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Soil moisture is connected to atmospheric moisture through direct evaporation and vegetation transpiration. This is a complex process that is in focus of numerous hydrological studies. We used the long-term measurements from a typical farming-pastoral ecotone in northern Hebei Province of China to study the relationship between the daytime and nighttime fluctuations of grassland surface soil moisture and evapotranspiration. The results show that the surface soil of grassland above 60 cm is the main water consumption area, while below 60 cm is the main water storage area. The 80–100 cm layer responds to precipitation 263 h later than that of the 0–20 cm layer. In the 0–20 cm and 20–40 cm layers where the moisture fluctuation is more obvious, the moisture movement in the lower layer occurs on average 7.85 h after the upper layer. The fluctuations were digitalized, and they were significantly correlated with potential evapotranspiration and actual evapotranspiration. This research suggests that soil moisture is affected by evapotranspiration and thus it always moves upward in the surface soil. When the evapotranspiration is large, the amount of water transport increases and it suggests that the use soil moisture fluctuation signals can be used to reflect evapotranspiration. This research hopes to provide a reference for a precise analysis of soil and atmospheric water resources and their interactions.
Wang et al. (Mon,) studied this question.