Effects of Soil Hydraulic Properties on Soil Moisture Estimation

Accurate quantification of soil moisture is essential to understand the land surface processes. Soil hydraulic properties influence water transport in soil and thus affect the estimation of soil moisture. However, some soil hydraulic properties are only observable at a few field sites. In this study, the effects of soil hydraulic properties on soil moisture estimation are investigated by using the one-dimensional (1-D) Richards equation at ELBARA, which is part of the Maqu monitoring network over the Tibetan Plateau (TP), China. Soil moisture assimilation experiments are then conducted with the unscented weighted ensemble Kalman filter (UWEnKF). The results show that the soil hydraulic properties significantly affect soil moisture simulation. Saturated soil hydraulic conductivity (K-sat) is optimized based on its observations in each soil layer with a genetic algorithm (GA, a widely used optimization method in hydrology), and the 1-D Richards equation performs well using the optimized values. If the range of K-sat for a complete soil profile is known for a particular soil texture (rather than for arbitrary layers within the horizon), optimized K-sat for each soil layer can be obtained by increasing the number of generations in GA, although this increases the computational cost of optimization. UWEnKF performs well with optimized K-sat, and improves the accuracy of soil moisture simulation more than that with calculated K-sat. Sometimes, better soil moisture estimation can be obtained by using optimized saturated volumetric soil moisture content K-sat. In summary, an accurate soil profile can be obtained by using soil moisture assimilation with optimized soil hydraulic properties.