Improving the Van Bavel-Hillel model for soil evaporation in grasslands by introducing vegetation coverage resistance

Modeling the soil evaporation under vegetation conditions is of theoretical and practical significance for water resources management in the Loess Plateau. In this study, a three-year field experiment was conducted in a bare land and three grasslands to measure soil evaporation using micro-lysimeters. The Van Bavel-Hillel model was then validated in the bare land. Based on this, the vegetation coverage resistance was proposed to reflect the comprehensive effects of vegetation, and it was applied into the Van Bavel-Hillel model to improve the model’s applicability under vegetation conditions. The results showed that the Van Bavel-Hillel model was effective in simulating evaporation from bare land, and the application of validated soil surface resistance and vegetation coverage resistance can make it perform well in the evaporation simulation in all studied grasslands. The obtained vegetation coverage resistances decreased linearly with the increase of soil moisture contents in all three grasslands, and the decreasing rates were similar in the M. sativa and P. giganteum plots, which were higher than that in the I. cylindrica plot. Soil surface resistances ranged between 533.4-746.5, 767.4-1154.7, and 133.4-1334.5 s/m in the I. cylindrica, M. sativa, and P. giganteum plots, respectively, and all showed the characteristics of first increasing and then decreasing during the growing season. When compared with natural grassland, M. sativa increased the soil surface resistance in all months, while P. giganteum reduced it in the early growing season, but increased it in the middle and late growing season. This research proposes a new idea for the simulation of soil evaporation under vegetated conditions, and provides a basic reference for water resources management in the vegetation restoration of the Loess Plateau.