Optimization Of Terrace Structure Based On Control Of Rainfall Runoff

Rainfall runoff controlling was an important way to solve the contradictory between drought and water shortage as well as soil erosion on Loess Plateau. In order to solve the problems of drought and water shortage and soil erosion for slope crops, while at the same time overcome the shortcomings of water conservation engineering optimization research on sloping field, this paper, based on research about response of runoff and sediment on typical slope and the concept of controlling rainfall runoff, discussed the optimization method of slope-separated terrace structure from the standpoint of total crop water requirement during the whole growth period and crop water demand in every growth period respectively, after the parameter selection and validation of WEPP model. The results showed as the following:1. Selection and analysis of model parameters. The sensitivity analysis for main parameters of soil document of WEPP model showed that, runoff that was not sensitive to changes of soil albedo rate, the initial saturated hydraulic conductivity, soil critical shear force, rill soil erodibility and inter-rill soil erodibility was sensitive to changes of effective hydraulic conductivity, which appeared to have inverse relationship between them; the amount of erosion that was not sensitive to changes of soil albedo rate, the initial saturated hydraulic conductivity and inter-rill soil erodibility was sensitive to changes of soil critical shear force, rill soil erodibility and effective hydraulic conductivity, which appeared to be positive relationship with rill soil erodibility and negative relationship with critical shear force and effective hydraulic conductivity. Using observational data of Ansai experimental station of soil and water conservation, the effective hydraulic conductivity, soil critical shear force and rill soil erodibility of fallow field and alfalfa field were confirmed to be 19.7mm/h, 3.5Pa, 0.026s/m and 22.7mm/h, 5.1Pa, 0.019s/m respectively.The research also showed: the above results were similar to the conclusions that have been published for the same place and slightly different from that in the south of Loess Plateau; the verification results of effective hydraulic conductivity and soil critical shear force were slightly smaller and rill soil erodibility was similar on hilly region comparing to the results in the south of Loess Plateau, which showed the hydraulic erosion more easily occurred on hilly region than in the south of Loess Plateau when the other conditions were same.2. Model validation. The test results for runoff and sediment on the fallow and alfalfa field using the selected soil parameters showed: WEPP model could basically reflect runoff and sediment production processes in the study area. The model prediction was better for fallow field than alfalfa field, for runoff than sediment and for small slope than steep slope. The WEPP model under parameter modification could be used as a tool for the research of different hydraulic erosions and transport processes on the study slope.3. Simulation about response of runoff and sediment on typical slope based on WEPP model. Under the same rate of change in rainfall conditions, the rate of runoff change was different from that of erosion. The rate of runoff chang on fallow field, alfalfa field and corn field were averagely larger 1.1%, 6.3% and 4.9% than that of erosion respectively, and as a result the effect of rainfall factor on runoff was slightly larger than erosion; alfalfa was a rainfall runoff controlling measure worthy promotion, which played a strong role on controlling sediment; the given runoff- erosion diagram under typical rainfall condition in different hydrological years could provide a reference for the design of controlling measures of rainfall runoff on hilly region of Loess Plateau.4. Optimization of slope-separated terrace based on the total crop requirement during the whole growth period. Because of the scarcity of water and the dislocation problem in time between the corn water requirement and natural runoff, slope-separated terrace was a better way to meet the off-site use for rainfall runoff on hilly region of Loess Plateau. Slope-separated terrace using natural interval slope directly as catchment area was more appropriate for the slope steeper than 10°and for terrace planting patterns of dates, maize and potato. When using the way of slope-separated terrace, the required length of interval slope was much larger for high quality and high-yield apple and millet, due to their higher water demand, so the model of slope-separated terrace combined with artificial catchment area was advised. This method was simple, but the relationship between crop water requirement in every growth period and timely water supply could not be accurately predicted.5. Optimization of slope-separated terrace based on crop requirement in every growth period. The individual slope-separated terrace which could not achieve the function of timely supply for crop water deficit was not a way to fully solve the dislocation problem in time between the corn water requirement and natural runoff, and in most cases the ratio of terrace width to interval slope length should be larger to collect the rainfall runoff, which seriously wasted land resources; However, the model of"water cellar + slope-separated terrace"could not only reasonably redistribute the rainfall runoff in space and time, but also well solve the dislocation problem in time between the corn water requirement and natural runoff. This model which achieved the function of timely supply for crop water deficit had the advantage of improving soil and water resources efficiency, increasing crop yields as well as preventing soil erosion, as a result it was a pattern of rainfall runoff controlling on slope suitable for promotion. The given table of the ratio of terrace width to interval slope length and the corresponding cellar volume could provide a reference for the design and planning of the slope-separated terrace on hilly area of Loess Plateau.The above results also showed that, using WEPP model for the calculation of precipitation, runoff, evaporation, soil water and erosion process, basing on crop water demand and concept of rainfall runoff controlling, was a rational method for structural optimization of slope-separated terrace.