Vertical distribution and temporal stability of soil water in 21-m profiles under different land uses on the Loess Plateau in China

Deep soil-water content (SWC) plays a crucial role in water-limited terrestrial ecosystems, because plant roots can extract soil water from depths of 20m or more. The distribution of soil water and its temporal variation in deep (>5m) soil profiles are not completely understood, partly due to the time and labor needed for their determination. We examined the vertical distribution patterns and temporal stabilities of soil water in 21-m soil profiles for two years under four typical land use types in the Liudaogou watershed of the Chinese Loess Plateau (CLP). The SWCs exhibited considerable variability over both depth and time under farmland, natural grassland, planted grassland, and shrubland. The soil profile could be partitioned into an active layer (0-2m) and a relatively stable layer (2-21m) based on the amount of temporal change in SWC. The mean available soil water contents (AWCs) among the land use types in the depth-time domain differed significantly (P<0.05), and followed the order: farmland (7.1%)>natural grassland (6.5%)>planted grassland (5.7%)>shrubland (4.9%). The mean available soil water storage for each 1-m-depth (AWS_1m) in the 0-21m profile, in the time domain, ranged from 50.3 to 71.4mm among the four land use types. Within the 21-m profile, as the depths of sub-profiles increased from 3m to 21m, the most temporally stable depths (MTSDs) of AWS_1m tended to become deeper in a step-like manner, producing ranges of MTSDs of 3-18m, 2-15m, 2-9m, and 3-20m under farmland, natural grassland, planted grassland, and shrubland, respectively. The ability of the MTSDs to estimate the mean AWS_1m in a soil profile was generally acceptable for each sub-profile, as indicated by the RMSD and RBIAS values obtained from a validation dataset, which ranged from 3.6 to 7.7mm and from 0.07 to 0.13, respectively, among the four land use types. The mean AWS_1m within the 21-m profile could, in general, be accurately estimated by measuring AWC to a depth of only 18m based on the temporal stability analysis. Land use greatly affected the vertical distributions and temporal stabilities of the AWC in the deep soils. The knowledge obtained from this information is vital for the sustainable use of water resources, rational management of various land uses, and scientific determinations of soil water in deep soils on the CLP and possibly in other fragile ecosystems covered by deep soils around the world.