PROPOSED LAND USE AND WATER

RESOURCES DEVELOPMENT

 

The catchment’s of the Mahaweli Ganga consists in the main of metamorphic rocks covered by fragmented sedimentary deposits. The thickness of these deposits varies greatly, but they are weathered and relatively soft, and so can be excavated without blasting to between 30 and 80 percent of their thickness. Gneisses and chamokites predominate on the dam sites, and the inter-bedding is only occasionally found. As a whole, therefore, the engineering-geology conditions can be rated quite favorable for the construction of high dams.

 

    The construction of sizeable transbasin canals will be complicated in mountainous areas and teen slopes, where a considerable amount of rock excavation will be necessary. On flat land, canals will only locally be cut in rock. Increased seepage from the canals should be expected fling terrain and where rocks are weathered. It is expected that seepage from distributory and from the field network will be within normal limits.

 

    In the upland part of the project area groundwater resources are not abundant. In the alluvial the lower Mahaweli basin and to the north of the project area, groundwater of reasonable is obtainable at depths of 10 to 20 feet. In the flood plain of the Mahaweli Ganga, ground-near the surface, reaching the topsoil in rainy periods and dropping by 3 - 5 feet in dry are sweet waters which may be used for paddy cultivation to reduce the irrigation duties. In the dry northern area, groundwater development could be an important factor in augmenting the irrigation supply and reusing irrigation water lost by seepage.

 

The hilly part of the Mahaweli basin is suitable for sitting storage reservoirs, but the construction of main irrigation canals will be expensive, particularly in the ancillary works required for the control age. In the lower eastern part of the project area, where the terrain is flat, flood control sits will be required. The middle left bank of the Mahaweli Ganga is also characterized by relatively flat relief favorable for irrigation ; the right bank basin has a more complicated terrain gravity irrigation here will be more difficult ; therefore overhead irrigation would be more applicable.

 

            The topography of the north-central part is generally favorable for surface irrigation; especially part of the territory, in the central part, the relief is more undulating and here again it may be necessary to rely for development on overhead irrigation.

 

2.         Proposed Land Use

 

Investigations into the soils, agronomy, and irrigation conditions of the project area, taking account of the additional information obtained in the second stage of the survey, led to the con-selection and distribution of irrigated crops should be guided by the following general

 

          (i).      The basins of the Mahaweli Ganga and the Maduru Oya should, as far as possible,. be used to grow the crops that consume most water, such as paddy, sugar cane and forage grass, since water is relatively cheap to supply in this area. The soils and topo­graphy are also favourable to these crops.

 

        (ii).      The upper right bank of the Mahaweli, the soils are very fertile, but the relief makes irrigation difficult. Sprinkler (overhead) irrigation appears more applicable here, and vegetables or other crops with modest water requirements should be grown, view of the greater expense of sup plying water in this way.

 

(iii)    In the north-central region, the supply of water is considerably more expensive. There­fore high-value crops such as cotton, chillis, groundnuts and maize appear more suited to these conditions.

 

Considering the above, the following general cropping patterns are suggested for the project area:

 

TABLE V-I

Cropping Pattern of New Irrigable Land

 


                             Cropping Pattern                                                 Percentage of
                                                                                                        irrigable Area

                  I.     Paddy (paddy-paddy-legume)                                           29
                 2.     Mixed (paddy-cash crops)                                                 16

                 3.     Cash crops in rotation                                                        50
                 4.     irrigated pastures             ..                                                   3
                 5.     Sugar cane                      ..                                                   2 

                                                                                                                 ____

                                                                                                                   100

 

 

It is assumed that land which is already irrigated will continue to grow paddy Assuming that cropping patterns in Table V-I are used, the available water supply (see Section 3 below) is suffi­cient to irrigate 900,000 acres during both Maha and Yala seasons. Of this, 653,800 acres will be new and 246,200 acres will come within the command of the proposed irrigation systems.

 

The area proposed to be irrigated includes all irrigable land available in the Mahaweli Ganga and Maduru Oya basins. The water supply to these areas, being closest, is the least expensive. Soil conditions are also favorable. The other lands to be irrigated, in the adjacent north-central part ot the island, have been selected from areas commanded by existing tanks. The use of the tanks to redistribute water supplied from the transbasin canals allows a reduction in the cross section of the transbasin canals which reduces the cost per mile by about half. Settlement is also facilitated if the new land is close to populated areas. The design and location of the proposed irrigation systems are summarized in. Chapter VIII. The systems, which are labeled alphabetically from A to M, area shown in Figure 2.

 

The net irrigated area within the boundaries of the proposed systems was determined by multi­plying the total area suitable for agriculture by the coefficient’ of land use.The total area for agricul­ture by the coefficient of land use. The total area suitable for agriculture was estimated from soils maps. while the land use coefficient was worked out from detailed surveys of three standard plots investigated in the second stage of the survey.

 

The Kaudulla standard plot, located in the lower left-bank basin of the Mahaweli, covers 12,200 acres of gross area suitable for agriculture and includes two zones: the ancient floodplain, which merges with the river delta, and the terrace above the floodplain. The average coefficient of land use for the whole plot is 61 percent: 57 percent in the upper lands, 65 percent in the lower lands and 70 percent in areas near the delta. The topographical and soil conditions of this plot are representative of system D-l. The upper part of the plot is similar in certain features to system B, while the lower part merging with the delta is comparable to system A. Accordingly. the land use coefficient of 57 percent was adopted for system B and 70 percent for A.

