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

Abstract

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 (θ). Clay content can be assumed to be temporally static, whereas θ and ECe are temporally dynamic. By imaging over time, we can attempt to tease apart these contributing factors. In nonsaline soils θ 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.

Temporal stability of electrical conductivity in a sandy soil

“…The coefficients in the exponential relationship between ECa and water content at each sampling location varied with the first PC value. These findings indicate that no unique relationship existed between ECa and SWC for the entire study field, as noted by Islam et al (2011), Pedrera-Parrilla et al (2014, and Robinson et al (2009).…”

Assessing the temporal stability of spatial patterns of soil apparent electrical conductivity using geophysical methods

“…were recorded before and after the first three ECa surveys (data not shown). The soil temperature did not change from 30 o C, which is consistent for tropical climates (Robinson et al, 2009). Therefore, no temperature correction was needed for the data.…”

“…Since the study site was topologically flat, the supposition was that EMI mapping can capture the dominant intrinsic physical soil property through repeated mapping, by employing a temporal stability analysis technique. Ranking and/or time-lapse ECa images were used successfully by Robinson et al (2009) to identify hydrologic subsurface patterns, soil texture, and water-holding capacity. The modified temporal and rank stability procedure described by Robinson et al (2009) was employed in this study to determine the stability of ECa s and ECa d across the field.…”

“…The sensor-field computer-GPS setup provided spatially exhaustive geo-referenced readings for ECa s and ECa d . Using the time lapse approach (Robinson et al, 2009), a total of nine ECa surveys were completed between 2009 and 2010. This was done to capture multiyear spatio-temporal and seasonal changes of ECa in the cocoa field.…”

“…Although, researchers have investigated the spatio-temporal stability of ECa in temperate regions (King et al, 2001;Nehmdahl and Greve, 2001), there has been limited research on tropical soils (Robinson et al, 2009). Even so, there are no reports of studies on cocoa soils using geophysical imaging to assess the temporal stability of spatial patterns of ECa to develop management zones.…”