Research on soil water dynamics of farmland in Mu Us Sand Land

Soil water is a key parameter in the interactions among the earth, atmosphere and hydrological cycle. Understanding soil water dynamics is critical in studying biophysical processes of the land surface. The Mu Us Sandy Land (MUSL) in northwest China is a dominant semi-arid climate region with scarce precipitation and surface water resources and groundwater as the main sources of water. Thus as water is the main factor restricting the development of the agriculture in MUSL region, it is vital to study soil water dynamics in farmlands to guide local agricultural production. Previous studies on soil water dynamics in MUSL and shrub areas of the region failed to take into account soil water dynamics during spring maize growing season, which is the main cultivation period in MUSL region. Therefore in this study, mainly based on field experiments, we examined soil water dynamics during spring maize season in MUSL by analyzing observed data for groundwater, soil water content, soil water potential, irrigation, rainfall, evaporation and maize transpiration. The results showed that groundwater clearly hydraulically correlated with soil water. Also significant correlations were observed between groundwater and soil water content at different depths, especially at 40-60 cm depth with a correlation coefficient >0.8. Besides, groundwater drawdown lowered soil water stable layer position and weakened hydraulic connections among layers, which limited soil water preservation. Meanwhile increasing maize water demand and declining groundwater table increased vertical variation in soil water content. Based on the field data for soil water content and soil water potential, we noted that the soil water dynamics tracked a curve of weak, strong and weak again during the period from seedling stage to dough stage. Also while about 20 cm soil depth was the main maize water absorption layer, the soil layer of 30-40 cm depth was relatively dry. Irrigation was applied only under water stress. Our study showed that it was possible to determine whether irrigation was needed or not during maize growth by analyzing soil water contents at the 30 cm and 40 cm soil depths. When soil water potential at the 10 cm depth dropped to ?0.18 bar, crop physiological processes induced hydraulic lift by maize root. By characterizing soil water dynamics during spring maize season in MUSL farmlands, our study provided essential reference database for agricultural water use along needed theoretical basis for water management in MUSL region.