Regional Assessment of Soil Water Salinity across an Intensively Irrigated River Valley

Extensive field investigations allowed depiction of soil water salinity within two large representative regions of a river valley that has been irrigated for more than 120 years. The nature and severity of the salinity problem is captured by hundreds of field surveys, encompassing tens of thousands of measurements and spanning 9 years. Soil water extract salinity, averaged over all surveys and all measured locations, is 4.1 dS m-1 in the Upstream Study Region and 6.2 dS m-1 in the Downstream Study Region. Variability over the measurements is substantial, with a coefficient of variation of approximately 0.51 Upstream and 0.39 Downstream. Relationships to soil and groundwater conditions also were explored, providing field-based insights into major contributing factors and into the value of measuring those factors as part of a salinity reconnaissance. A broad survey, like that described herein, affords a sense of the magnitude of economic loss, in terms of crop-yield reduction, that is exacted by irrigation-induced salinity. Evaluation of the measurements in relation to an estimated average threshold for crop-yield reduction indicates that approximately 42% of the more than 122,000 locations surveyed over both regions had ECe values exceeding the corresponding threshold. Average yield reductions due to soil water salinity are approximately 6% over surveyed locations Upstream and 17% over locations Downstream. Moreover, survey results point toward general targets for irrigation and drainage planning and form a basis for the development of more detailed solutions using computational modeling.