3. WATER REQUIREMENTS AND SUPPLIES

 

3.1   Earlier Studies

 

3. 1. 1    Water balance studies for the diversion of Mahaweli waters to develop 231,000 acres of land in areas H, I (H), D and G were carried out by Messrs. Engineering Consul­tants Incorporated (E. C. I.), Dever, Colorado. On the basis of these studies the capacities of the Polgolla and Bowatenna tunnels were fixed at 2,000 c.f.s. and 1,000 c.f.s. respectively. The water requirements for the farms were computed on the basis of tentative data on evapo-transpiration and cropping patterns available at that time.

 

3. 1. 2    Since then more experimental data on evapo-transpiration requirements and field water losses have been obtained from field studies carried out at the Agricultural Research Station, Maha Illuppallama. Cropping patterns for the project area have also been determined based on such factors as land classification and climate, market demands and the requirements of the national plans. A fresh water balance study has therefore been carried out using the modified values for the project farms as well as the existing lands in acres H & 1(H) under Stage I.

 

3.2  Water Requirements

 

Irrigation water requirements have been evaluated from the consumptive use data presented in the U.N.D.P. report of 1968 where the Penuan’s formula was selected because it fitted the observed results of trials made at the Maha Illuppallama Research Station. The farm losses have been evaluated on the basis of the research data, at 30 - 50%. The consumptive use and the average water requirement per acre of farm area for the assumed standard cropping pattern are given in Table 14.

 

3. 2; 1    The total water requirements for the farms (consumptive use plus field losses) in areas H and 1(H) including the existing lands under Stage I assuming no rainfall is 915.6 thousand acre feet.

 

3. 2. 2    Water losses in conveyance from reservoir to farmgate is assumed to b~. 30% of the reservoir releases. The losses due to evaporation in the reservoir are estimated at 6.0 inches per month for the period April to September and 4.5 inches per month for the other six months. These values are based on the results of field tests carried out by the Department of Irrigation.

 

2. 2. 3    The water losses from the farms and channel network is partly retrieved in the lower reservoirs of the system for reuse. It is assumed that 30% of the water lost is retrieved for reuse within the system as return flow.

 

3.3  Water Supply

 

3.3.1      The water requirements for the farms are not partly from the effective rainfall in the farms, partly from the inflow into the reservoirs from the local catchments and partly by diversion of the Mahaweli waters through the Bowatenna- Kala Oya tunnel.

 

3. 3. 2    The effective precipitation is based on the arithmetic mean monthly rainfalls recorded at Maha Illuppallama and Anuradhapura. These values are also shown in Table 14.

 

3. 3. 3    The local inflows into the major reservoirs within the system H & I (H) from their individual catchments were computed by Messrs. ECI for the 25 year period 1945-68 and was published in their report. The catchments areas, inflows from local catch­ments, capacities of the major reservoirs and the extents of farm area under them are given in Table 13.

TABLE 13 — Inflows into Major Reservoirs and their capacities

 

 


                                  Catch-      Av :         Max:     Mm:      Gross       Nett         Farm area in
      Reservoir                ment      inflow       inflow   inflow     capa-       capa-               acres
                                     area      103 Ac.    103Ac. 103Ac.    city           city         Stage    Stage

_________________Sq. Ml.        ft.            ft.         ft.      103Ac.ft. 103Acft.       1          2

Area H

Kandalama        ..           38          25.1        65.2         4.9       27.4         23.8       4,000     14,000
Kalawewa         ..         325         166.4      395.3       57.4      100.0         96.8
Mahallluppallama                                                                    4.5           4.0       13,000    57,000
Kattiyawa          .            53          12.3        31.8         2.7         2.8           2.6
Usgala Siyambalan­-
   gamuwa         ..           71          41.7        98.7       15.0       22.0         20.8       1, 500        -

Rajangana         ..         622         103.4      245.6      35.9       81.6         76.6     17, 000
Angamuwa        ..           50          33.0        85.7         6.3       12.8         10.4

 

Area IH

                                                                                              

Nachchaduwa                                                                       45.3         44.0 

Nuwara Wewa ..           275        108.3      468.9       32.4       36.1         31.2        10,000       —

Basawakkulama                                                                     1.9           1.9

Tisawewa          ..                                                     2.9         2.9

 

Total     ..                          562.2    1,391.2    154.6    337.2      315.0    45,500      71,000

3. 3. 4    The quantity of diversion from the Mahaweli Ganga into the area H & 1(H) is adopted from the water balance studies of Messrs ECI. These studies were prepared on the assumption that the service areas D & 0 would receive priority for the Mahaweli waters. The average annual diversion for the 24 year period 1945-68 would be 409.0 thousand acre feet.

