Irrigation management and soil salinity diagnosis: a study case in Pakistan

Monitoring soil salinity has always been of a great concern in the context of policies related to the management and rehabilitation of irrigation systems. This is particularly the case in Pakistan that has the largest contiguous irrigation system in the world. During the last 50 years, several national agencies have been assessing salinity hazards through surveys based on geo-chemical analyses or expertise. However, there are many differences between the extent and location of salt-affected soils reported by these agencies. Differences are primarily explained by differences in criteria and methods used to classify soils, and to assess salinization and sodification. Also, these surveys do not allow a clear identification of the impact of soil salinity on the functioning of an irrigation system or on its agricultural productivity. Either specific farming practises enable water users to control salinity and limit its impact on crop yields; either agricultural productivity is limited by other major constraints such as irrigation water availability, water-table depth or non-water related farm constraints. The present study compares three different soil salinity assessment methods. To better understand the differences between the results obtained by applying these methods, a detailed analysis is performed to identify the marginal influence of irrigation management and soil physical conditions on soil salinity, soil visual appearance and crop status. The study is carried out in sample areas of the Chishtian Sub-division, a 70,000 hectare irrigation system located in the South-Punjab, Pakistan. The main source of irrigation in this sub-division is canal water, complemented by groundwater pumped by farmers to compensate for canal water supply inadequacy and unreliability. The command area of the Chishtian Sub-division is characterised by primary salinity, mostly decreasing in areas where canal water is available, while the use of poor quality groundwater contributes to an increase in soil salinity and sodicity. Through a Geographical Information System, irrigation management indicators are linked to results of a visual salinity survey, to farmers' peception of salinity and sodicity, and to a soil map. Statistical analyses are performed to establish the relationships existing between the different soil and salinity variables investigated and irrigation management indicators.