EVALUATION OF WATER QUALITY AND IRRIGATION SUITABILITY ACROSS CLIMATIC ZONES IN THE WALAWE RIVER BASIN, SRI LANKA

Abstract

The quality of surface water is vital for maintaining sustainable agriculture, particularly in regions that heavily depend on irrigation. The Walawe River Basin, an essential hydrological system in Sri Lanka, spans multiple climatic zones and supports extensive agricultural activities. However, there is a lack of research concerning the spatial variation in irrigation water quality across these zones. This study aims to examine surface water quality and its suitability for irrigation in the intermediate, dry, and arid zones of the Walawe River basin during the dry season. A total of 38 surface water samples were collected in March 2024. In situ measurements included pH, electrical conductivity (EC), and turbidity. At the same time, laboratory analyses determined the concentrations of anions and cations utilizing various spectrophotometric techniques and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). To assess the suitability of the water for irrigation, several established indices were calculated, including the Irrigation Water Quality Index (IWQI), Sodium Adsorption Ratio (SAR), Per cent Sodium (Na%), Kelly’s Ratio (KR), Permeability Index (PI), and Magnesium Hazard (MH). Statistical differences across various climatic zones were evaluated using a General Linear Model. The results revealed significant spatial variability across several parameters, with the arid zone displaying higher pH values, electrical conductivity (EC), alkalinity, chloride, fluoride, and sulfate levels (p < 0.05). This variation is likely attributed to limited rainfall and increased evaporation. While most sampling sites fell within acceptable limits for irrigation use, the elevated sodium percentage (Na%) and lower permeability index (PI) values observed in the dry and arid zones indicate potential long-term risks to soil permeability and structure. The Irrigation Water Quality Index (IWQI) categorized all samples from the intermediate and dry zones as highly suitable. In contrast, 10% of the samples from the arid zone were downgraded to medium suitability. Overall, this study highlights the impact of climatic variability on irrigation water quality within the Walawe Basin. The findings advocate for implementing region-specific water management practices and ongoing monitoring programs to support sustainable agricultural development and mitigate potential future degradation of soil and water resources.