Groundwater and Soil Moisture Dynamics in Humid Tropic Hillslopes With Contrasting Soil Depths and Topographic Conditions

Abstract

Abstract Soil and bedrock characteristics play important roles in groundwater (GW) and soil moisture dynamics along hillslopes. Compared to temperate climate regions, runoff in the humid tropics remains poorly understood, being broadly characterized by deeply weathered bedrock and thick soils with rich clay content. To better understand subsurface runoff processes in humid tropics, GW and soil moisture were monitored in two adjacent hillslopes with different underlying soil depths and land cover (forest and oil palm). The monitoring results showed that the average depths and temporal variations of GW varied substantially between the two sites. At the forest site, where the topography is comparatively steeper and covered with a shallower soil layer, the GW at the foot of the slope was more responsive to rainfall. Alternatively, the comparatively gentle slope and deeper soil layer of the palm oil site produced GW patterns that responded more slowly to rainfall. To elucidate the predominant controlling factors, a physically based hydrologic model was employed whose parameters were estimated from the field observations, and calibrated further to represent the observed patterns. Subsequently, a numerical experiment was conducted by varying the model parameters. The findings indicated that soil depth and saturated hydraulic conductivity have important roles in the dynamic response of GW; whereas soil water retention curves were also prominent determinants of surface soil moisture. The results also supported the importance of lateral saturated subsurface flow in soil layers, leading to the rapid responses of GW at the forest site, while such dynamic patterns did not appear in thicker soil layers, indicating different subsurface flow mechanisms, even at adjacent hillslopes.