 

3.4   Water Balance

 

3. 4. 1  A fresh water balance study was carried out at monthly intervals for the 24 year period 1945-68 adopting the revised values for field water requirements.

 

3. 4. 2    The study indicates that the existing reservoirs (with the projected increase in the capa­city of the Kala Wewa to 100,000 acre feet) have sufficient storage capacity to meet the requirements of Stage I as well as those of the proposed lands under Stage II. Water shortages varying from about 1.05 to 12.3% of the corresponding annual requirements will be experienced in ten years out of the 24 year period of study as shown in Table 15.

 

TABLE 15

 

Water Shortages

 


                                Year                       1945     1947     1948     1950     1953     1956     1957     1966     1967     1968
                                Total demand
                                 in 103 ac.ft           1296     1288     1265     1329     1240     1336     1264     1279     1284     1265

 

Shortage in

103 ac. ft        114.1          13.7    155.3    74.6    74.5      144.2       70.1     71.1    154.0      57.5

 

Shortage is

                                     I of demand     8.8      1.05      12.3          5.6        6.0        8.6        5.6       5.6       12.0        4.5

 

 


3. 4. 3        It will be seen that there will be no shortage during 14 out of the 24 years while the shortage will be between 0 & 9% during 8 out of the 24 years and between 10 & 12.5% during the balance 2 years.

 

3. 4. 4    The study also indicates that the shortages occur during the period July to September when the flow in the Mahaweli Ganga is low and that the spill go from the local reser­voirs occur during the period December - February. The quantities spilled each year are generally small in comparison to the corresponding shortages. Hence further increase of the capacities of these reservoirs will not be very effective in red acing shortages.

 

3. 4. 5    It is expected that the indicated water shortages could be overcome by efficient water use.

 

3.5   Water distribution

 

3. 5. 1    The main and distributory channels will flow almost all the time. Constant and con­trolled discharges will be made possible by water level regulators and baffle modules.

 

3. 5. 2    Turn out area grouping of 10 to 15 farms is considered as the basic irrigation unit within which the full discharge of the field channel will be directed for issue to the farms on a rotation basis. The conveyance capacity of each field channel has been standardised at one c.f.s. Turn out discharge will be one c.f.s. at peak periods and can be reduced according to requirements. When the required discharge falls below 0.5 c.f.s. instead of reducing the discharge, two turn out areas each receiving one c.f.s. can be combined for purposes of rotation.

 

3. 5. 3    The time of irrigation at peak periods is taken as 16 hours per day (5-21 hours) for upland crops and 24 hours for paddy. Some turn outs irrigate both paddy and upland crops. While the upland crops can receive irrigation water during the day time, the paddy will be irrigated during the night. This will most often be the case since paddy fields are scattered in almost all turn out areas.

 

3. 5. 4        In the case of turn out areas where only upland crops are grown, water supply will have to be interrupted at night. The distributary discharge can be conserved at night by either reducing the flow in the distributary and carrying the excess water in the main channel to a regulator tank, or carrying the excess water in the distributary itself to a regular tank. This type of operation requires that village tanks located at strategic points be maintained for a flexible operation.

 

3. 5. 5    Water issue to the fields is effected on the basis of a predetermined irrigation calendar prepared for the whole network at the beginning of each cultivation season. In the early stages of the operation of the project, it is desirable that the distribution of water to the turn outs be in the hands of the operating staff. The responsibility for distribu­tion within a turn out area will be vested with the farmers. The design of the network provides for measurement of the water up to the turn out. The quantity delivered at a turn out will be measured both for purposes of efficient control as well as for payment on the basis of quantity delivered if so required.