Modernization of the left bank irrigation system of the Kaudulla reservoir of Sri Lanka

The Kaudulla reservoir (the Kaudulla tank) and the areas irrigable from the tank fall under Mahaweli system ‘D’ and are situated in Medirigiriya District Secretariat Division of the North Central Province of Sri Lanka (Figure 1). Drought is frequently experienced in the region; for example, 1,110 people from 350 families in Medirigiriya were affected by drought in the latter part of 2017 (Jayarathne 2016). Water scarcity for irrigated paddy cultivation in the area has led to poor cropping intensity, especially in the Yala (April–August) season, which is one of the two seasons in which paddy is cultivated every year, the other being Maha (November–March). An irrigation water management study implemented to investigate the efficiency under normal operating conditions at Kaudulla indicates that water management is comparatively efficient during Yala even with the limited water resource, but that rainfall over the demand area could be used more effectively as a substitute for irrigation water during the Maha season (Holmes et al. 1978). A case study carried out centering on the Wadigewewa and Nikahena small tanks in the irrigation area strongly recommends an increase of the tank capacities to store the ample rainwater received in the rainy season (Rekha 2014).

The existing cultivation areas amounting to 7,160 ha have the potential to receive water from the left bank (LB) canal of the Kaudulla reservoir (the Kaudulla tank). However, only 3,790 ha are supplied directly by the left bank canal at present, and the rest is by 35 rainfed small village tanks. Limited storage capacities of the tanks, low rainfall in the dry season and regulated inflow to the Kaudulla tank have caused a significant temporal variation in paddy yield in the Kaudulla LB command area. It is against this background that this study is being carried out to propose appropriate modernization of the existing irrigation system to increase the yield derived from the LB command area. The study develops a model for water management in the left bank area to select the best intervention to maximize the return from investment while producing satisfactory yield under limited water availability, to support the farming families. The HEC-HMS model was applied to calculate the catchment yields of 35 village tanks. The results of the crop water requirement generated from the CROPWAT model were used with WEAP to evaluate the water balance and demand area coverage to identify the best investment for improving irrigation water supply to maximize the return. The best intervention among a few feasible structural interventions was identified based on the economic return.

A calibrated and validated rainfall–runoff model generated using HEC-HMS was applied to calculate the catchment yields of 35 village tanks under the left bank canal of the Kaudulla tank (Figures 1 and 2; Table 1). The results of the crop water requirement generated from the CROPWAT model were used with WEAP to evaluate the water balance and demand area coverage.

An analysis was carried out using the CROPWAT and WEAP model applications to identify the best investment for improving irrigation water supply to maximize the return.

LB canal irrigation areas are supplied with water by different methods. The cultivation area at the side of the first section of the LB canal is supplied with water directly from the canal through the anicut (weir) system. The rest of the area falls under the command areas of 35 tanks and is supplied with water from the tanks, and these are supplemented by the LB canal of the Kaudulla tank to varying degrees depending on the availability of water in the Kaudulla tank.

Accordingly, two cases are considered for the analysis of irrigation water requirements in the LB canal irrigation area:

• 1.

  The irrigation demands of the area under minor tanks are met only by the tanks that are under improvement of the system independent of water from the Kaudulla tank, except for the anicut system. Simulation is done considering that the tanks receive inflow from their own catchments.

Five sub-cases are considered for simulation with augmentation of tank capacities under two options as per Table 2 and Figures 1 and 2.

• 2.

  The unmet irrigation demands of the area under minor tanks under Case 1 above are also met from the Kaudulla tank as shown in Figure 3.

Both cases are financially analyzed using the net present value method to ascertain the feasibility of the proposals.

The proposal for the new canal system is shown in Table 3 for supplying water to the village tanks to supplement the water release requirements in the season from these tanks. Feeder canals FC1–FC13 are considered for Case 1, and FC1–FC25 are considered for Case 2. Most of the feeder canals are tributaries of the Ambagasoya stream.

The study carried out for the period of 2013–2017 reveals that the water available in the Kaudulla tank, which is fed by its own catchment and from the controlled releases from the Minneriya tank at present, is not sufficient to cultivate 3,790 ha of paddy area fed by the left bank canal in the Yala season (Figure 4). Enhanced diversion from the Minneriya tank is unlikely in the near future due to the limited water allocation to the tank from the Mahaweli river development scheme.

Therefore, under this situation, it is not realistic to consider feeding the minor tanks from the Kaudulla tank during the water scarce months in the Yala season. Thus, the demand areas of the existing village tanks have to depend on their own catchment yields to cultivate 3,370 ha of land. Different investment options were considered, including the augmentation of eight selected tanks and construction of new canals.

