Detecting soil salinity changes in irrigated Vertisols by electrical resistivity prospection during a desalinisation experiment
Increasing soil salinity threatens the sustainability of irrigation schemes in several rice growing areas of the world, particularly in western Africa. Desalinisation is needed to sustain irrigated rice cultivation. However, to keep track of the effectiveness of any desalinisation method, monitoring of soil salt content is required. The objective of this study was to assess the ability of non-destructive electrical resistivity prospection to monitor salinity changes (spatial and temporal) in a Vertisol profile during a desalinisation experiment in Kollo, Niger. Desalinisation was induced by successive ponding and flushing cycles on a 10m×10m square plot. Seven data acquisition campaigns were performed between July 13 and September 24 2009. In each campaign, (i) apparent electrical resistivity measurements were performed with two electrode spacings (10 and 30cm) and were transformed into the inverse apparent electrical conductivity (ECa), (ii) soil samples were extracted from three soil layers (0–10, 10–20, and 20–40cm). Desalinisation was more efficient for the topsoil layer than for the deeper ones. Mean electrical conductivity of 1:5 aqueous extract of soil samples (EC) decreased from initial values of 2.0, 2.8 and 2.9dSm−1 to 1.3, 2.2 and 2.6dSm−1 respectively at 0–10, 10–20 and 20–40cm. The relative decrease in salt stocks was estimated at 35.7, 22.7 and 11.4% of the initial amounts in these respective soil layers. Mean ECa measured at the soil surface also decreased over time, and the highest correlation (R=0.57) was observed between ECa30cm and EC of the upper soil layer (0–10cm). ECa survey allowed the detection of spatial and temporal salinity changes with a high spatial resolution. Relative decrease in ECa over time was an indicator of the desalinisation process. This appears useful to adjust water supply for saline soil reclamation, particularly in regions with high water scarcity.
