Temporal instability of soil moisture at a hillslope scale under subtropical hydroclimatic conditions

Abstract

Temporal instability, a condition of soil moisture with unstable spatiotemporal patterns, may occur when the relative roles of the controlling factors change. It impacts the applicability of the temporal stability in the prediction of soil moisture. However, the occurring frequency, degree and underlying hydrological conditions of the temporal instability are far from clear. This study aimed to examine the presence of temporal instability at the hillslope scale and to characterize and quantify the frequency, degree and underlying hydrological conditions contributing to the phenomenon. Sloping crop lands under subtropical hydrological conditions are known for their frequent transition between dry and wet periods and were chosen for this study. Soil moisture was monitored at 5 locations and at seven soil depths down to 2.2 m along a 160 m hillslope over a period of a year. Relative difference analysis was employed to evaluate the temporal stability of soil moisture. Representative locations (RLs) mainly appeared at locations with a mild slope and relatively high sand content. Daily RLs were not consistent among the study period, which has the highest proportion to be the same to those full dataset based. These proportions were depth dependent, the lowest (40%) at moderate soil layers (i.e. 50 and 80 cm) and the highest (95%) at deep soil layers (i.e. 180 and 220 cm). An obvious temporal instability was observed by the extensive ranges in the daily mean relative difference (MRD) values. If the whole dataset was used to calculate the mean soil moisture of the hillslope, it would be overestimated by 16.1% or underestimated by 18.2%. The frequency of temporal instability was low and varied over time and soil depths. For example, 17% of the time for this period was temporally unstable at 20 cm and 2% time of the study period was unstable at 220 cm. These low proportions confirm that the temporal stability remains a valid and robust tool in mean soil moisture prediction. The rainfall after a long drought period or a storm after successive rainfall probably caused the temporal instability. Therefore, caution should be exercised during the strong dry-wet transition period.