The effect of salinity on water productivity of wheat under deficit irrigation above shallow groundwater

Abstract

Saline groundwater is often found at shallow depth in irrigated areas of arid and semi-arid regions and is associated with problems of soil salinisation and land degradation. The conventional solution is to maintain a deeper water-table through provision of engineered drainage disposal systems, but the sustainability of such systems is disputed. This shallow groundwater should, however, be seen as a valuable resource, which can be utilised via capillary rise (i.e. sub-irrigation). In this way, it is possible to meet part of the crop water requirement, even where the groundwater is saline, thus decreasing the need for irrigation water and simultaneously alleviating the problem of disposing of saline drainage effluent. Management of conditions within the root zone can be achieved by means of a controlled drainage system. A series of lysimeter experiments have permitted a detailed investigation of capillary upward flow from a water-table controlled at shallow depth (1.0 m) under conditions of moderately high (5 mm/day) evaporative demand and with different levels of salinity. Experiments were conducted on a wheat crop grown in a sandy loam soil. Groundwater salinity was held at values from 2 to 8 dS/m while supplementary (deficit) irrigation was applied at the surface with salinity in the range 1–4 dS/m. Our experiments show that increased salinity decreased total water uptake by the crop, but in most treatments wheat still extracted 40% of its requirement from the groundwater, similar to the proportion reported for non-saline conditions. Yield depression was limited to 30% of maximum when the irrigation water was of relatively good quality (1 and 2 dS/m) even with saline groundwater (up to 6 dS/m). Crop water productivity (grain yield basis) was around 0.35 kg/m 3 over a wide range of salinity conditions when calculated conventionally on the basis of total water use, but was generally above 1.0 kg/m 3 if calculated on the basis of irrigation input only.