Soil Water Dynamics Characteristics And Nutrient Loss In The Degraded Grassland From A Slope In The Water-wind Erosion Region

In order to understand the soil water depletion and replenishment after returning farmland to grassland on the sloping, and the influence of antecedent soil water contents on soil erosion and nutrient loss in the water-wind erosion region of the Loess Plateau. By using the runoff cell location field observations of the soil water contents of soil profiles and soil erosion and nutrient loss began in 2003, analyzed the impact of dynamic changes in soil moisture sloping cropland to grassland conversion years later and wet years to restore the pre-conditions and the influence of antecedent soil water contents on soil erosion and nutrient loss. The main conclusions were as follows:(1) The soil water content of the grassland(alfalfa) decreased distinctly and water content was close to the wilting point after alfalfa was planted for many years. Despite of having more runoff, the soil water content was in a higher level in the farmland. The average water content of grassland, abandoned land and farmland were 7.4%, 12.6%, 12.5% respectively at the beginning of the growing season in 2010. Soil water was recharged to different level under different land use types in the wetter year, which rain was 669.4 mm, and the recharging depth in the wetter year was deeper than normal year. The soil water content of alfalfa land, abandoned land and farm land were restored to 10.0%, 14.5%, 15.5%, and the soil moisture increased by 35.1%, 15.1%, 24.0% respectively. As a result, the soil water content decreased significantly by returning farmland to grassland, and the distinct soil water content recovery was observed only in the wetter year. But the alfalfa and abandoned grass can effectively reduce the runoff and sediment yield in the slope, in the five years of study, only three records of the runoff and sediment yield, and a total of 23 times of the runoff and sediment yield in the hilly land.(2) Soil water content was one of many factors controlling runoff on the Loess Plateau. The antecedent soil water contents could be an important factor in determining runoff amount and sediment yields. This study was carried out to determine if the Hydrus-1D model could accurately predict the soil water content and reasonable values of antecedent soil water contents. Based on measured data collected during a four-year period, the Hydrus-1D model was found to accurately simulate soil water content in the upper soil layer(0 ~ 6 cm) of farmland on a watershed slope with an average EF value, 0.64. The model provided continuous parameter values, including antecedent soil water contents, which could be used to accurately predict runoff and sediment yield from hill slopes. Runoff and sediment yield increased with the increase of rainfall amount and with antecedent soil water contents. When the antecedent soil water contents was greater than 0.12 cm3 cm-3, the probability of generating runoff could be as great as 85% under the rainfall conditions of the study area and period. Therefore, the Hydrus-1D model was considered to be a viable means by which to determine antecedent soil water contents and other parameters that could then be used in models to predict erosion and runoff on the Loess Plateau.(3) Rainfall characteristics become the determinants of soil erosion and nutrient loss in the water-wind erosion region of the Loess Plateau. Runoff and sediment increased with increasing rainfall. With the increase of the amount of sediment runoff increased, they were a very significant linear relationship. In general, large runoff with the same rate of runoff would carry more sediment. Compared with the experimental plot soil particles, silt clay and silt content increased slightly.(4) Effective phosphorus content in runoff changes over time was stable, the difference was not big. Nitrate nitrogen and ammonium nitrogen content fluctuation changed over time. Total nitrogen and total phosphorus content in the sediment with time difference was not big. Effective phosphorus also changed over time, ammonium nitrogen and nitrate nitrogen content changed trend over time. On the other hand, organic matter content over time. Nutrient content in runoff with increasing antecedent soil water contents increased, but this relationship was no good linear relationship between them. Nitrate content and available phosphorus content increased with antecedent soil water contents increases. When the antecedent soil water contents increased, ammonium nitrogen content did not increase. Nutrient content in the sediment would not be affected by the antecedent soil water contents, and with the increase of rainfall and rainfall intensity.