Formulating and assessing best water and salt management practices: Lessons from non-saline and water-logged irrigated fields

Abstract

To address the on-site and off-site salt load impact associated with non-saline and water-logged irrigated fields a well-considered strategy is required. This implies minimal salt mobilisation and additions through irrigation, no crop yield losses due to excessive salt in the root zone, and minimal irrigation-induced drainage and leaching. The aim was to formulate best water and salt management practices to achieve this strategy and conduct an extensive assessment on whether they are implemented before problems appear. Weekly and seasonal data (2-years) from 19 fields (28 measuring sites) were used. Crops included barley, wheat, groundnuts, maize and lucerne grown in a semi-arid climate. Dominant soils were sandy loam, loamy sand and sandy with lateral moving shallow groundwater tables (depth>1.2 m and electrical conductivity<250mSm−1), while electrical conductivity of primary water sources is < 100mSm−1. Good decisions included the use of centre pivot compared to flood irrigation, irrigation schedules that ensured soil matric potential for maximum crop transpiration and limited yield losses due to water logging and soil salinity. Poor decisions were incorrect sprinkler design and excessive pumping pressure and limited use of rainfall and capillary rise to irrigate less. Growing season rainfall-plus-irrigation were a mean 55% more than the quota (annual water allocated to farmers), and 35%, 39%, 60%, 106% and 67% more than a “conservative” estimate of barley, wheat, groundnuts, maize and lucerne transpiration, respectively. The magnitude of variation in this oversupply were similar than the mean values. Approximately all salts applied through irrigation were leached from the root zone, while minimal re-use of drainage water was made (only 3 fields). In terms of resource use efficiency and preventing environmental degradation the evidence is overwhelming for improved decisions at field level. However, given sufficient supply of water, continuation of the status quo, might be sustainable in the long term.