Dynamic variations in profile soil water on karst hillslopes in Southwest China

Precipitation in the karst regions of southwest China is plentiful, however, soils are shallow and highly permeable, which results in rapid transportation of surface water through a soil profile. Therefore, the presence and amount of water stress in a soil layer can be unpredictable. This study will investigate the soil water conditions on karst hillslopes in southwest China through dynamic in situ observations of profile soil moisture (θ). Three profiles at two slope positions on two shrub-grasslands (S1 and S2) were selected, respectively. At each profile, TDR 100 probes were set at 10, 20, 30, 50, and 70 cm and for 100 cm, only on downslopes. Soil samples (undisturbed and disturbed) were collected to measure basic physical and chemical properties. Precipitation and θ were monitored manually each week between July 15, 2012 and November 25, 2014. Results show that soil properties, which have an important effect on θ, varied among slope positions and between the two hisllslopes. A higher clay content, which led to greater water holding capacity, explained a higher θ in downslopes and on S2. Shallow soil layers (0-30 cm), especially in the coarse textured soil layers of S1, were susceptible to water stress due to the limited availability of soil water. Water stress was low during water supplying (December through April) and relatively stable periods (May to June). However, during water consuming periods (July to November), profile θ decreased greatly which resulted in severe water stress for the whole profile. Although precipitation was seasonally uneven, based on weekly observations, profile θ on both hillslopes was not temporally active (coefficients of variation of θ was smaller than 20%) and the variance of each depth was similar. Generally, the effects of precipitation on θ can last two (0-50 cm) to five (70-100 cm) weeks. Nevertheless, θ of downslope deep soil, which was (or near) saturated all year round, was barely affected by precipitation. These results can provide valuable information for hydrological models and rational strategies for ecological restoration designing in karst regions.