Limnology project at Mahaweli reservoirs:I. some physical properties of Kotmale, Victoria and Randenigala rservoirs

Abstract

The Physical properties of Kotmale, Victoria and Randenigala reservoirswere studied since 1987 upto now and this paper presents the data obtained during1987.

The transparency values changed with the reservoir and the season. Thehighest frequency percentage of Secchi depth observed at each reservoir were.Victoria 1.21-1.61m, Randenigala 2.51-3. Olm and Kotmale 2.10-2.30m, indicatinghighest values at Randenigala and lowest values at Victoria. Accordingly themean euphotic limits were 5.00m at Victoria, 6.25m at Randenigaia and 5.lOmat Kotmale. The highest differences in temperature gradients were observedfrom surface to about 25m depths in all three reservoirs, in the deeper layers thetemperature gradient is distinctly low. The highest conductivity values wereobserved closer to the dams of all three reservoirs indicating the accumulation ofdissolved ions towards the dams. Range of 30-87pslcm at Kotmale, 85-139pslcm at Victoria and 110-188 pslcm at Randenigala. The highest values wererecordedfrom Randenigala. The highest conductivity values were correlated withthe water rentenion levels of the reservoirs. Fluctuations in pH values with theseasons follow a similar pattern in all three reservoirs. The mean pH values ofthe three reservoirs were in between 6.0 and 8.7. In all three reservoirs, the pHgradient was higher above 10m depth whereas below this depth it remained moreor less constant.

The high dissolved Oxygen concentrations were closely linked with theeuphotic zones of the three reservoirs. The mean Oxygen concentration calculatedduring 1987 investigation period from surface to 5m depth were 4.75-7.50mg]; at Kotmale, 4.70-6.60 mgjl at Victoria and 4.70-6.65 mgfl at Randenigalareservoirs. Dissolved concentrations were very low below the depth of 20min all three reservoirs. However there was no evidencefor the presence of deoxygenated layers in all three reservoirs.