The unmet irrigation demands of the area under minor tanks of the existing system that are not dependent on water from the Kaudulla tank and are instead fed from inflows from their own catchments, for the period 2013–2017, are shown in Figure 5.

The analysis reveals that the annual unmet water demand from 2013 to 2017 varies from 33.75 to 51.86 MCM. The highest unmet demand is shown in the year 2015. Figure 6 shows the monthly unmet demand in 2015.

The maximum monthly unmet water demand of 16.58 MCM is seen in July 2015 of the Yala season.

A proposal for a new canal system is shown in Table 3. Feeder canals FC1–FC13 are considered for Case 1, and FC1–FC25 are considered for Case 2. Most of the feeder canals are tributaries of the Ambagasoya stream.

Figure 7 illustrates the results of the simulation carried out for monthly unmet demand for 2015 with a proposed improvement to the canal system of Case 1-b. The improvement reduces the unmet water demand from 16.58 to 16.35 MCM in July 2015, which is the most water-stressed month of the Yala season.

Figure 8 is the result of a decrease of unmet water demand to 16.35 MCM for the proposed new canal system with the augmentation of tank capacities without inundation of upstream infrastructures as described in Table 4 for July 2015 in the Yala season.

Figure 9 shows the decrease in unmet water demand to 14.78 MCM in July 2015 in the Yala season as a result of the proposed new canal system with augmentation of tank capacities of Option 1 in Table 4.

Figure 10 shows a further decrease in unmet water demand to 11.37 MCM in July 2015 in the Yala season because of the proposed new canal system with augmentation of tank capacities of Option 2 in Table 4.

The demand area coverage shown in Figure 11 of 12 selected demand sites varies from 0 to 100% in five cases analyzed in Case 1 for July 2015.

Figure 12 shows the unmet demand for Case 2 for 100% crop intensity.

Case 2 is shown to be the most favorable to reduce the water scarcity in the study area. However, this option is possible only if sufficient water is supplied from the Kaudulla tank to the minor tanks. It is evident from Figure 3(1) that even the supply of irrigation water from the Kaudulla left bank canal to the left bank irrigated area is not fulfilled in the Yala season. Hence, it is not practical to increase the cropping intensity by 100% as described in Case 2, unless there is an enhancement of diversion from Minneriya or Minneriya-Kanthale Yoda Ela to supplement these demands through the Kaudulla tank.

The following analysis is carried out based on HSR 2016 (Government of Sri Lanka approved schedule of rates −2016) for construction.

Prime cost for the construction of the proposed feeder canal = Rs. 17,120 per meter length

(Capacity: 0.58 m³/s and Reinforced Cement Concrete-lined (RCC) Canal: 1.2 m × 0.75 m)

Prime cost for augmentation of tank capacity by 1 MCM = Rs. 5,000,000

Prime cost for land acquisition including infrastructures = Rs. 6,000,000/ha

It is assumed that the inflation rate of 7.6% as at 2017 remains unchanged.

Table 5 shows the summary of financial analysis for Case 1.

Payback periods calculated using the net present value by considering 7.6% inflation rate for prime cost and turnover from the yield increment are shown in Table 5. Case 1-e shows that yield can be increased by Rs. 205 million by increasing the total cultivation area in the season through irrigation using minor tanks alone without depending on the water from the left bank canal of Kaudulla tank. That is, an increased area coverage and minimum payback period by augmenting tanks and constructing the proposed canals in Case 1-e.

An economic analysis was carried out using the net present values, and it was found that the option that includes the construction of a new canal system and the augmentation of capacities of village tanks, namely Nikahenawewa, Elabatuwewa, Babiyawewa, Halmillawewa, Meegaswewa, Migollawewa, Pathokwewa and Wadigewewa, from the total capacity of 3.8–20 MCM would be the most appropriate option. This intervention (a) increases the income from yield by 205 million with a payback period of 12 years and (b) also increases the area coverage of cultivation under the left bank canal cultivation area of the tank in the Yala season.

Irrigation water management in the left bank canal development area under the Kaudulla reservoir in the North Central Province of Sri Lanka was analyzed by the application of the HEC-HMS, CROPWAT and WEAP models. The best intervention among a few feasible structural interventions was identified based on the economic return.

Accordingly, the construction of a few new canals and augmentation of capacities of village tanks, namely Nikahenawewa, Elabatuwewa, Babiyawewa, Halmillawewa, Meegaswewa, Migollawewa, Pathokwewa and Wadigewewa tanks, to increase the total capacity from the present 3.8 to 20 MCM was found to be the most appropriate option. This intervention increases the income from the yield by 205 million with a payback period of 12 years.

All relevant data are included in the paper or its Supplementary Information.