Using Remote Sensing Techniques for Appraisal of Irrigated Soil Salinity

Waterlogging and salinization are the twin evils of the irrigated agriculture in arid and semi-arid areas, which reduce the productivity of agricultural lands adversely. Managing salinity so as to minimize its environmental impact is a prerequisite for the long-term sustainability of irrigated agriculture. It necessitates establishing fast monitoring systems that facilitate taking actions. Remote sensing appears to offer several advantages over the conventional ground methods used to map and monitor soil salinity. This paper describes an integrated approach to assess soil salinity using remotely sensed data. This encompasses spatial analysis of ground truth and satellite data. The study area is located in the District of Faisalabad in Pakistan. The ground truth data of soil salinity from selected sampling points is tied to the corresponding pixels from the satellite image bands. Remotely sensed data based salinity indices (band combinations) and principal components using principal component analysis (PCA) are developed to find out the occurrence pattern of the salinization. Out of the six salinity indices developed, the following proved to be the most promising when compared with ground truth data. Salinity Index = (B-2 x B-3)/B-1 Where; B-1, B-2 and B-3 are bands of the satellite, IRS-1B LISS-II, in spectral range of band 1 (0.45-0.52 mu m), band 2 (0.52-0.59 mu m) and band 3 (0.62-0.68 mu m) respectively. Regression methods were applied to find the best correlation between salinity data and corresponding pixels on the satellite images. The resulting images were categorized into three distinct classes of saline soils; slightly saline, moderately saline and strongly saline. Using supervised maximum likelihood classification, the images were classified with an overall accuracy of more than 90%. Based on the classified results, saline soils covered 8.7%, 14.2% and 6.6% of the image area in 1991, 1994 and 1997 respectively. The relationship between salt affected soils, waterlogged soils and groundwater quality revealed that 60 to 70% of the salt-affected soils occurred in the zone of shallow water table within 200 cm from ground surface, which is indicative of waterlogged situation. The groundwater in the saline areas is also of hazardous quality, which restricts plant growth except high salt-tolerant crops. [GRAPHICS] Figure A shows the probability density of salt-affected soils with intensity ranging from zero to unity. "Zero" indicated normal soils and above zero to unity represented intensity of salt-affected soils. Using satellite data, the principal component analysis (PCA) and the salinity indices are found to be promising techniques for assessment of saline soils. In the scenario of water scarcity (restricted irrigation supplies in supply channels) due to persistent drought, the reuse of poor quality ground water for irrigation and the failure of tile drainage system in the area are likely to disturb the water ecosystem resulting in increased risks of further environmental land degradation.