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Sri Lanka has 103 river basins that have water flows starting from the central highland.
Most of the river basins have already been used for many purposes such as irrigation,
hydro-power generation, industries and urban w ater supplies. In addition, the most
upper basins have undergone high utilization by human activities such as upland
cultivation, river w ater withdrawal, road construction and human settlements.
From a hydrological point of view, all these activities directly and indirectly affect the
stream w ater flow to the low er basin. In the case of Mi Oya, for a long time, ft has been
evident that there is a lack of water for a number of large irrigation reservoirs and
hundreds of small tanks located in the lower basin. Hydrologically, the upper catchment is
the main source of w ater to the main flow in turn to the low er basin. W hen w ater from
the upper catchment is drastically changed, it affects the lower basin w ater use. Thus, ft is
essential to adopt a better w ater management system. Based on this concept, a model
for basin management was built up using hydrological parameters and socio-economic
data, derived from a research carried out from 2010-2015. The model was applied to
the Mi O ya upper basin and explored how basin w ater balance has been affected by
human interference. This paper first presents a model developed, and afterwards
discusses what measures should be adopted to support m ore w ater to be drained to the
lower stream. Hydrological parameters namely rainfall, temperature and evaporation
w ere collected from the nearest stations in addition to site measurements in the field.
Catchment w ater yield (W y) was done by using formula W y = (P-E) x A w here P, rainfall
E, evaporation A, area of the catchment. W ater discharge (Q ) of the streams was
computed using floating method Q = V a x A where Va flow velocity where A crosssection of the stream x depth. Adequacy of w ater in the reservoirs was calculated by
water level studies of reservoirs together with farmer's experiences. It was revealed that
the total volume of w ater produced by the upper catchment has been considerably
(4 0 % ) decreased due to human activities. The prominent activity is the blocking out of
small streams by farmers for their own purposes such as the construction of private tanks
and newly made paddy fields. It is also evident that removing vegetation cover for chena
cultivation has caused higher w ater evaporation from the soil resulting in low w ater
discharge to the drainage system. The model proposed measures three types mitigations,
improving and controlling to ensure the increase of w ater derived from the upper basin
to the lower basin.
