Comparison of three models fitting the soil water retention curves in a degraded alpine meadow region.

Soil water retention curve (SWRC) plays an important role in simulating soil water movement and assessing soil water holding capacity and availability. Comparison of fitness between different models to determine the best SWRC model of specific regions is required. In this study, three popular models, van Genuchten, Brooks Corey and Gardner model, were selected for comparing in a degraded alpine meadow region on the eastern Tibetan Plateau. Fitness, error distribution along with key parameters were compared. For each soil horizon, the soil moisture content at all soil water potentials decreased consistently with degradation, thereby integrally moving the SWRCs of all soil depths downward with degradation. The differences in SWRCs across various degradation degrees diminished along with soil depth and soil water potential. The Adj.r values of van Genuchten, Brooks Corey and Gardner models ranged in 0.971-0.995, 0.958-0.997, and 0.688-0.909, respectively. The van Genuchten and Brooks Corey models significantly (p < 0.05) outperformed the Gardner model, and have no significant differences in fitness. The fitness of all three models showed no significant changes with degradation. Regardless of degradation degree and soil depth, the fitting error of van Genuchten and Brooks Corey models was mainly distributed in the higher (from -100 hPa to -500 hPa) and lower (below -10000 hPa) potential sections. With regard to the parameters of van Genuchten and Brooks Corey models, the field capacity (θs), and permanent wilting moisture were highly coherent with Adj.r values of higher than 0.98, while the curve shape parameter (θr), and air entry pressure of the Brooks Corey model were much lower than those of the van Genuchten model with Adj.r values of lower than 0.91. The SWRCs with varying degrees of degradation are best fitted by both van Genuchten and Brooks Corey models but cannot be fitted by Gardner model. Soil water holding capacity decreased with degradation especially in the top soil (0 cm to 30 cm), but the curve shape of all SWRCs did not change significantly with degradation.