The proposed plan of development has
two basic aims:
(i)
the maximizing of agricultural production, which has become a pressing
necessity on account of the growing population.
(ii) the generation of hydropower
in large quantities, sufficient to meet the growing needs of industries.
The former would be achieved by the
introduction of double cropping and improvements III cultural practices;
but the major obstacle to this is the lack of reliable supply of water
in adequate quantities. Thus the control and management of the water resources
of the Mahaweli Ganga and Its tributaries is
the key to the overall plan of development.
2. The Approach
The surveys of the Scheme have revealed that
soils suitable for irrigation extend over an area of about 1.5 million
acres. The water resources available after lull development (about 5.6
million acre feet per year) would however only be enough for year-round
irrigation of about 0.9 million acres. In other words, the limiting factor
is the availability of water and mint of land. To ensure that the available
water resources are distributed as well as possible throughout the project
area, it has been decided to base the Master Plan for development on the
following general principles:
(i) Having first meet
the irrigation requirements of the lands in the Mahaweli Ganga and Maduru Oya basins, the remaining water may
he diverted for irrigation in the north central part.
(ii) The expansion of irrigation to new areas will
be considered only after an adequate water supply is assured for the existing
irrigation scheme within the boundaries of the irrigation systems.
(iii) New irrigated lands should be located within
the command of the newly designed tanks which cater lot the existing irrigation
schemes.
(iv) Water supply for hydro-electro
irrigation should be provided in adequate quantities all through the year
and in not less than 85 percent of years.
(v) Water supply for hydro-electric stations at
designed rates to generate firm power should he available in not less
than 90 percent of years.
3. Main Features of the Master Plan
The survey of the water resources of
the Mahaweli Ganga and its tributaries shows
that, alter providing fir irrigation of all available lands in the Mahaweli
basin itself, roughly 40 percent of the regulated water would remain,
which could be diverted to the north-central part of the island. Of the
900,000 acres which could be supplied with water for irrigation, a little
more than half lie in the Mahaweli and Maduru
Oya valleys and the remainder in the north-central part of
the island. Of this 900,000 acre area, 246.000 acres are presently cultivated
and 654,000 acres are mostly uncultivated.
The Master Plan envisages storing some
six million acre feet of water in reservoirs located on the Mahaweli Ganga,
its tributaries and the Maduru Oya.
Eleven of these reservoirs would include power stations near the dams;
and another power station is planned to be located on a Transbasin
canal. The installed capacity of
these stations would be 508 megawatts. The total output of firm power
would be 2,037 million kWh per year and be corresponds to about half of
what is required by Ceylon
by 1090.
The main reservoirs would be Randenigala and Victoria for the irrigation supply of the
Mahaweli basin; and Moragahakanda, Polgolla and Kotmale for the north-central
part, (see Chapter VIII). The regulation of river flows with the help
of these reservoirs would result in the additional benefit of flood control;
over 200 square miles of the lower flood plain and delta would be reclaimed
and the agricultural areas around two northern townships would be protected
from the inundation from which they periodically suffer.
The irrigation areas proposed in the
Master Plan are grouped into 14 irrigation systems. Eight of these (A,
B, C, D-l, D-2, E, F, 0), with a total irrigated area of 470,000 acres,
would be located in the basins of the Mahaweli Ganga
and Maduru Oya; the remaining six systems
(H, I, L, M, K, J), would be in the north-central part. The layout
of the proposed works is shown in Figure 3.
4. Estimated
Water Requirements for the Irrigation Systems
Irrigated agriculture in Ceylon
is restricted mostly to paddy cultivation
and during the last 5-10 years, a certain amount of data on water
releases from tanks has been recorded. This, together with data from experimental
investigations, has been used for determining water requirements for paddy.
Irrigation duties for other crops have
been worked out from potential consumptive use (see Section 2, Chapter
IV) and monthly transpiration coefficients. Since no data are available
for these coefficients in Ceylon
, use has been made of the results of research in Arizona
and Texas
.
The estimated irrigation duties for
different cropping patterns are shown in Table VII-l below.
TABLE VII - 1
Water
Consumption for Cropping Patterns
