| As the water source area in the dry temperate zone of China, the forest/vegetation inmountains such as Qilian Mountain should be restored for soil protection and using the foresthydrological benefits. However, unreasonable afforestation will lead the runoff reduction. It'snecessary to control and regulate the interrelationships between water and forest/vegetationbased on the environmental characters and the needs of society development. In order tounderstand the mechanism of the interaction between forest/vegetation and hydrologicalprocesses, and to quantify the effect of the heterogeneity of physical conditions on thehydrological processes and water balance in watershed, as well as to evaluate the hydrologicalresponse to vegetation transition, especially the watershed function of water yield, the model ofTOPOG, which was developed based on the environment conditions of tropic region, was usedin Pailugou small Watershed of Qilian Mountains in this paper. Calibration and validation ofTOPOG model was done using the meteorological and hydrological data meaused at the outletof the watershed in 2001 and 2002. Based on the simulation of the hydrological processes suchas canopy interception, evapotranspiration and runoff during growing season (May～Sep), theeffect of physical condition such as elevation, aspect and slope on the water balance as well asthe contribution to watershed runoff generation of different watershed parts was analysed.Seven scenarios with different forest coverage of Picea crassifolia of 0.41％, 9.19％, 12.31％, 36％, 36.15％, 45.65％and 46.33％were established. Then the responses of hydrologicalprocesses of the watershed for these vegetation scenarios were simulated, and the effect of thevegetation transition on water balance of watershed, especially on runoff generation werecalcualted. The main results were showed as following:1. The applicability and limitation of TOPOG in temperate regionThe canopy interception simulated by TOPOG is similar to that measured when therainfall amount in an event is ranged within 5～25 mm in growing season (May～Sep). In theforest of Picea crassifolia, the simulated values of total evapotranspiration, transpiration fromoverstory canopy, forest floor evapotranspiration are almost the same to that simulated bystatistical equations developed by Chen and Wang for spruce forest on slopes with elevation of2730～3100 m. For grassland on sunny slope, the evapotranpiration simulated by TOPOG isclose to that calculated by Wang's statistical equation when the elevation is within 2700～2800m, since the statistical equation is based on the observed data of this elevation. The runoff volume during growing season simulated by the "yield" mode of TOPOG, in which thehydrograph is not calculated, nearly equals to that measured at the outlet of PailugouWatershed. However, for simulating all hydrological processes in mountainous watershed oftemperate zone, some hydrological processes existing in the temperate zone, such as snowmeltand soil frozen, should be added in the TOPOG model.2. The effect of topographic conditions on the ratio of canopy interception, transpiration, soil evaporation, and runoff to the precipitation from May to SepThe averaged canopy interception ratio of Picea crassifolia forest is 0.38, and itdecreases with increasing elevation, but showes no big difference among slope aspect andslope gradient. The ratio of transpiration varies among slope aspects, such as the ratio is 1.0 onwest slope (the highest case) and 0.38 on east slope (the lowest case). The ratio of soilevaporation is about 0.01, which does not change with elevation, slope aspect and slopegradient. There is big difference of runoff ratio among slope aspects, e.g. the highest value of0.26 is on the north slope and the lowest one of -0.44 is on the west slope.The average of canopy interception ratio of shrubs is around 0.2, but it increases withincreasing elevation. The ratio of transpiration of shrubs is 0.4～0.8, increasing with risingelevation. There is a little difference between slope aspects, for example, it is 0.6 on northwestslope and 0.57 in the northeast slope. It also declines when the slope turns steeper. The soilevaporation ratio varies in the range of 0.012～0.03, which rises with increasing elevation. Theratio of potiental runoff decreases with risen elevation and increases with growing slopegradients, having a variation range of-0.06～0.45.The canopy interception ratio of grassland is 0.1, showing no difference with elevation, slope aspect and slope gradient. The transpiration ratio is 0.4～0.6, which slightly decreaseswith elevation and is bigger on northwest slope than on northeast slope. The soil evaporationrate, which does not change with elevation, aspect and gradient, is 0.07～0.12. The potentialrunoff ratio is 0.15～0.42, which is the highest on northwest slope and does not changeaccording to elevation, aspect and gradient, too.3. The spatial distribution of runoff in watershedSpruce forest on the shady slope within the elevation range of 2650～2900 m acts as awater-consuming area, since the potential runoff in growing season (May～Sep) is less thanzero. Spruce forest on the shady slope within the elevation range of 2900m～3200m is awater-balancing area, since the evapotransporation is close precipitation. Spruce forest on the shady slope within the elevation range of 3200～3300 m is a runoff-genertating area, becausethe runoff in growing season (May～Sep) is 51.0 mm in average. For grassland, the slope withan elevation less than 3000 m, it is runoff-genertating area, with runoff of 104.2～152.0 mmin growing season (May～Sep). As the shrub-located area, the upper part of the watershedwithin the elevation of 3300～3700 m, is a runoff-genertating area, with the runoff of57.9～157.3 mm. But if it increased to the elevation range of 3700～3770 m, it turns to be awater-consuming area, the potential runoff is-15.3 mm.4. The influence of vegetation on water balance of the watershedThe canopy interception in growing season (May-Sep) under different vegetation isdifferent as the following: spruce forest (176.8 mm)＞shrub (95.8 mm)＞grassland (50.8 mm).For example, the interception of spruce forest at the elevation of 2700 m is 171.2 mm, which is120.5 mm more than that of grassland growing on the same elevation, as well as 183.2 mm atthe elevation of 3300 m, which is 86.5 mm more than that of shrub on the same elevation.The transpiration during growing season (May-Sep) under different vegetation varieslike that: the shrub (304.7mm)＞spruce forest (298.7mm)＞grassland (242.5mm). Forexample, the transpiration of spruce forest at the elevation of 2700 m is 252.3 mm, which is41.1 mm more than that of grassland on the same elevation.The soil evaporation in growing season (May～Sep) of different vegetation is in a orderof: grassland (38.4 mm)＞shrub (10.4 mm)＞spruce forest (5.1 mm). For instance, the spruceforest on the elevation of 2700 m is 4.9 mm, which is 29.4 mm less than that of grassland, and5.5 mm at the elevation of 3300 m, which is 4.4 mm less than that of shrub on the sameelevation.The average of potiential runoff under different vegetation is in the order of: grassland(112.7 mm)＞shrub (77.2 mm)＞spruce forest (-7.5mm). And the potential runoff depth (-19.5mm) under spruce forest on the elevation of 2700 m is 134.7 mm less than the grassland on thesame elevation. When the elevation is 3300 m, the potential runoff depth of spruce forest is51.1 mm, 101.3 mm less than shrub on the same elevation.5. The effect of vegetation variation on water balance of watershedWhen the spruce forest coverage increases 1％of the watershed area, the average canopyinterception in the whole watershed increases 1.3 mm, and the average transpiration increases3.2 mm, but the average soil evaporation decreases 0.3 mm during the growing season (May～Sep). Thus the potential runoff of the watershed decreases 4.2 mm as well as the peak discharge decreases 0.004 m~3/s.
|
PDF Full Text Request