Vertical distribution and influencing factors of soil water content within 21-m profile on the Chinese Loess Plateau

In arid and semiarid regions that have deep soils, plant root systems can extract soil water to a depth of 20m or more. An accurate evaluation of soil water conditions in such regions is essential in order to improve the understanding of the role of soils as a water pool and to design scientifically based water management strategies. However, the vertical distribution of soil water and its storage in deeper layers are unclear due to the shortage of field data. In this study, soil water content (SWC) and related soil (i.e., soil organic carbon, soil particle composition) and plant (i.e., root depth, root mass) properties were determined to a depth of 21m at 11 sites across the Loess Plateau of China. Soil water storage (SWS) and available soil water storage (AWS) were calculated for each 1-m thick soil layer in the profile as well as for the whole profile. Mean values for SWC, SWS, and AWS in the soil layers had a similar distribution pattern down the profile, which could be divided into three statistically significant different sub-layers where the parameter decreased (0–2.5m), then increased (2.5–5m), and increased with fluctuations (5–21m). The amount of SWS and AWS in the 21-m profiles varied from 1639mm to 5669mm and from 831mm to 2953mm, respectively. In the root zone, the vertical distribution and quantity of soil water were significantly influenced by land use and plant characteristics (i.e., root mass); while below the root zone, soil texture became a more important factor. Compared with the upper soil layers, the deeper soil layers provided a larger water pool and thus potentially had a stronger buffering capacity, which could mitigate the effects of seasonal or inter-annual drought on plants. Understanding this information is important to regional water budgets, sustainable land management and the restoration of the ecological environment on the Loess Plateau and possibly in other water-limited ecosystems around the world.