Spatiotemporal hydrographical modelling in a GIS environment
Ranjith Premalal De Silva
Department of Agricultural Engineering
University of Peradeniya, Sri Lanka.
John C. Taylor
Silsoe College, Silsoe, Bedford, MK45 4DT
United Kingdom.
The Mahaweli is the longest river in Sri Lanka,
covering a catchment area of 10,400 sq. km. The upper catchment is defined as
the area above 150 m elevation from mean sea level encompassing an area of 3124
sq.km. The Mahaweli Development Programme, a major undertaking in the Upper
Mahaweli Catchment Area (UMCA) was implemented with the aims of providing water
to the dry zone of the country through a massive diversion scheme and also
generating hydropower. Under this development programme, a series of large
reservoirs was constructed across the main water course at Kotmale, Polgolla,
Victoria, Randenigala and Rantembe. The construction of these reservoirs
inundated a considerable agricultural land area (5400 ha) and the accompanied
developments were responsible for the displacement of several settlements (14000
families).
The changes made on the catchment environment are very
prominent due to both the short time span in which they occurred and the high
magnitude of the impact. The displaced peasants have been settled in the
unsuitable terrain. Steep slopes have been put under cultivation without proper
conservation measures. Agricultural objectives have been derived to obtain
short-term benefits without any concern on the sustainability of the land and
water resources.
Present Problems and a Future Crisis
Critics now say that the hydrological regime has been adversely affected due to indiscriminate land
use changes. It is also pointed out that the river flows have diminished
significantly during the last two decades. Productivity of the agricultural land
has declined. This has directly affected the income status of the farmers thus
creating social problems. Further, reservoir siltation and eutrophication demand
considerable maintenance expenditure. Frequent landslides have threatened human
life and infrastructure. If these problems are not properly resolved, a future
crisis is inevitable and that will jeopardise the expectations of the Mahaweli
Development Programme.
Mitigation Concepts
A large-scale afforesttion/reforestation scheme has been introduced considering the environmental benefits
of the forest cover in terms of the hydrological stability and the
sustainability of the catchment resources. Forestry planning strategies have
mostly been focussed on the physical land capability and sociological
considerations. However, the hydrological consequences of reforestation and the
other land resource conservation measures could lead to high water losses from
the catchment system. In contrast, trees at proper locations can intercept water
from clouds and improve the water yield. Hence, it is required to find strategic
locations for tree plantations to ensure positive water budget in the catchment
while providing extensive benefits in the other sectors. This created the need
for a comprehensive model to simulate the hydrological dynamics of the catchment
to answer ‘what if’ questions for different conservation scenarios.
Hydrologicl Modelling and GIS
Most of the hydrological
models are numerical and computer based, and assume some form of spatially
averaging process for parameter definitions. The lack of recognition of spatial
diversity at catchment sale has been a serious constraint hampering the
simulation, validation and practical application of the hydrological modelling
results. In contrast, GIS is well suited for spatial modelling with
large and complex databases. However, the present GIS have an inherent
limitation of representing time in its spatial data structures. Hence, GIS and
hydrological modelling can be considered as complimentary. GIS could benefit
from the temporal modelling capabilities of hydrological models and hydrologic
models can benefit from the spatial modelling capabilities of GIS. Within this
conceptual framework, this study was focussed on developing a spatiotemporal
hydrological model in GIS to investigate and analyse the hydrological dynamics
and behaviour of UMCA in Sri Lanka.