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.
