| The impacts of soil and water conservation on river flow and soil-hydrology of Jia-lu-he, Tu-wei-he, Pian-guan-he and Qiu-shui-he catchments have been analyzed by the simulation and on-spot measuring methods according to data of during 1950's to 1990's.The selected catchments of the above four all are in the key area of soil and water conservation on the Loess Pleatu. The main results are as follows.The trend, reasons and critical year of annual river flow' change have been researched by rank correlation and maximum deviation division. The results showed that each river flow has a tendency to gradual decrease mainly by soil and water conservation in the above four catchments. The critical year of the river flow decrease was 1974,1974,1977 and 1974 for Jia-lu-he, Tu-wei-he, Pian-guan-he and Qiu-shui-he catchments, respectively.Each river flow has been divided into prophase and anaphase period according to the critical year. The impacts of soil and water conservation on river flows have been analyzes by comparison of the runoff amount of prophase and anaphase period. The results indicated that the precipitation producing waters and annuals fluctuate of runoff amount in each catchment have been decreased mainly by soil and water conservation. Amount of runoff in the four catchments reduced from 26.8% to 69.0%. The runoff coefficient felled from 20% to 56%. The variation coefficient dropped from 17% to 76%. Furthermore, soil and water conservation also resulted in the underground flow decrease but its decrease extent is smaller than of surface flow.The season changes of the river flows and its response to soil and water conservation have been researched. In a year the change of river flow is drastically and season distributes of runoff in river is mainly centralize in the summer in Jia-lu-he, Qiu-shui-he and Pian-guan-he catchments. For example, the runoff amount from July to August accounts for 41%, 58% and 57% of the annual runoff amount in the above three catchments, respectively. One exception is Tu-wei-he catchment where underground runoff is the primary part of river flow. Its runoff season distribution is much uniformity. Its runoff amount of July to August accounts is 24% of the annual runoff amount. In a year river flow has two peaks that respectively are March and August. There is different decrease in the different months by soil and water conservation. The maximum and the minimum months of reduction occur in August and January, respectively. The continuance maximum decrease of three months is from July to September in all four catchments. The runoff reduction of the three months accounts for 51.5% to 76.5% of the annual runoff decrease. The comprehensive control of soil and water loess makes the seasonal distribution of river flow more even by reducing more in flooding season and less innon-flooding season.The paper proposed a method to analyze the impact of the integrated control of soil and water loess on flood flow in a large-scale catchment. Results showed, taking Jia-lu-he catchment as example, that the integrated control of soil and water loess could reduce the flood-peak flow and flood water amount, that the flow process has been prolonged and that the peak flow was postponed.The paper also proposed a new method to build a comprehensive precipitation-runoff model based on soil and water conservation practices. The concepts of the precipitation scale of catchment and soil and -water conservation measure's scale were introduced. The methods of calculating these scales were also proposed. With these proposed methods, comprehensive precipitation-runoff models respectively have been developed for the four catchments. These models can be used to estimate impact of control measures on river runoff, and to what degree these measures effect the runoff production in river catchments at any given year.The paper also analyzed the interaction between vegetation and soil water resources. A diagnose index to evaluate the dry soil layer presented based on vegetation productivity. According...
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