Time-lapse geophysical imaging of soil moisture dynamics in tropical deltaic soils: An aid to interpreting hydrological and geochemical processes

A fundamental, and often intriguing question, in hydrology is where does the water go? This becomes particularly difficult to observe when water arrives at the ground surface and infiltrates into soils. The development of rapid, campaign-style imaging methods that do not need to be left in situ are therefore of great interest in tracking subsurface hydrological redistribution. We present a novel geophysical imaging approach identifying spatiotemporal variation consistent with soil water redistribution in a tropical deltaic soil. The intention is to provide additional insight into spatiotemporal soil hydrological/biogeochemical processes. The bulk soil electrical conductivity response (ECa) is primarily controlled by the clay content and type, the ions retained in the soil solution (ECe), and the soil water content (theta). Clay content can be assumed to be temporally static, whereas theta and ECe are temporally dynamic. By imaging over time, we can attempt to tease apart these contributing factors. In nonsaline soils theta is the major contributor to temporal changes in ECa. By exploiting an intensive rainfall event (75 mm), with time series spatial ECa measurements, before and after the event, we were able to identify zones of water depletion and accumulation and to provide an indication of the time required for the soil to return to its prior state. In addition, locations with more clay and salts were identified through response surface-directed soil sampling. We found important spatiotemporal variation across the level 4 ha field site that from visual inspection appeared uniform.