| In loess hilly region, soil water is the only resource that supplies plant growing. With the changing of many factors, such as precipitation, crown interception, crown transpiration, soil evaporation, runoff and seep, soil moisture is changing. Precipitation is the only income of soil, but evaporation and transpiration are major expenditures. Hippophae Rhamnides and Caragana Microphylla are t\vo kinds of tree species that can conserve soil and water, improve ecological environment on the Loess Plateau. Studying water balance and soil moisture ecological characteristic of these t\vo kinds of trees, we will acquire a lot of experience to guide our ecological environment construction and recognize the ecological and environmental effect of planted forest. By using field experiment and interior analysis, the main results are as follows: L Soil moisture ofHippophae Rhamnides and Caragana Microphylla has clearly changed with vertical and seasons. There have three layers in soil moisture when it is changing with the depth of soil. (1) Soil water dynamic layer (0-3 Ocm): this layer is affected by weather factors and the roots system. (2) Soil water utility layer (Hippophae Rhamnides 30-24Ocm; Caragana Microphylla, 30-200cm): this layer is mainly used by the roots system for resisting dry weather condition. (3) Soil water supplementary layer (Hippophae Rhamnides240-800cm; Caragana Microphylla 200~800cm); the change of soil water in this layer is very low; it is changed only when the weather is very dry. The change of soil moisture ofHippophae Rhamnides and Caragana Microphylla land in the growing season is as follow: Soil moisture is highest in the earlier period of growing season and lowest in the end of it. But to the land of different slope ofHippophae Rhamnides, the result is different. In the middle of growing season, soil moisture is the highest; in the end of the growing season, it is lowest. There are also have many factors to effect soil water, such as the length, the position and gradient of slope. Above 300cm, soil moisture of the middle slope is highest, long slope and short slope are same. Below 300cm, long slope is the highest, the follow is middle slope, and short slope is the lowest. Seeing from the position of slope, soil water in upper part higher than central section and centralsection is higher than lower part.2. The crowns of Hippophae Rliamnides and Caragana Microphylla have remarkable hydrological effects. They can intercept a lot of precipitation. Capability of the crowns changes with precipitation. Annual intercepting rate of crown of Hippophae Rhamnides is 20.72%; the rate of crown of Hippophae Rhamnides that was handled in 1998 is 13.23%; Caragana Microphylla's rate of crown is 42.67%.3. Hippophae Rhamnides land and Caragana Microphylla has remarkable effect on reducing runoff compared with barren slope. In the year of 2002, runoff of Hippophae Rhamnides land is only 14.68% of barren slope. Runoff of the Hippophae Rhamnides land that was handled in 1998 is 66.78% of barren slope. Runoff of Hippophae Rhamnides land is decreased along with its age. Compared with barren slope, its average runoff is 59.51%. In this givwing season, land of Caragana Microphylla didn't have runoff; this result indicates Caragana Microphylla has remarkable effect on reducing runoff.4. Expenditure of soil water is higher than income. This kind of result makes soil water cycling of Hippophae Rhamnides (Caragana Microphylla) land is not balance. In the whole growing season, water consumption of different forestland is not equal. Handled Hippophae Rhamnides is highest (668mm); the follow is Caragana Microphylla, (648.8mm); Hippophae Rhamnides is the lowest (638mm).5.Because of negative cycle of soil water, there form a dry layer in different forestland. When the depth of observation is 800cm, the dry layer in handled Hippophae Rliamnides and in Caragana Microphylla are ]80cm~800cm, in Hippophae Rhamnides is J80~680cm, in barren land, it is 200cm~500cm.
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