The spatial variability of soil water storage and its controlling factors during dry and wet periods on loess hillslopes

Soil water storage (SWS), a critical parameter in hydrological processes, is an effective water source for vegetation growth in the semi-arid Loess Plateau of China. Its spatial pattern at various soil depths along transects and temporal changes in the dominant environmental factors that affect SWS are essential to ensure the sustainability of vegetation restoration efforts and achieve an accurate understanding of hydrological processes on the Loess hillslope. In this study, we investigated SWS at depths of 0-4 m at a total of 54 points on three hillslopes covered with artificial forest, natural forest and natural grass during four observation periods. The results reflected clear seasonal trends in SWS. A substantial water deficit occurred during the severe drought year of 2015. SWS at depths of 0-1 m increased and SWS at depths of 1-4 m decreased from after the rainy season of 2015 to before the rainy season of 2016 (a near-normal drought year), and SWS at depths of 0-4 m maintained its resemblance to conditions that occurred during the rainy season of 2016. These results may indicate that drought conditions affect variations in SWS. In addition, topography and vegetation type were the dominant factors controlling SWS in the different soil layers. SWS at shallow soil depths was mainly affected by topography, while SWS at deep soil depths was mainly controlled by vegetation type. During the dry season, slope aspect was the most important factor controlling SWS at shallow soil depths due to the effects of slope aspect on snowmelt and wind evaporation. On the other hand, during the wet season, the slope gradient was more important in terms of its effect on SWS than slope aspect at shallow soil depths due to the effects of slope gradient on infiltration and runoff.