Abstract

Irrigation water management in the paddy cultivation area under the left bank canal of the Kaudulla reservoir (Kaudulla tank) in the North Central Province of Sri Lanka has become a serious issue due to limited water availability and inefficient water distribution infrastructure at present. Insufficient storage capacities of the village tanks in the cultivation area, low rainfall during some months and regulated inflow from the transbasin diversion to Kaudulla tank have had a significant impact on yield in the two cultivation seasons of the year. In this article, modernization of irrigation infrastructure in the command area was investigated for effective utilization of limited available water. The results of the calibrated and validated Hydrologic Engineering Centre - Hydrologic Modeling System (HEC-HMS) model and Crop Water and Irrigation Requirements Program (CROPWAT) model were used with Water Evaluation and Planning Model (WEAP) to evaluate the water balance and demand to identify the best investment for improving irrigation water supply to maximize the return. Economic analysis was carried out using the net present values for different modernization options. Accordingly, the construction of a new canal system and augmentation of the capacities of village tanks from the present total capacity of 3.8–20 MCM was found to be the most appropriate option. This intervention will increase the income from yield by 205 million with a payback period of 12 years in the Yala season.

HIGHLIGHTS

  • Insufficient storage capacities of minor tanks, low rainfall during some months in the tank catchments and regulated diversion to the Kaudulla tank have had a significant impact on the cultivation area under Kaudulla tank situated in the North Central Province of Sri Lanka.

  • A calibrated and validated HEC-HMS model and the CROPWAT model were used with WEAP to evaluate the water balance and demand to identify the best investment for improving the irrigation system based on the economic return.

INTRODUCTION

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).

Figure 1

Existing irrigation system used for the WEAP model.

Figure 1

Existing irrigation system used for the WEAP model.

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.

METHODOLOGY

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.

Table 1

Details of minor tanks

No.Tank nameCoordinates
Capacity (MCM)Irrigable land (ha)Catchment area (ha)
Easting (m)Northing (m)
Damsopura wewa 231,116 328,505 0.116 371 53 
Aluthwewa 233,321 323,600 1.111 216 254 
Illukpitiya wewa 233,787 326,535 0.284 102 93 
Gaminipura wewa 234,809 326,912 0.289 72 175 
Rambawewa 233,093 327,614 0.305 88 94 
Meegaswewa 234,236 332,918 0.261 85 139 
Pathokwewa 234,214 334,368 0.174 56 53 
Nikahenawewa 236,371 333,835 0.222 76 202 
Sathpaththini 234,368 327,228 0.033 22 58 
10 Elabatuwewa 237,997 332,665 0.347 78 242 
11 Baybiya wewa 231,974 331,352 1.406 128 726 
12 Kumbukunawa 235,681 331,505 0.958 71 366 
13 Wadiga wewa 235,208 336,386 0.579 187 155 
14 Kuda wewa 235,810 332,170 0.044 28 24 
15 Badde wewa 234,172 333,500 0.043 15 17 
16 Jayagampurawewa 230,154 329,648 0.048 32 44 
17 Ekamuthu wewa 230,206 329,119 0.030 25 33 
18 Rota wewa 234,569 330,037 0.173 262 177 
19 Naripotawewa 236,809 332,975 0.059 32 47 
20 Kadurugaswewa 232,616 336,034 0.102 74 95 
21 Aruna wewa 234,501 324,480 0.100 159 73 
22 Wedikachchiya 225,089 332,873 0.579 48 63 
23 Halmillawewa 226,406 333,418 0.740 172 97 
24 Weeragollawa wewa 229,735 335,670 0.050 50 70 
25 Migollawawewa 231,579 335,525 0.056 97 107 
26 Dambagodawela wewa 236,589 326,387 0.180 44 13 
27 Palliyagodella wewa 235,476 325,394 0.320 337 73 
28 Unakatuwa wewa 237,414 335,471 0.055 12 43 
29 Kumarakanda wewa 238,034 336,341 0.060 203 139 
30 Gallendiwala wewa 233,346 336,108 0.040 24 110 
31 Tank T 221,570 331,824 0.035 26 28 
32 Tank U 222,321 333,040 0.045 47 68 
33 Tank W 224,551 335,025 0.030 44 107 
34 Tank V 226,649 334,823 0.065 49 104 
35 Tank S 228,155 335,218 0.055 39 54 
No.Tank nameCoordinates
Capacity (MCM)Irrigable land (ha)Catchment area (ha)
Easting (m)Northing (m)
Damsopura wewa 231,116 328,505 0.116 371 53 
Aluthwewa 233,321 323,600 1.111 216 254 
Illukpitiya wewa 233,787 326,535 0.284 102 93 
Gaminipura wewa 234,809 326,912 0.289 72 175 
Rambawewa 233,093 327,614 0.305 88 94 
Meegaswewa 234,236 332,918 0.261 85 139 
Pathokwewa 234,214 334,368 0.174 56 53 
Nikahenawewa 236,371 333,835 0.222 76 202 
Sathpaththini 234,368 327,228 0.033 22 58 
10 Elabatuwewa 237,997 332,665 0.347 78 242 
11 Baybiya wewa 231,974 331,352 1.406 128 726 
12 Kumbukunawa 235,681 331,505 0.958 71 366 
13 Wadiga wewa 235,208 336,386 0.579 187 155 
14 Kuda wewa 235,810 332,170 0.044 28 24 
15 Badde wewa 234,172 333,500 0.043 15 17 
16 Jayagampurawewa 230,154 329,648 0.048 32 44 
17 Ekamuthu wewa 230,206 329,119 0.030 25 33 
18 Rota wewa 234,569 330,037 0.173 262 177 
19 Naripotawewa 236,809 332,975 0.059 32 47 
20 Kadurugaswewa 232,616 336,034 0.102 74 95 
21 Aruna wewa 234,501 324,480 0.100 159 73 
22 Wedikachchiya 225,089 332,873 0.579 48 63 
23 Halmillawewa 226,406 333,418 0.740 172 97 
24 Weeragollawa wewa 229,735 335,670 0.050 50 70 
25 Migollawawewa 231,579 335,525 0.056 97 107 
26 Dambagodawela wewa 236,589 326,387 0.180 44 13 
27 Palliyagodella wewa 235,476 325,394 0.320 337 73 
28 Unakatuwa wewa 237,414 335,471 0.055 12 43 
29 Kumarakanda wewa 238,034 336,341 0.060 203 139 
30 Gallendiwala wewa 233,346 336,108 0.040 24 110 
31 Tank T 221,570 331,824 0.035 26 28 
32 Tank U 222,321 333,040 0.045 47 68 
33 Tank W 224,551 335,025 0.030 44 107 
34 Tank V 226,649 334,823 0.065 49 104 
35 Tank S 228,155 335,218 0.055 39 54 
Figure 2

