et al. (1981), Fedorov (1977, 1981) and summarized by others (A. A. Molchanov, S. A. Bratsev, A. V. Lebedev, etc.) This programme has produced revised estimates of evaporation from various forests and made it possible to detect the influence of forest species and tree age, soil conditions, and climatic factors on evaporation. Relationships have been developed between evaporation and meteorological factors, heat balance, and biological characteristics.

Evapotranspiration components of mature forests of different types are summarized in table 3. The age of the conifers (spruce and pine) is 100 to 120 years and of the deciduous species (birch and aspen) 50 to 60 years. The absolute values (mm) indicate evaporation in the southern taiga subzone in the European USSR. The relative values (lower part of table) indicate evaporation partitioning in the two neighbouring forest subzones, central taiga and mixed forests, over an area of about 2,000,000 km². For conifers, more moisture is consumed by mature spruce forests (490 mm) and less is transpired by pine (450 mm).

Considerable changes among the evaporation components occur during the year. In winter, conifers intercept and evaporate 20 mm more than deciduous forests. This largely explains the difference in the amounts of snow stored in conifer and deciduous forests by the start of the snowmelt.

TABLE 3. Components of average annual evapotranspiration in mature forests of the southern taiga subzone of European USSR (normal values of water balance components: P= 700-750 mm, E=450-500 mm, R= 250 mm).

E_forest = total evapotranspiration; E_tr = transpiration by trees; E_s = evaporation from soil and ground vegetation; E_p = evaporation of precipitation intercepted by tree crowns.

During spring, evapotranspiration in conifer stands is also greater than in deciduous forests (by 15 mm). Differences in snow storage and springtime evaporation cause the main differences in spring snowmelt runoff: water yield from deciduous forests is 20 to 30% greater than that from conifer stands.

During the warm period (May to October), deciduous forests evaporate and transpire 95% of the total annual evapotranspiration, whereas for conifers the proportion is 85 to 90%. During dry summer periods, evapotranspiration in deciduous forests falls to 90% and in conifer forests to 75% of the annual evapotranspiration. This reduction is attributed mainly to the reduction of the evaporation of precipitation intercepted by the crowns.

The relations between the components of evapotranspiration during the whole life of conifer and deciduous species is shown in figure 1. Mature deciduous forests transpire 100 mm (25%) more than mature conifers (table 3), whereas young and middle-aged conifers (40 to 60 years) transpire the same amount of water as deciduous species of the same age. Maximum differences between E_p and E_s components occur in young and middle-aged forests. These differences for conifers are the reverse of those for deciduous forests. Differences in transpiration (E_tr) are less. In old forests of different types (140 to 160 years) evapotranspiration is 10% (40 mm) less than in mature forests.

The annual regime of soil moisture and ground water is strongly influenced by the sum of the E_tr and E_s components, since they operate mainly during the warm period. The difference between the sum of E_tr and E_s in deciduous and conifer forests amounts to 25-30% (100 mm) in young and middle-aged stands, whereas in old forests the difference is 57%. Soil drying and lowering of ground-water tables in deciduous forests is therefore much greater than in conifer stands. During dry summer periods all the reserves of soil moisture and ground water are transpired by young and middle-aged deciduous trees, which greatly reduces summer low flow in comparison to runoff from conifer-covered areas.

In the southern and central taiga subzones of the European USSR deciduous species predominate in young and middle-aged stands, while conifers dominate in mature forests. This happens because by the time conifers become mature, deciduous species (birch and aspen) senesce and die.

During the life of a forest, evapotranspiration and its components tend to change, whereas precipitation is relatively constant (fig. 1). In clear-cut areas evaporation is lowest (60% of evaporation from mature 100-year-old forest). Young and middle-aged forests transpire 10 to 20% more water than mature forests and 20 to 30% more than old 140-year-old forests. Therefore, when analysing the hydrological effects of forests one should take into account both the forest species and its age. These factors are frequently neglected, only the area under forest being taken into account, leading to contradictory estimates of forest impact on water resources.

