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

- The temporal stability of AWS1m in 21-m profiles under four land uses was examined.
- Total storage of AWS in 21-m profiles differed among four land uses (P < 0.001).
- Land use greatly affected soil water vertical distributions and temporal stability.
- AWS1m MTSDs in sub-profiles from 0-3 m to 0-21 m increased in a step-like manner.
- Mean AWS1m estimation accuracy using the MTSD was acceptable for all sub-profiles.

SummaryDeep soil-water content (SWC) plays a crucial role in water-limited terrestrial ecosystems, because plant roots can extract soil water from depths of 20 m or more. The distribution of soil water and its temporal variation in deep (>5 m) 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 (0-2 m) and a relatively stable layer (2-21 m) 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 (AWS1m) in the 0-21 m profile, in the time domain, ranged from 50.3 to 71.4 mm among the four land use types. Within the 21-m profile, as the depths of sub-profiles increased from 3 m to 21 m, the most temporally stable depths (MTSDs) of AWS1m tended to become deeper in a step-like manner, producing ranges of MTSDs of 3-18 m, 2-15 m, 2-9 m, and 3-20 m under farmland, natural grassland, planted grassland, and shrubland, respectively. The ability of the MTSDs to estimate the mean AWS1m 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 7.7 mm and from 0.07 to 0.13, respectively, among the four land use types. The mean AWS1m within the 21-m profile could, in general, be accurately estimated by measuring AWC to a depth of only 18 m 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.