Proposed irrigation system used for Case 1.

Figure 2

Proposed irrigation system used for Case 1.

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.

Method of analysis

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.

Figure 3

Proposed irrigation system used for Case 2.

Figure 3

Proposed irrigation system used for Case 2.

Table 2

Options for Case 1

ItemTank nameCapacity (MCM)
ExistingProposedPotential increase without acquisition of additional landsIncrease through acquisition of additional lands
Option 1 Nikahena 0.222 0.44 2.56 
Elabatuwewa 0.347 2.5 0.71 1.79 
Option 2 Babiya 1.406 1.45  2.55 
Halmiila 0.74 0.84 1.16 
Meegaswewa 0.261 1.5 0.42 1.08 
Migollawa 0.056 0.24 3.76 
Pathowewa 0.174 0.21 0.79 
Wadigewewa 0.579 0.74 1.26 
ItemTank nameCapacity (MCM)
ExistingProposedPotential increase without acquisition of additional landsIncrease through acquisition of additional lands
Option 1 Nikahena 0.222 0.44 2.56 
Elabatuwewa 0.347 2.5 0.71 1.79 
Option 2 Babiya 1.406 1.45  2.55 
Halmiila 0.74 0.84 1.16 
Meegaswewa 0.261 1.5 0.42 1.08 
Migollawa 0.056 0.24 3.76 
Pathowewa 0.174 0.21 0.79 
Wadigewewa 0.579 0.74 1.26 

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.

Table 3

Proposed feeder canal system

Feeder canalLength (m)FromTo
FC1 708 Kudawewa Naripotawewa 
FC2 264 Naripota Nikahenawewa 
FC3 209 Meegaswewa Baddewewa 
FC4 584 Baddewewa Pathokwewa 
FC5 2,726 Rotawewa Meegaswewa 
FC6 148 Kumbukunawawewa Kudawewa 
FC7 1,639 Gaminipurawewa Dambagodawelawewa 
FC8 1,281 Rambawewa Illukpitiyawewa 
FC9 1,143 Rambawewa Sathpaththiniwewa 
FC10 1,015 Illukpitiya wewa Anurawewa 
FC11 1,726 Ambagasoya bifurcation Migollawa wewa 
FC12 365 Rotawewa Kumbukunawa 
FC13 248 Ekamuthuwewa Jayagampurawewa 
FC14 15,108 LB Canal bifurcation Migollawa wewa 
FC15 383 FC 14 bifurcation Tank T 
FC16 746 FC 14 bifurcation Tank U 
FC17 3,544 FC 14 bifurcation Wedikachchiya wewa 
FC18 291 FC 14 bifurcation Weeragollawa wewa 
FC19 434 FC 14 bifurcation Tank S 
FC20 1,232 FC 14 bifurcation Tank V 
FC21 936 FC 14 bifurcation Tank W 
FC22 3,273 LB Canal bifurcation Damsopura wewa 
FC23 2,857 FC 22 bifurcation Rambawewa 
FC24 3,100 FC 23 bifurcation Rotawewa 
FC25 700 FC 22 bifurcation Ekamuthuwewa 
Feeder canalLength (m)FromTo
FC1 708 Kudawewa Naripotawewa 
FC2 264 Naripota Nikahenawewa 
FC3 209 Meegaswewa Baddewewa 
FC4 584 Baddewewa Pathokwewa 
FC5 2,726 Rotawewa Meegaswewa 
FC6 148 Kumbukunawawewa Kudawewa 
FC7 1,639 Gaminipurawewa Dambagodawelawewa 
FC8 1,281 Rambawewa Illukpitiyawewa 
FC9 1,143 Rambawewa Sathpaththiniwewa 
FC10 1,015 Illukpitiya wewa Anurawewa 
FC11 1,726 Ambagasoya bifurcation Migollawa wewa 
FC12 365 Rotawewa Kumbukunawa 
FC13 248 Ekamuthuwewa Jayagampurawewa 
FC14 15,108 LB Canal bifurcation Migollawa wewa 
FC15 383 FC 14 bifurcation Tank T 
FC16 746 FC 14 bifurcation Tank U 
FC17 3,544 FC 14 bifurcation Wedikachchiya wewa 
FC18 291 FC 14 bifurcation Weeragollawa wewa 
FC19 434 FC 14 bifurcation Tank S 
FC20 1,232 FC 14 bifurcation Tank V 
FC21 936 FC 14 bifurcation Tank W 
FC22 3,273 LB Canal bifurcation Damsopura wewa 
FC23 2,857 FC 22 bifurcation Rambawewa 
FC24 3,100 FC 23 bifurcation Rotawewa 
FC25 700 FC 22 bifurcation Ekamuthuwewa 

