Soil water replenishment by rainfall: a case study of headwater forests in terraced agroforestry systems

Headwater forests function as key hydrological regulators, enhancing soil water conservation and mitigating erosion in hilly agroforestry systems. Although the role of multi-layered forest structures in regulating the hydrological cycle is widely recognized, the specific mechanisms by which headwater forests govern soil water infiltration and replenishment in terraced landscapes remain unclear. Based on in-situ hydro-meteorological data from 2020–2021 collected in fir, bamboo, and mixed forests in the Longji Terraces of southern China, the replenishment of soil water and its seasonal dynamics under different rainfall intensities were examined. The results showed that: (i) The mixed forest maintained significantly higher soil water content (SWC) and exhibited lower annual variability compared to the bamboo and fir forests. (ii) The replenishment of soil water decreased as the antecedent SWC increased across all soil layers in the bamboo and fir forests, and the mixed forest displayed minimal interlayer differences in replenishment and the weakest sensitivity to rainfall intensity. (iii) The replenishment efficiency of soil water decreased with rainfall intensity in the fir and mixed forests, while the bamboo forest demonstrated deep activation potential during torrential rain events. These findings can help to better understand the dynamic response of soil water to rainfall in complex agroforestry ecosystems, and provide a reference for the creation of regional ecological landscapes.