Observations of electrical resistivity tomography and groundwater level under an irrigated terraced paddy field in northern Taiwan

In an irrigated paddy terraced field, subsurface return flow within the plots potentially occurs as long-term ponding water may lead to shallow groundwater uplift to the soil surface. However, subsurface water remains hidden from direct observations. Using an irrigated paddy terraced field located on Ewei Mountain, Taipei, as the study area, we investigated subsurface water during both irrigation and non-irrigation periods by employing electrical resistivity tomography (ERT) and monitoring the groundwater level in wells. The ERT results provided two-dimensional (2D) electrical resistivity sections. The sections show that the subsurface moisture was higher during the irrigation period than during the non-irrigation period. The ERT results indicate the possible location of the shallow groundwater layer and the presence of preferential flows beneath the embankments. Information gathered from rock cores, groundwater level records, and ERT was used to delineate the boundary between the topsoil layer and other geological materials. Rainfall and irrigation can dramatically increase the groundwater table, although it hardly exceeds this boundary. Our findings suggest that considering the subsurface return flow within plots may not be essential in this study area. The investigation with ERT helped assess whether the subsurface return flow needs to be considered in the irrigation management of a terraced paddy field.