Evidence of Subsurface Lateral Flow In a Forested Catchment As Revealed by Real Time Soil Moisture Monitoring.

Subsurface lateral flow has been observed to contribute substantially to hillslope and

catchment runoff. Understanding occurrence and intensity of subsurface lateral flow is fundamental to conceptualize subsurface water flow and predict the contaminate transport and runoff. In this study, we monitored soil moisture dynamic at different depth and location along a concave hillslope to investigate the evidence of the subsurface lateral flow. By comparing the storage increase at each horizon and total storage increase at each site with rainfall, we showed the strong and weak evidence of subsurface lateral flow. The results indicate that there was a threshold behavior between the rainfall and total storage increase and the threshold value will increase from wet season to dry season (from 3.1-5 mm to 5.5 -7.9 mm), which indicate the effect of initial condition on the lateral flow and imply that more rainfall is needed to initialize the lateral flow in the dry season. The results also indicate that the upslope site 53C and valley floor site 15 has higher frequency of occurrence of strong subsurface lateral flow (12.3 and 11.5 respectively) than the other sites. This may be due to the steep slope and relative deep soil for site 53C and large contribution area for site 15.  This implied that infiltrated water to bedrock through the shallow upslope site 74 may laterally supplement the deep soil layer in the site 53. In addition, we also found that the layers that subsurface lateral flow most likely occurred were quite different for different site. This was due to the different soil type, soil depth and hillslope location.  This study provided the better understanding of the temporal and spatial evolution of lateral flow in subsurface in the Shale Hills Catchment and will also improve future hydrological model development.