Estimating irrigation-induced changes of water and salinity levels in Karagan Lewaya lagoon, Sri Lanka
Date
2002-09
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University of Agricultural Sciences
Abstract
The imminent extension of the large-scale Uda Walawe irrigation scheme in the south of the
tropical island of Sri Lanka will have serious ecological effects on the development area. The
International Water Management Institute is trying to assess these effects by a long-term
study, in which the pre-development status is investigated and the changes are monitored in
biological, hydrological and water quality and socio-economic surveys. The study described
here contributes to the hydrological assessment, focusing on a coastal lagoon called Karagan
Lewaya and its small catchment area in the south of the development area. The lagoon will
receive high quantities of drainage flows, which could deteriorate its quality as a habitat for
migratory birds and lead to flooding of settlements.
With the objective of improving the understanding of the hydrological dynamics and
forecasting the impact of irrigation development, a simulation model was set up. Applying a
simple spreadsheet modeling approach, a model of the lagoon was developed and combined
with an adapted existing runoff model of the catchment area. The water balance model
generates simulated lagoon water levels; a simulation of salinity levels was also attempted.
Various scenarios concerning irrigation and lagoon management were defined.
In order to collect data for the development and improvement of the models and to be able to
monitor the future changes, the previously established hydrometric monitoring network was
extended. Additional information was obtained from topographical maps and remote sensing
images.
Resulting from the lack of long data series needed for a calibration of the model, the
simulation results remain preliminary. It was possible, however, to investigate the effects of
different scenarios and to show general trends:
Drainage flows from the extended irrigation scheme will lead to a significant rise of water
level and decrease of salinity in the lagoon, with less fluctuations than at present. To prevent
flooding of town areas close to the lagoon, an existing outlet channel will have to be opened
during long periods. These changes will lead to a deterioration of the quality of Karagan
Lewaya as a habitat for birds, but could be of benefit for the local settlers (washing, fishing,
cattle bathing).
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Keywords
LAND USE, ECOSYSTEMS, AGROECOLOGY, WATER MANAGEMENT, ENVIRONMENTAL IMPACT ASSESSMENT, IRRIGATION SYSTEMS, SALINATION, SIMULATION, MATHEMATICAL MODELS, GROUNDWATER, WATER QUALITY, WETLANDS, HYDROLOGY