Modified Numerical Approach to Estimate Field Capacity

Field capacity is one of the most commonly used soil parameters in irrigation systems and agriculture. Methods for determining field capacity are either time, pressure, or model based. The numerical approach to estimate field capacity using a model-based definition is found to be effective in its ability to predict field capacities; however, this modeling-based approach is difficult to run for some soil as the numerical model requires an extremely small time increment close to zero and some oscillations are produced while the top part of the soil column approaches near saturation. In this study, an approach using the Richards' equation in conjunction with the modified van Genuchten-Mualem (MMVG) model to simulate variably saturated flow in estimating field capacity is developed. The MMVG model is found to be useful to avoid the numerical oscillations, and there is less effect from the negligible flux associated with the estimate of field capacity. For estimating the real-world soils' field capacity, a combined soil data set was developed from two wide-ranging databases. A new neural-network pedotransfer function (PTF) was developed to match data set values and objected values calculated in the MMVG model. Using this numerical approach together with the new PTF also showed better agreement with the filed measurements than benchmarked pressure heads and the model-based method with the original MVG models. The dependency of field capacity on soil texture was shown by the contour diagrams. The potential application of this approach was shown in the distribution of field capacity in China. DOI:10.1061/(ASCE)HE.1943-5584.0000662. (C) 2013 American Society of Civil Engineers.