Abstract:
The Deduru Oya reservoir project which is under construction is aimed to improve the livelihood of
farmers in parts of the North Western Province by increasing the productivity of its land by regulating and
diverting the Deduru Oya water by two main canals. The proposed Left Bank (LB) canal which is 44.1 km long
with a discharge capacity of about 7.1 m3
/s at the intake will supply water to augment about 136 existing
storage-based minor irrigation systems in the left bank of the Deduru Oya river. This study is focused on the
analysis of the diversion requirements of the proposed reservoir. For each of the 136 rain-fed minor
reservoirs(tanks) that will be supplied water by the LB canal, the relevant catchment areas, storage areas, natural
streams, land use patterns and cascades were identified for modeling the system. Topographic, geology and land
use details were collected from the digital data of the Survey Department of Sri Lanka and Arc GIS 9.2 was
used as a tool for spatial analysis. HEC-HMS (Hydrologic Engineering Center– Hydrologic Modeling System)
version 3.0.1 was calibrated and verified for the Tittawella Tank, which is a minor tank in Kurunegala District
and also for the Deduru Oya river Catchment. HEC-HMS model was then used to develop rainfall runoff model
for each of the rain-fed irrigation tank catchment and the Deduru Oya reservoir catchment. WEAP21 version
3.43 (Water Evaluation And Planning) model was used to study the water requirement from LB canal. The LB
development area would require 62 MCM annually through the LB canal as a supplement to meet the LB
irrigation demand of the existing and proposed new development areas in dry year while annual RB trans-basin
diversion is 79 MCM, and annual downstream release for hydropower, irrigation and environmental
requirements is 502 MCM in 2009. With long term forecast rainfall data, the WEAP model can be used for
predictability studies in the Deduru Oya LB development areas. The model developed is a useful tool for
planning water resources development in the Deduru Oya reservoir project.
