Physical structure and rainfall controls on subsurface hydrological connectivity in hillslope-riparian-stream continuums

The hydrological connectivity of hillslope-riparian-stream (HRS) continuums is crucial for runoff generation and solute transport. The achievement of water resource protection and water quality improvement requires a systematic understanding of the structure and rainfall controls on HRS connectivity. Herein, two HRS continuums with different soil depths and slopes (HRS-1: thin soil depth and steep slope; HRS-2: thick soil depth and gentle slope) were established. We monitored the soil moisture from the surface to the soil-bedrock interface at 15 min intervals from March to June 2021. The HRS connectivity was analyzed based on soil saturation conditions, and partial least squares regression (PLSR) was used to reveal the relationships between rainfall and HRS connectivity. The results showed that the time required to establish hydrological connectivity in HRS-1 was shorter than that in HRS-2, which indicated that the contribution to runoff of the HRS continuum with a thin soil depth and steep slope was dominant during the early stage of rainstorm. As rainfall intensity increased, the required time was shortened exponentially due to the changes in hydrological connectivity patterns. In addition, the higher connectivity strength (i.e., the magnitude of HRS connectivity) was observed in the HRS-2 than that in the HRS-1 during heavy rainfall events. The PLSR analysis showed that rainfall amount, 30 min maximum rainfall intensity, 15 min maximum rainfall intensity, and rainfall duration were important controls affecting connectivity strength. Rainfall amount and peak rainfall intensity exerted more important effects than did antecedent soil moisture on the connectivity strength. Furthermore, there was a clear rainfall threshold for HRS connectivity, from 14.8 mm in HRS-1 to 21.1 mm in HRS-2. The increased soil depth and reduced slope enhanced the rainfall threshold of HRS connectivity. Our results indicate that the physical structure of the HRS continuum exerts a primary control on the rainfall threshold.