Modeling The Effects Of Soil And Climate On Soil Moisture Spatial Variability

Soil moisture is an important state variable in terrestrial ecosystems.Due to the complex interaction with surrounding environment,soil moisture tends to display significant spatial variability.Exploring the spatial variability of soil moisture is of great importance for understanding the ecohydrological process.With regard to the study of soil moisture spatial variability(SMSV),especially at the regional scale,there is a great inconsistency between the research results,so further research is needed.In this study,a one-dimensional process based vadose zone model with generated soil hydraulic parameters was utilized to analyze the impact of variability in precipitation and soil hydraulic properties on main controlling factors of SMSV and then the interplays between soil moisture and actual evapotranspiration(ET_a)and groundwater recharge(GR)was investigated.The simulation results revealed that(1)The impact of precipitation on mean relative difference(MRD)of soil moisture increased with the increase of variability in precipitation and the decrease of variability in soil hydraulic parameters,especially under wet climatic conditions.(2)For clay loam and silt loam,the soil hydraulic parameters were the main controlling factors of SMSV when the soil hydraulic parameters varied in a larger range;however,when the variability in soil hydraulic parameters decreased,under wet climatic conditions,the impact of soil hydraulic parameters(mainly residual soil moisture contentθ_r and saturated soil moisture contentθ_s)on MRD weakened,which lead to a significant increase of the impact of precipitation on MRD.The precipitation exceeded the soil hydraulic parameters and became the main controlling factor when the variability in precipitation increased to a certain extent.In contrast,the soil hydraulic parameters were always the main controlling factors of SMSV under the arid climatic conditions.(3)For the sand,since the soil moisture was mainly strongly influenced byθ_r,the soil hydraulic parameters(especiallyθ_r)were the main controlling factors of SMSV under any climatic conditions.(4)The controls of soil hydraulic properties on ET_a and GR greatly weakened under arid climatic conditions due to limited water supplies;whereas,the distribution of MRD still largely depended on soil hydraulic properties under the same climatic conditions(especiallyθ_r).As such,the correlations of MRD with ET_a and GR also varied significantly with climatic conditions.(5)Due to the complex interactions of soil moisture,ET_a,and GR with soil hydraulic properties,soil samples with average moisture conditions did not necessarily produce average values of ET_a and GR.The results of this study adequately indicated that the impact of precipitation and soil hydraulic properties on soil moisture spatial variability is closely related to the variable variability.Moreover,the results of this study underscore the importance of using soil moisture information from multiple sites to infer areal average values of ET_a and GR,even with the knowledge of representative soil moisture monitoring sites.