Abstract:
Magalla is a reservoir which is located at Nikaweratiya area in Deduru Oya river
basin. Deduru Oya river basin which is in dry zone of Sri Lanka has severe flood during rainy season
and lean flow during dry season. Magalla is one of the ancient tanks in the basin and major source of
water is receiving through Ridi Bendi Ela canal. There is a diversion scheme just downstream of the
Deduru Oya reservoir which is named as Ridi Bendi Ela weir. Water is diverted to the Ridi Bendi Ela
canal from the weir and length of the canal is 21 km. Runoff water which is coming from Magalla
reservoir catchment also significant, other than Ridi Bendi Ela canal inflow. There is a proposal to
augment water to Magalla through Deduru Oya multipurpose reservoir development project.
Objective of the study was to develop reservoir simulation model for effective irrigation water
management of Magalla reservoir. Nikaweratiya, Wariyapola and Ridi Bendi Ela rain gauge stations
are selected for the study and daily rainfall data from 1991 – 2012 was statically analysed. Crop water
requirement calculated using CROPWAT 8.0 software. HEC-HMS 4.0.1 software selected to model
rainfall runoff and reservoir simulations. Magalla model was developed while using initial and
constant loss method, Clark unit hydrograph method, recession base flow method and Muskingum
routing method. Model was calibrated and validated for continuous and event based simulations.
Simulated and observed values of the model were statistically verified and results revealed that the
calibrated model is capable of capturing the seasonal characteristics of stream flow and storages
satisfactorily. The calibrated model with the parameters of respective sub-catchments can be used to
find water diversion requirements from Deduru Oya river to achieve 100 % cropping intensity in Magalla scheme.
