Spatial and Temporal Variation of Irrigation Water Quality of a Small Tank Cascade System in Up-stream of Malala Oya River Basin, Thanamalwila, Sri Lanka

Abstract

Limited attempts were made using Canadian Council of Ministers of the Environment (CCME) water quality index (WQI) to interpret irrigation water quality conditions of tank cascade systems (TCS) in Sri Lanka. The present study attempts (i) to investigate the temporal and spatial variations of the irrigation water quality of a small TCS in Malala oya basin in Thanamalwila catchment, Monaragala district and (ii) to determine the WQI recommended by CCME. Representative tanks of TCS for upper (Sinhalayagama wewa and Podi wewa), middle (Maha wewa) and lower (Bagmuwa wewa) locations were selected to understand the nutrient dynamics along the cascade during Maha (wet) and Yala (dry) seasons. Hydrochemical parameters of pH, electrical conductivity (EC), Total Dissolved Solids (TDS), NO3 -—N and NH4 +—N, PO4 3-, CO3 - and Cl- were determined applying standard analytical methods. The results reveal that the levels of pH and salinity have been increased in the middle and tail tanks of TCS in both dry and wet seasons. The Bagamuwa wewa possessed the highest values in EC, TDS, Cl-, NO3 -—N and NH4 +—N, indicating the unsuitability for irrigation. Further, Bagamuwa wewa and Podi wewa discovered extraordinary high values for PO4 3- in the dry spell beyond the permissible limits of 0-2 mg/L. The normalized values of CCME WQI for each tank in the cascade showed fair and good water quality conditions in upper cascade tanks while marginal conditions in middle and lower cascade tanks. The Bagamuwa wewa obtained the lowest CCME value indicating the marginal use for irrigation in both seasons. Accordingly, it was clear that the irrigation water quality decreases along the cascade where a temporal and spatial variation was clearly identified. Therefore, CCME WQI can be used to understand the temporal and spatial variations of water quality as an effective tool for a small TCS.