Impact of non-drained irrigated rice cropping on soil salinization in the Senegal River Delta

Abstract

Soils in the Senegal River Delta are naturally saline due to the presence of a shallow saline water table (electrical conductivity>20 dS m −1; water table depth between 1 m and >2 m throughout the year). Private irrigation schemes for rice in the Delta entail low investments and generally lack a surface drainage infrastructure. Such schemes are often abandoned after a few years, which is commonly blamed on soil salinization because of lack of drainage. However, no surveys have been conducted so far to sustain this assumption. The objective of this study was to obtain quantitative information on the impact of non-drained irrigated rice cropping on soil salinization in the Delta. We selected a 2000 ha study area with predominantly private non-drained irrigation in the Gorom region in the Delta. Soil salinity was assessed for 118 sample fields at the onset of the wet rice growing season using an electromagnetic conductivity meter (EM38) and conventional laboratory techniques (0 to 0.3 m depth). The number of years under rice cropping for individual fields was obtained using satellite imagery from previous years (1992–1995) and a geographical information system. EM38 measurements were poorly correlated with root zone EC lab data, which was attributed to a low root zone water content at the time of measurement (i.e., end of the dry season). EM38 measurements reflected, therefore, mainly subsoil salinity and could not be used to predict root zone salinity. Contrary to common thinking, 1:5 soil extract conductivity (EC lab) at 0–0.30 m depth declined with increased cropping intensity. Measurements conducted shortly before the start of the growing season resulted in an average EC lab of 2.2 dS m −1 for four years of rice cropping without drainage, and in an average EC lab of 8.9 dS m −1 for zero years of rice cropping. Rice cropping therefore reduced soil salinity, even when conducted without drainage. Results are explained by the leaching effect of the ponded water layer in an irrigated rice field, temporarily transporting salts downwards and blocking capillary rise of salt from the water table to the soil surface. EC lab determined for rice fields at harvest ranged from 0.4–1.7 dS m −1 for 0–20 cm depth. Comparison with previous data showed that a yield decline due to salinity can be expected if EC lab>0.8 dS m −1. Installation of surface drainage facilities may, therefore, in some cases be worth the investment in the Delta.