Study On The Laws Of Soil And Water Loss In Small Watershed In Gulled Area Of Loess Plateau

Based on the observational long series data of precipitation, runoff, sediment in Nanxiaohe gully and Yanwachuan watershed, the characteristics of water and sediment, the laws of soil and water loss were studied in the gully area of loess plateau in this thesis. The main findings are as fallows:(1)In the gullied area of loess plateau, the rainfall which could produce runoff usually occurs from April to October, the erosive rainfall usually occurs from May to September. While the July to September's runoff took 70% of the annual runoff, the July to September's erosion amount account for 91% of the annual erosion amount. The rainfall-runoff happens six to seven times in a year, the average precipitation about 200mm, which takes a percentage of 48.8% to 54.1% of the rainfall happened in the flood season. The erosive rainfall happens two or three times in a year, the average precipitation about 100mm, which takes a percentage of 32.6% to 58.7% of the rainfall -runoff. The rainfall-runoff is most common below 40mm, taking 70.6%of the total amount of rainfall, especially concentrated from 10mm to 20mm, amount for 25% of the total rainfall-runoff.(2)Slop length and slop degree, which are the main terrain factor influence soil and water erosion, take the important place to cause rill erosion when other conditions are defined. Under the comprehensive effects of precipitation, slop length and slop degree, water and soil loss is positive correlated to these three factors. So shorting and decreasing the length and the degree of the slop are the main measures to prevent rill erosion and ephemeral gully erosion.(3) runoff-Rainfall easily happened on the slope of wasteland, the order was followed as: sloping wasteland>sloping farming land>woodland>grass; the runoff is easily happened on the sloping farming land, the order was followed as: sloping farming land> grass> slope wasteland> woodland; The sediment was easily occurred on sloping farming land, the order was followed as: sloping farmingland> slope wasteland>grass>woodland.(4)The serious soil and water loss was usually occurred by several storms in a year, for example, the strongest rainstorm happened in Dongzhuang gully watershed takes about 40% of the total annual runoff, the sand transportation account for 55% of the annual amount in this individual rainfall. The same condition also happened in Yanwachuan watershed, about 43% runoff of the annual runoff and 55% sand transportation of the total amount of this year coursed by a single rainfall.(5)It is found that there is an remarkable correlation between the soil erosion single rainfall, average rainfall intensity and the biggest rainfall intensity (I₃₀) by using multifactor analysis method. The annul erosion has better correlation with the biggest rainfall intensity (I₃₀ ), flood season rainfall, maximum 10 daily rainfall and maximum 30 daily rainfall. In a small watershed, the correlation relationship between rainfall and annual erosion in a non-managed basin was better than that in a managed basin. There is a remarkable relationship between individual rainfall and single-soil erosion, annual rainfall, annual soil and water erosion. The linear relationship in those factors is more conspicuity than that in power function, it is far better than rainfall sediment relationship.