 

I.   The coefficient of land use (CLU) is the ratio of the net area on which crops ate actually grown (NA) to the gross area suitable for agriculture. (GA) which includes land occupied by roads, settlements and

                 

irrigation canals, and land which is inconvenient to irrigate. CLU =    NA                                                                                                                                                                      GA

 

The Kalawewa standard plot, (6,600 acres) is located in system H. The soils and topographic conditions of this plot are roughly representative of other central systems of the non-central part. The land use coefficient in this plot is 52 percent under gravity irrigation and reaches 70 percent under sprinkler irrigation. For the non-central part systems, the coefficient of 50 percent was adopted so that the best lands for irrigation could be selected in the future.

 

The third standard plot, Ulhitiya Oya (3,700 acres), is representative of the southern part of system C, where owing to the topography irrigation is possible only by overhead sprinkler. The coefficient of land use in this plot is 70 percent under sprinkler irrigation.

 

From the above considerations, the following distribution of irrigated land is proposed:

 

TABLE V-2

Land Proposed for Irrigation

 


                                                                       Area                       New Area by cropping pattern

         Basin             Systems

                                                           Total      Existing   New           1       2       3        4         5

 


1,000 acres

Mahaweli Ganga A, C, D-l, D-        2;347.9     102.5     245.4      118.9 25.5 68.0 18.0 15.0

MaduruOya            B                          124.9         6.8     118.1        42.7 20.0 55.5     -           -

NCP Rivers         H, I, K, L, M, J       427.2     136.9     290.3        31.8 59.4 199.1    -           -

 


3,.        Proposed Development of Water Resources

 

The main potential source of water for irrigation and power in the area surveyed is the Maha­weli Ganga, which has a mean yield of 6,400,000 acre feet per year at the lowest site of diversion, Kandakadu. To develop the natural yield of the Mahaweli Ganga, its tributaries, the Maduru Oya, 4 other rivers, and to generate hydro-elect5ric power, fifteen dams have been designed. It has been established that by regulating the yield of the Mahaweli Ganga and its tributaries, it will be possible obtain a regulated flow, at 90 percent frequency, of 4,300,000 acre feet per year. Water diversion structures have been designed on the Mahaweli Ganga, the Amban Ganga, and the right bank tributaries of the main river, to obtain, apart from the regulated yield, 404,000 acre feet of the unregulated flow (side inflow). The total assured water release is therefore 4,704,000 acre feet, or 73 percent of the main yield of the basin. Further, it has been estimated that an additional quantity of water can be by regulating the smaller rivers and making use of the existing tanks. Design studies for on the rivers Maduru Oya and Ulhitiya Oya showed it would be possible to store 142,000 acre feet on the Maduru Oya site and 45,000 acre feet on the Ulhitiya Oya site. The present regulated yield from rivers in the north-central part of the island, together with the smaller rivers in the Mahaweli Gana basin, which have not been included in the above estimate, amounts to 689,000 acre feet.

 

        The total utilizable water resources of the surveyed area amount to 5,580,000 acre feet per year. This quantity of water would be sufficient to irrigate approximately 700,000 acres of paddy or, alterna­300,000 acres under cotton, groundnut, chillies or other high value crops.

 

        Table V-3 shows the yields of the various proposed source of water supply, including the fifteen dam which have been designed. All except four of these dams (the Rotalawela, Kalu Ganga, Maduru Oya, Ulhitiya Oya) are to be used to generate electricity as well as to supply water.

 

        Besides the multipurpose units listed in Table V-3, another power station is proposed on the trans basin canal to Kala Oya. The parameters of the twelve power stations are summarized below:

           

            Installed capacity                                                               508    MW
            Firm power                                                                       233    MW
            Average annual energy output                                         2,611    million kWh
            Firm annual energy output                                               2,037    million kWh

 

The above account of the proposed use of water and power resources is based on the finalization of designs during the second stage of the survey. The principal change made to the earlier designs has been to exclude one of the 16 hydro units designed earlier, the Hassalaka Oya, which was clearly uneconomical.

 

TABLE V-3

 

Yield of Main Proposed Sources of Supply

(1,000 acre feet)

 


Proposed Reservoirs on                 Regulated    Existing and Proposed                         Local  

Mahaweli Ganga and                      Net Row                Tanks                                         Yield

its tributaries

 


1-2  Victoria Randenigala                     1,392        Tanks in system D- 1, G (Minneriya­-         

                                                                       Kantalai)                                                      75
Kotmale-Polgolla                            475         Tanks in system C                                        14
Moragahakanda                          1,396         Parakrama Tank (D-2)                                 10

         Rotalawela*                                   120         Vakaneri Tank                                             43

         Kalu Ganga*                                  172         Tanks in system H (Kala Oya)                    195

         Upper-Lower Uma Oya                 317         Tanks in system I (Malwatu Oya)                156

         Heen Ganga                                   160         Tanks in system J (Parangi Aru-Pali Aru)      43
         Taldena                                          134         Tanks in system K(Kanagarayan Ant)            2
         Pallewela                                        134         Tanks in system L (Ma Oyal                         51

         Bowatenne                                       20**      Tanks in system M (Yan Oya)                    120

 


         Total                                            4300         Total                                                         689

 


   *Hydro units without power stations.

**Reservoir storage not included’ in the sum.

 

 


Anicuts                        Side                 Proposed Reservoirs on other                              Local

                                    Inflow                          Rivers                                                         Yield

 


Kandakadu                  266                  14  Maduru Oya*                                                      142

Angamedilla                  110                  15  Ulhitiya Oya*                                                        45

Right Bank tributaries     28

 


      Total                      404                              Total                                                             187

 


Grand Total            4,704                           Grand Total                                                   876

 


      Total utilizable water resources in the project are =                                                        5,580

 

     

 * Hydro units without power stations.