Simulation Of Soil Water Dynamics In Red Soil Sloping Farmlands Using The Hydrus Model

Soil hydraulic properties and water dynamics in sloping farmlands are the basis for field water management. Difficulties in the measurement cause the blindness in soil and water conservation measures and seasonal drought defense measures in the red soil sloping farmland in South Hubei. Simulations of soil water dynamics using models is a common substitute. In this paper we studied and investigated the usability of HYDRUS in red soil sloping farmland of south HuBei, discussed the temporal variability of surface soil hydraulic properties, and illustrated the characteristics of water dynamics under different soil and water conservation measures.The experiment was carried out in a red soil sloping farmland plot during the main summer maize growth seasons in2011and2012. Experiment treatments included bare land (B), crop (P), bare land+straw mulch (BS), crop+straw mulch (PS), and crop+herbage strip(PM). Meteorological data, soil water contents (in depths of5,15,30,45and60cm), hydraulic parameters, and crop information were collected. We simulated the water dynamics under different treatments using the HYDRUS model with three types. They are1) sensitivity analysis,2) simulations of the whole experiment periods and stages,3) simulations of treatments with straw mulch. The conclusions are as follow:Significant temporal variabilities occured in the surface soil hydraulic properties, after large rainfall events, surface soil hydraulic conductivity and water content under the same suction decreased, the decrease in B was greater than those in P and PM. As the temporal variability of soil hydraulic properties affected the water cycle of sloping farmland, more attention should be paid on it when soil water dynamics is simulated and predicted.For the output items of the HYDRUS model (soil moisture, landsurface runoff, rainfall infiltration, and evapotranspiration), the most sensitive parameters were soil hydrologic parameters, and then crop parameters and meteorological parameters. The last one were geography parameters and root water uptake parameters. In order to improve the efficiency and accuracy of the HYDRUS model, it is necessary to obtain accurately and optimize the following parameters:saturated soil water content (θs), saturated hydraulic conductivity (Ks), parameter n, inverse of the air-entry value (Alpha), rainfall, radiation flux and leaf area index (LAI).The HYDRUS model can well simulate the soil water dynamics in root zone of the red soil sloping farmland. It shows the availability of HYDRUS in this region. Whether under treatments of B or P, or treatments with herbage strip or straw mulch, the values of determination coefficient (R2) between the simulated and measured water contents in different soil layers were all larger than0.5; and the values of root mean square error (RMSE) were below0.03cm3/cm3; and the values of relative error (RE) were below4%. Simulation results, with relative high sensitivity, could reflect the difference between treatments. When simulating soil water dynamics in treatments with straw mulch, considering the surface straw mulch as a special layer of soil medium will be a better way to get better simulation results. HYDRUS is a good tool to simulate the water dynamics in the red soil sloping farmland.As temporal variability of soil hydraulic properties in topsoil affects the input parameters of model, soil water dynamics in the whole experiment periods and stages were simulated respectively. Simulation by stages could get better results. For example, the values of R2between the simulated and measured water contents of different soil layers under treatment of bare land were larger than0.6; the values of Nash-Sutcliffe efficiency coefficient (NSE) were larger than0.1; and the values of RE and RMSE were very low. This simulation method increased the efficiency and accuracy of the modeling in rainy periods, which could be an important reference for other similar areas.Measured and simulated results showed that the conservation measures had no significant effect on soil moisture in rainfall periods, but significant effect occurred in dry periods. Between the treatments, smaller differences of soil moisture occurred in deep layers, while largeer differences happened in topsoil. For example, in B, P and PM treatment, the mean soil moistures in the depth of5cm increased were0.1840cm3/cm3,0.2192cm3/cm3,0.2222cm3/cm3respectively, and the fluctuation of moisture by weather reduced in turn; the variation coefficients (CV) of soil moisture were33.19%,31.84%,30.40%, respectively. Results provide an important basis for the soil and water conservation and seasonal drought defense in the red soil sloping farmland.