Pre- and Post-Irrigation Mapping of Soil Salinity with Electromagnetic Induction Techniques and Relationships with Drainage Water Salinity

Irrigated agriculture may impair negative "on-site" (i.e., soil salinization) and "off-site" (i.e., salts exported in irrigation return flows) effects. The aim of this study was to assess these effects, and the relationships between soil-and drainage-water salinity in a basin before and aft er the development of irrigation. Soil salinity (ECe) was estimated through electromagnetic induction (EMI) techniques (ECa) and the corresponding calibration equations, and drainage water salinity (ECdw) was measured in the outlets of 22 watersheds delineated within a 505-ha study area before (Year 2006) and aft er (Year 2008) irrigation development. Apparent soil EC was about 50% higher in 2008 than in 2006 due to increasing soil water contents, but estimated ECe was about 15% lower in 2008 than in 2006 due to salt leaching induced by irrigation. Drainage water EC increased by one order of magnitude from the upper (ECdw = 0.4 dS m(-1)) to the lower (ECdw = 4.5 dS m(-1)) reaches of the drainage network. The mean ECdw of the 22 watershed drainage outlets increased from pre-irrigation 2006 (2.4 dS m(-1)) to post-irrigation 2007 (3.1 dS m(-1)) and 2008 (2.7 dS m(-1)) years. Drainage water EC significantly depended (P < 0.001) on surface-weighted EC (ECe*) of each watershed, but the linear regressions were different in 2006 and 2008. The transformation from dryland to irrigation led to the leaching of salts and its export in the irrigation return flows. A compromise needs to be attained in the study area to achieve the beneficial "on-site" leaching of salts while minimizing the detrimental "off-site" export of salts.