Comparisons between Forests and Fields

The most reliable experimental data on evapotranspiration from forest stands of various types and ages in comparison to evaporation from treeless areas were obtained in the 1960s and in the 1970s by Molchanov (op. cit.). He carried out water-balance investigations, taking into account deep percolation of water. According to his (1970) experimental results, spruce forest (with density of 0.7-1.0) of the northern taiga on loamy clays aged from 80 to 160 years evaporates annually 250 to 370 mm (300 mm on average), whereas meadows evaporate 300 mm and clover 370 mm. Taiga in central regions (Vologda district), consisting of spruce and birch stands on loamy clays aged from 40 to 140 years, evaporates 320 to 450 mm (depending on the age), or 400 mm on average; evaporation from meadows and clover fields during the same years was 375400 mm. In the steppe zone (Lugansk district) forest belts 20 to 50 years old evaporate 460 mm and the surrounding fields 435 mm. Thus, according to these data (supported by other of Molchanov's findings), northern taiga forests evaporate 10% less than the fields and grasslands around them. In the central part of the European USSR, evapotranspiration from fields and forest is the same, on average, whereas in the south, forests evaporate about 5-10% more than the surrounding open land. The latest waterbalance studies carried out by the State Hydrological Institute (Gidromet. 1976, 1980; Vodogretsky 1979; Krestovsky, Postnikov, and Sergeeva 1979; Fedorov 1977) agree with Molchanov's comparative estimates, but indicate that his absolute evaporation values are somewhat underestimated since he used uncorrected data from precipitation gauges. Corrections for wind effect, initial moistening of the bucket, and evaporation from the bucket may be as large as 100 to 200 mm in regions where the proportion of snow and fine rain is large.

FIG. 1. Variations in evapotranspiration components during the period of growth of new forest in the southern alga subzone of the European USSR;
E = 475 mm.
E_forest = evapotranspiration;
E_tr = transpiration by trees;
E_s = evaporation from soil and ground vegetation;
E_p = evaporation of precipitation intercepted by tree crowns.
Forest types: 1 = conifer; 2 = deciduous mixed, mainly conifers after 70 years; 3 = deciduous.

Effects of Land Use

Evaporation from forests and fields, estimated over a long period, accounts for 50 to 80% of annual total precipitation. Minimum loss of precipitation by evaporation occurs in the northern taiga regions, and maximum losses are typical of the foreststeppe regions of the European USSR.

Evaporation from agricultural fields follows the annual physiological cycle of plant development. Since meteorological conditions change slightly over 10 or 20 years, the annual evapotranspiration for fields also varies slightly during 10 or 20 years (the coefficient of variation of annual evapotranspiration values, C_v, is 0.15-0.25). Therefore, when comparing water consumption by fields to that of forested areas, the average evaporation from fields for 10 to 20-year periods may be used.

Evaporation from treeless plains of the European USSR ranges from 300 to 350 mm y^-1 within the northern taiga subzone and from 500 to 550 mm y^-1 in the mixed forests and forest-steppe zones (Gidromet. 1976, 1980; Zubenok 1976). In the southern taiga subzone of the European USSR evaporation from treeless areas ranges from 450 to 500 mm and is on average 475 mm y^-1 in most of the region. This estimate is supported by numerous water-balance investigations (Krestovsky 1969b; Fedorov 1977).

TABLE 4. Ratios of annual evaporation from forests of different age to annual evaporation from fields in the southern taiga subzone of European USSR

Evapotranspiration from treeless areas (475 mm y^-1 includes evaporation from grassland (475 mm), marshy areas (490 mm), winter crops (cereals and perennial grass, 520 mm), summer crops (cereals and industrial crops, 460 mm), row crops (potatoes, vegetables, 440 mm), and bare fallow (400 mm). For vast treeless areas (many thousands of hectares) crop rotation is relatively constant and this allows us to use the average value of Evapotranspiration from fields. For small treeless watersheds (up to 1,000 ha) Evapotranspiration from agricultural fields may not equal the regional evapotranspiration .

Evapotranspiration from fields may be either smaller or greater than evaporation from forest; it depends on the age of the forest (table 4). Forests transpire greater amounts of moisture than open land only when 30 to 100 years old. Evapotranspiration from forest is less than that from fields when the forest is 2 to 20 years and over 120 years old. Mature forest (100-120 years) evaporates on the average the same amount as fields but substantially less compared to winter crops (they occupy 30-40% of the treeless area) and substantially more than tilled crops and bare fallow. Mature pine forest evaporates 7-10% less than spruce forest and 5% less than fields. Such differences in evaporation from forest and fields are also observed in neighbouring subzones of the European USSR.

Therefore one cannot answer directly the question "Which is greater, evaporation from field or from forest?" because the question arises "What sort of field and what kind of forest?" Depending on the type and the age of the forest, the type of crops, and the meteorological conditions the differences in evaporation from fields and from forest vary within rather wide limits.

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