Spatial heterogeneity characteristics of soil-epikarst thickness in a typical karst dolomite small watershed

Objective The thickness of soilepikarst is an important index affecting the hydrological process of critical zones. The spatial heterogeneity is critical understanding the structural evolution mechanism of the key zones of the Earth and the water conservation function of the key zones of the Earth. Methods Based on the 1.4 km2 area of the Mulian catchment in Huanjiang, Guangxi, the 1 731 soil-epikarst thickness samples were obtained through electrical resistivity tomography (ERT) with 45 sampling lines.The spatial distribution pattern of soil and epikarst thickness and its influencing factors were investigated. The results showed that the average thickness of soil and epikarst was 1.15 m and 6.44 m with strong and medium variance, respectively. Results The results of geostatistical analysis indicated that the spherical model and exponential model could reflect the spatial structure characteristics of soil and epikarst thickness, respectively. Soil thickness showed moderate spatial autocorrelation, a long range and good spatial continuity. However, the epikarst presented strong spatial autocorrelation and spatial dependence with a short range. Soil thickness was affected by multiple environmental factors (topographic wetness index, vertical curvature, curvature, aspect, slope, elevation, coverage, outcrop ratio and NDVI), while the thickness of epikarst was more affected by soil thickness and vegetation type. Conclusion These results are helpful for understanding the evolution of the soil-epikarst zone and provide a scientific basis for the spatial prediction of soil-epikarst thickness in karst areas

Preferential Flow in Different Soil Architectures of a Small Karst Catchment

In karst regions, soil architecture varies along topographical locations, resulting in marked differences in infiltration rates. However, the relationship between soil architecture and preferential flow (PF) is still unclear. In this study, dye tracing was used to investigate PF and the dominant effects of five types of soil architecture in a small karst catchment. These soil architectures included deep clay soil in farmland (CSF) and deep clay soil in shrubland (CSS) in the depression; shallow clay soil with minimally weathered, slanted bedrock on downslope locations (CSWD); shallow sandy loam soil with highly weathered bedrock on midslope locations (SLSM); and sandy loam soil with rock fragments on upslope locations (SLSU). The results showed that macropores and cracks were the major channels that resulted in a high degree of PF in depressions and downslope locations. Preferential flow also continued along the rock–soil interface in the downslope locations. Finger flow mainly appeared in mid- and upslope locations, and cracks mainly appeared in the surface soil layer. However, down- and upslope locations showed lower PF. We observed that PF might not contribute to the different infiltration rates in different topographical locations. Tillage in depressions damages macropores, thereby minimizing vertical percolation. The high saturated hydraulic conductivity () of the soil matrix covered the development of PF in the mid- and upslope locations, and the decrease in weakened the formation of PF as a result of heterogeneity of soil properties. These findings demonstrate that infiltration-excess runoff may occur in depressions and downslope locations and that saturation-excess runoff may occur in mid- and upslope locations

Evaluation of remote sensing-based evapotranspiration estimates using a water transfer numerical simulation under different vegetation conditions in an arid area

Daily actual evapotranspiration (AET) and seasonal AET values are of great practical importance in the management of regional water resources and hydrological modelling. Remotely sensed AET models and Landsat satellite images have been used widely in producing AET estimates at the field scale. However, the lack of validation at a high spatial frequency under different soil water conditions and vegetation coverages limits their operational applications. To assess the accuracies of remote sensing-based AET in an oasis-desert region, a total of 59 local-scale daily AET time series, simulated using HYDRUS-1D calibrated with soil moisture profiles, were used as ground truth values. Of 59 sampling sites, 31 sites were located in the oasis subarea and 28 sites were located in the desert subarea. Additionally, the locally validated mapping evapotranspiration at high resolution with internalized calibration surface energy balance model was employed to estimate instantaneous AET values in the area containing all 59 of the sampling sites using seven Landsat subimages acquired from June 5 to August 24 in 2011. Daily AET was obtained using extrapolation and interpolation methods with the instantaneous AET maps. Compared against HYDRUS-1D, the remote sensing-based method produced reasonably similar daily AET values for the oasis sites, while no correlation was observed for daily AET estimated using these two methods for the desert sites. Nevertheless, a reasonable monthly AET could be estimated. The correlation analysis between HYDRUS-1D-simulated and remote sensing-estimated monthly AET values showed relative root-mean-square error values of 15.1%, 12.1%, and 12.3% for June, July, and August, respectively. The root mean square error of the summer AET was 10.0%. Overall, remotely sensed models can provide reasonable monthly and seasonal AET estimates based on periodic snapshots from Landsat images in this arid oasis-desert region