Studies on Infiltration at Various Tropical Soil Textures

Abstract

Understanding how water infiltrates and how fast water moves through soil play important role for agricultural planning such as development of plant irrigation, fertilizer and soil nutrition movement, surface and subsurface water pollution, and groundwater recharge. Because of the available data that obtained from direct measurement are very limited, therefore it is necessary to apply indirect method for simulating infiltration process in soil by taking into account soil physical and hydraulic properties which is collected during soil sampling as the predicting variables. The model used in simulation must be able to describe distribution of pressure head and soil water content along with the infiltration process. Objectives of the research are to optimize soil water retention function parameter of various tropical soil textures; to assess numerical simulation for solving the problem of one dimensional infiltration into a deep and homogeneous soil; and to simulate cumulative infiltration of various soil textures. Soil texture was classified based on International Society of Soil Science (ISSS) classification using distribution of sand, silt, and clay fraction. Soil water retention function of Van Genuchten was optimized using 165 soil sample data collected from Flores, Kotawaringin Barat, Samarinda, Kutai, dan Gorontalo. Combining error terms by a root-mean-square and changing to the finite difference form of the equation variables was applied to analyze error of water retention and hydraulic conductivity function. Numerical procedures of Crank-Nicolson semiimplicit discretization combined with Newton method and elimination technique of Thomas algorithm was used to solve one-dimensional Richards-Darcy’s equation. The Green and Ampt, and Philip infiltration model was used to find the length of wetted zone and sorptivity at each soil textures. Source code of programming was written in BASIC using Visual Basic for Application in Microsoft Excel. Soil water retention function parameter was optimized for 10 ISSS soil textures that is heavy clay, sandy clay, sandy clay loam, sandy loam, sand, light clay, clay loam, loam, silty clay, and silty clay loam. Result of simulation showed that Richards-Darcy’s equation was able to simulate infiltration process in soil by taking into account soil physical and hydraulic properties as the predicting variables. The output of simulation includes tabulated values and graphics representing the cumulative infiltration and the pressure head, water content, and unsaturated hydraulic conductivity profiles at different times. The results showed that sand, loam, sandy loam, heavy clay, and light clay soil textures have the largest to smallest cumulative infiltration and depth of wetted zone with time elapsed respectively.