Water balance to identify Lunugamvehera reservoir management requirements

Abstract

Lunugamvehera reservoir constructed across the Kirindi-Oya in the Southern Sri Lanka, is considered as a prominent water body not only because of its large size but also due to widespread accusations that this reservoir has failed to serve its farmers.

Water balance of a reservoir can easily identify the fluctuations of reservoir storage through the fluctuations of inflows to the reservoir, seepage from the reservoir, evaporation and water extractions for purposes such as cultivation and domestic.

Recent studies on small irrigation tanks have shown a possibility of the actual evaporation and seepage from a tank to far exceed the estimates done at the planning and design stage. The Irrigation Department which is the organization managing this reservoir has been measuring water inflows, reservoir water levels, and reservoir water usage through the sluices. These data along with evaporation data of the area were incorporated into a reservoir water balance to compare the variation and effects of water accounting components in the system. Water balance model constructed keeping in line with the Irrigation Department guide lines, identified the necessity of accurate water release measurements for efficient management of reservoir storage.

The water balance model outputs fitted to the observed data provide the opportunity to analyze the sensitivity of evaporation and seepage volumes from the reservoir thus providing management information. Reservoir water levels were compared with the estimates done using standard procedures for the assessment of the deviation from a manager's expectations, thus revealing management options such as strengthening the measurements and a review of forecasting methods.