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| Kathmandu Saturday April 13, 2002 Chaitra 31, 2058. |
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Effect of land-use changes in
Kathmandu Valley
By MADHAV NARAYAN SHRESTHA
There has been a growing need to quantify the
impact of land-use changes from the standpoint of anticipating and minimizing potential
environmental impact. Changes to streamflow quantity and quality that result from land-use
changes need to be established. Kathmandu valley is an ancient valley with a varied
history. Significance in its historical development are the rise of conurbation and the
design of Pagoda style architecture and high rise temples with stepped plinth basements.
After 1952, a new phase of development began with remarkable changes in social status and
the migration of people to the valley. The general trends of urbanisation remained slow
until the mid sixities. Only in the seventies, infrastructures like road networks and
water supply systems started to develop rapidly. As a result, with its three cities --
Kathmandu, Lalitpur and Bhaktapur -- saw population rising rapidly and haphazardly. It is
necessary to systematize the human settlement, implement the town planning more
scientifically and carry out land use in a proper manner. There are various development
plans for the valley, namely construction of an outer ring road, an urban settlement
development plan and construction of link roads on the banks of major valley rivers. The
configuration of the valley is more or less circular with a watershed area of 651 square
kilometres. The valley is an intramontane basin and the valley floor occupies about 55
percent of the area with 35 percent foothills and the remaining 10 percent is mountainous
areas.
A study using the Geographical Information
System (GIS) with Remote Sensing data for the four years1978, 1984, 1990 and 1996 found
different types of land use. It found that the Hydrological Soil Group (GSG) type A, which
has high infiltration rate (i e sand/gravel), accounted for 46 percent of the area of the
valley, and this was found mostly in the northern and western parts of the valley. HSG
type B, which has moderate infiltration rate, accounts for 25 percent. Similarly, HSG type
C and D, which have slow and very slow infiltration rates (ie clay) accounts for 7 percent
and 22 percent respectively.
The most visible change due to land-use change
is in surface waterflow. The effect of land-use change has been evaluated on the
hypothesis that when the same rainfall pattern is routed through different land-use
changes, the resulting difference in the stream flow will indicate the effect of the
land-use change. Compared with the flow that occurred in 1978, the percentage increment in
peak flow was found to be 14 percent and average percentage increment was 15 percent from
land-use of 1996 due to premonsoon rainfall pattern. Similarly, the percentage increment
in peak flow has been found to be 13 percent and average percentage change in streamflow
12.2 percent. Due to the post monsoon rainfall of 1978, the percentage increment in peak
flow was 14 percent and average change was 13 percent. The study shows that the trend of
land-use change was significant in 1990 and difference in land-use changes between 1990
and 1996 has been found to be less. Percentage changes in peak flow and average flow by
superimposing the monsoon events of different years were the almost same. Thus the
percentage changes in peak flow and average changes in streamflow values depends upon
land-use changes irrespective of storm events and the time. The average daily monsoon flow
is increased by 12 percent when there is 9 percent deforestation and 17 percent
urbanisation. In some tributaries, the peak flow value is found increased by 20 percent
although the monthly rainfall depth was observed decreased by 3 percent. It clearly shows
the land-use change effect on stream flow production due to deforestation and
urbanization, which is the most essential assessment for sustainable land-use planning. It
is found that the percentage change in stream flow due to land-use change is almost
constant for different land-uses irrespective of the rainfall pattern (different years)
and irrespective of time of occurrence ( pre-monsoon, monsoon and post-monsoon).
Considering the new urban development plan
proposed by successive governments and which consists of 27 urban settlement development
plans for the 35,300 ropan area under Kathmandu Metropolis, Lalitpur sub-metropolis,
Bhaktapur Municipality and Madhyapur-Thimi Municipality, and a proposed 66.15 km outer
ring road (second alternative proposed by the Department of Roads, and planned to be
constructed at the foothills of the valley) along with 22.25 km of radial connecting road,
the daily average streamflow values are found to have increased by 16.5% during the
pre-monsoon season, 8% during the monsoon and 5% during the post-monsoon seasons, when
compared with streamflow values observed in 1978.
Most invisible change due to land use traced
after long duration due to land-use is in groundwater storage. The deposits within the
valley are predominantly lacustrine. Most of the recharge area (from which water
penetrates to join groundwater storage) is on the northern portion of the valley. Most of
the rechargeable areas are confined to the high flat plains and alluvial low plains
because the exploitation of groundwater seems to be difficult in the surrounding high
mountains. Due to gradient, the rainfall is converted quickly into streamflow that
infiltrates the ground and joins the nearest tributaries.
The groundwater is being used for water supply
and its contribution is around 50%. The extraction from tubewells under the Nepal Water
Supply Corporation (NWSC) amounts to over 80% of the total extraction in the valley. The
total pumpage rate by private as well as gas wells is smaller compared to that of NWSC
wells. The groundwater levels (storage) have been in a nearly steady condition in the
early 1980s, because no large well was operated at that time. Extraction of ground water
by pumping has increased since 1984. The net well field in the valley has been found to be
6.16 sq.km. The study shows a 2m increment in drawdown ( lowering of groundwater table) in
1984 due to extraction of groundwater compared to that of 1978. In 1989, the drawdown
increased by 4m compared to that of 1984. The ground water storage is found to be
decreasing sharply since1986 and facing a continuous drop through nonstop extraction. Some
recharge areas, which are in the northern part of the valley, are rapidly converting to
urban areas.
The extraction of groundwater is also increasing
to fulfil the demand of water. The study shows three distinct trends of drawdown: Minor
decreasing trend from 1977 to 1981, an increasing trend from 1981 to 1985 and a sharp
increasing trend after 1986. As the surface water available is almost constant in the
valley, extraction of groundwater is the only alternative to fulfill demand due to
urbanization. This may be one of major reasons for increasing drawdown. The total drawdown
calculated from the water balance study of the valley is found to have increased by 2.75 m
in 1984 and 7.5 m in 1989 compared to 1978. Drawdown due to groundwater extraction is
found increasing by 2m in 1984 and 6m in 1989 compared to 1978. Hence drawdowns of 0.75 m
in 1984 and 1.5 m 1989 can be attributed to hydrological modification due to land-use
change. The effect of land-use modification is more predominant in groundwater than in
surface water. This is due to the fact that the extraction of groundwater to fulfil the
demand of growing urbanization is increased and the portion of water infiltrating into
groundwater is reduced due to increase in imperviousness.
Information on existing land-use, its spatial
distribution and its changes is essential prerequisite for planning and management. Land-
use planning and land management strategies hold the key for the development of any
region.
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