Abstract:
Agriculture uses more water when compared with other water users. Insufficient water
resources in a country would create additional issues of governance due to poor food security
for its people and lack of water for the sustenance of the environment. Water shortages
especially for agriculture are most felt in the dry zone in Sri Lanka and most of the farmers
are failing to cultivate full extent in both Maha and Yala season. Irrigation Department
Guideline (ID 1984) which is the base for reservoir operation, planning and management in
Sri Lanka, has the need to improve its methods by identifying suitable parameters and
operational options suited for field applications. There are only limited studies of reservoir
operation practice in Sri Lanka. Twenty years (1997-2016) of reservoir operation data of
Namal Oya reservoir at Ampara District, Sri Lanka were analyzed at a weekly time scale to
compare the practice and the guideline to critically evaluate the requirements for better water
management with a view of achieving water security and thereby reaching food security.
This work is an evaluation of irrigation reservoir water management practice to make
recommendation for efficient water management in order to achieve water and food security
for farming communities in the dry zone of Sri Lanka. A weekly water balance model
according to the Irrigation Department guideline was developed for the reservoir system
while including the behavior of the catchment area and the practice of cultivation in the
command area. The model development was carried out using spreadsheets. A weekly crop
water requirement was also developed to check observed water release which were
compared with the crop water model estimations to verify the adherence to the guidelines.
These results were then compared with the actual water releases to evaluate the variations,
influence of parameters and the field level cultivation practices. Inflow model was also
developed based on Irrigation department guideline and a monthly 2 Parameters model and
were later compared with observed storage. The comparison of model developed with ID
guideline and the water use plans of the Namal Oya Irrigation department office revealed the
average annual difference of observed and calculated water release is 1091 Ha.m where 392
Ha.m in Maha season and 699 in Yala season and observed annual water release is 2098
Ha.m where 705 Ha.m in Maha and 1391 Ha.m in Yala which indicating the Namal oya
Irrigation reservoir are releasing 50% more water than the observed values in a water year.
The model results and the actual practice demonstrated that the overall efficiency of the
irrigation scheme is estimated based on trial and error method and the value is 55%. The
most sensitive parameters in the water balance inflow, sluice release and seepage. The study
indicated that if the efficiency level can be increased by 70%, the annual water demand will
be reduced from 2654.82 Ha.m to 2055 Ha.m which enables to served nearly 496 Ha more
command area each water year. The key parameters that need attention are inflow andsluice
discharges. Consideration of practical advantages and the need for water security leads to
recommending to incorporate the present practice with an update of ID guideline.