Spatial Variability Of Characteristic Function Of Soil Water Movement On A Forested Hill-slope

It is a critical step to study spatial variability of soil water retention curve for understanding runoff generation mechanism and developing physically-based, distributed watershed hydrological models. The objectives of the study are to quantify the spatial variability of characteristic function of soil water movement, and explore its relations with soil physical properties over a forested plantation hill-slope.Twelve evenly spaced soil profiles(10mx20m) were selected over a 32degree Chinese PinefPinus tabulaeformis) plantation forested hill-slope in Miyun Reservoir catchment for the study. Seventy-two undisturbed cores for soil water retention measurement and samples for root content and stoniness analysis were collected from those profiles at depths, with two replications each, 0-20cm, 20-40cm, and 40-60cm, respectively. Additional 8 undisturbed cores and samples were collected at two depths of 60-80cm and 80-100cm from two profiles with 2 replications each in order to explore vertical variability of soil water movement characteristics. Soil water retention curves were measured in the laboratory by using pressure chamber. Van Genuchten model fits slightly better the observed water content and water potential data than Brook-Corey model by using RETC6.0 package. Therefore, traditional statistics and geo-statistics were used for spatial variability analysis of soil water movement function parameters of fitted Van Genuchten models and its relations with soil physical parameters such as root content, porosity, stoniness, and bulk density.Moderate spatial variability was observed for saturated water content 9 s with Cv of 13.82%, 12.57%, and 19.62% for 0-20cm, 20-40cm, and 40-60cm, respectively. When fitting semivariogram with exponential model, integral scale of 9 s was 17m for the depth of 20-40cm. However, there were no spatial structures observed for 9 s at other soil depths. Log-normal distribution and strong spatial variability parameter awas observed. Cv of a are 117%, 70.38%, and 135.33% for soil layers of 0-20cm, 20-40cm, and 40-60cm, respectively. Unlike 9 s, there were no spatial structure for the soil layer of 40-60cm while the integral scales of exponential semivariogram of 0-20cm and 20-40cm were 19m and13m, respectively. The parameter of n was found normally distributed and slightly spatial variability existed with Cv of 5.33%, 6.06%, and 3.78% for 0-20cm, 20-40cm, 40-60cm, respectively. When fitting the semivariogram with spherical model, the integral scale of n was 21m, 21m, 16m for soil layers of 0-20cm, 20-40cm, 40-60cm, respectively. It is found that s is positively correlated with In a for all soil layers and negative correlation between In a and n as well as between 9 s and n for 0-20cm,20-40cm.Correlation analysis between retention curve parameters and other soil physical parameters revealed that n and In a are negatively related with bulk density and stoniness, respectively, while 9 s is positively correlated with soil porosity.