Preliminary investigation of the water quality of irrigation reservoirs of the Kala Oya river basin, Sri Lanka

Abstract

Most ancient reservoirs of Sri Lanka have been constructed for irrigating agricultural lands in the past and fisheries development in these reservoirs has been a relatively recent advancement. Due to the multiple uses of reservoir resources, hydrological regimes are bound to vary from reservoir to reservoir, which may influence their water quality. A long-term research project has therefore been carried out to determine the optimal water allocation for multiple uses in 10 reservoirs of the Kala Oya river basin in Sri Lanka. In this paper, preliminary results of this long-term project are presented with a view to investigate possible inter-relationships among major limnological parameters, which are possibly influenced by hydrological regimes. Twelve water quality parameters specify in 10 reservoirs were measured from June 2013 to May 2014. When the 10 reservoirs were treated as a single group, the reservoir water quality did not show any significant inter-relationship among the parameters measured. However, multivariate statistical analysis (cluster analysis and non-metric multidimensional scaling) clustered the 10 reservoirs into 3 groups, so that the possible inter-relationships among limnological parameters could be explored. Results indicate that except in two reservoirs with high flushing rates, significant linear relationships were evident. Accordingly, chlorophyll-a positively related with total phosphorus and dissolved phosphorus with a negative relationship of Secchi disk depth to total phosphorus. Principal component analysis (PCA) indicated that in 4 upstream reservoirs of the Kala Oya basin, reservoir water quality was dominated by Secchi disk depth with a negative influence of total phosphorus. However, three reservoirs downstream were dominated by limnological parameters such as chlorophyll-a, Dissolved oxygen, pH, temperature and organic weight. Nitrate level dominated the limnological condition in the Rajanganaya reservoir while the Siyambalangamuwa reservoir was dominated by alkalinity and conductivity. Dissolved phosphorus and seston weight were important water quality parameters in Ibbankatuwa reservoir. These variable limnological parameters indicate that the reservoir management should be viewed from the biological productivity perspectives, through investigation of possible reasons for such variability.