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.

土壤水分通过直接蒸发和植被蒸腾作用与大气水分相连。这是一个复杂的过程，是众多水文研究的重点。我们利用河北省北部典型农牧交错带的长期测量数据，研究了草地表层土壤水分昼夜波动与蒸散量之间的关系。结果表明，60 cm以上草地表土为主要耗水区，60 cm以下为主要蓄水区。80-100 cm 层对降水的响应比 0-20 cm 层晚 263 h。在水分波动较为明显的0～20 cm和20～40 cm层，下层水分运动平均发生在上层之后7.85 h。波动被数字化，它们与潜在蒸散量和实际蒸散量显着相关。这项研究表明土壤水分受蒸散作用的影响，因此它总是在表层土壤中向上移动。当蒸散量较大时，水分输送量增加，表明利用土壤水分波动信号可以反映蒸散量。本研究希望为土壤和大气水资源及其相互作用的精确分析提供参考。当蒸散量较大时，水分输送量增加，表明利用土壤水分波动信号可以反映蒸散量。本研究希望为土壤和大气水资源及其相互作用的精确分析提供参考。当蒸散量较大时，水分输送量增加，表明利用土壤水分波动信号可以反映蒸散量。本研究希望为土壤和大气水资源及其相互作用的精确分析提供参考。