RESULTS AND DISCUSSION

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.

Figure 4

Unmet water demand under the Kaudulla left bank canal.

Figure 4

Unmet water demand under the Kaudulla left bank canal.

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.

WEAP model results

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.

Figure 5

Unmet demand for Case 1-a from 2013 to 2017.

Figure 5

Unmet demand for Case 1-a from 2013 to 2017.

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.

Figure 6

Monthly unmet demand for Case 1-a for 2015.

Figure 6

Monthly unmet demand for Case 1-a for 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 7

Monthly unmet demand for Case 1-b for 2015.

Figure 7

Monthly unmet demand for Case 1-b for 2015.

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.

Table 4

Scenarios for Case 1

CaseDescription
1-a Present condition of tanks and canal system as in Figure 1  
1-b Proposed improvement to the canal system as in Figure 2  
1-c 1-b and alternate by raising of spill level without inundation of upstream of eight tanks in Table 2  
1-d 1-b and augmentation of proposed tank capacities of Option 1 as in Table 2  
1-e 1-d and augmentation of proposed tank capacities of Option 2 as in Table 2  
CaseDescription
1-a Present condition of tanks and canal system as in Figure 1  
1-b Proposed improvement to the canal system as in Figure 2  
1-c 1-b and alternate by raising of spill level without inundation of upstream of eight tanks in Table 2  
1-d 1-b and augmentation of proposed tank capacities of Option 1 as in Table 2  
1-e 1-d and augmentation of proposed tank capacities of Option 2 as in Table 2  
Figure 8

Monthly unmet demand for Case 1-c for 2015.

Figure 8

Monthly unmet demand for Case 1-c for 2015.

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 9

Monthly unmet demand for Case 1-d for 2015.

Figure 9

Monthly unmet demand for Case 1-d for 2015.

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.

Figure 10

Monthly unmet demand for Case 1-e for 2015.

Figure 10

Monthly unmet demand for Case 1-e for 2015.

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 11

Demand area coverage for Case 1.

Figure 11

Demand area coverage for Case 1.

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

Figure 12

Unmet demand for Case 2.

Figure 12

Unmet demand for Case 2.

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.

Financial analysis

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.

Table 5

Summary of cost–benefit for the proposed system

No.DescriptionPrime cost (Rs. million)
Yield (Rs. million)Yield increment (Rs. million)Payback period using enhancement (years)
CanalTanksLand acquisitionTotal
Case 1-a       
Case 1-b 206   206 >100 
Case 1-c 206  212 >100 
Case 1-d 206 25 390 621 71 71 15 
Case 1-e 206 81 1,266 1,553 205 205 12 
No.DescriptionPrime cost (Rs. million)
Yield (Rs. million)Yield increment (Rs. million)Payback period using enhancement (years)
CanalTanksLand acquisitionTotal
Case 1-a       
Case 1-b 206   206 >100 
Case 1-c 206  212 >100 
Case 1-d 206 25 390 621 71 71 15 
Case 1-e 206 81 1,266 1,553 205 205 12 

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.

CONCLUSIONS

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.

DATA AVAILABILITY STATEMENT

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

REFERENCES

Holmes
D.W.
,
Batchelor
C. H.
,
Gunston
H.
&
Dawson
R. J.
1978
Water Management Study at Kaudulla Irrigation Scheme, Sri Lanka
.
The Irrigation Department
,
Sri Lanka
.
Jayarathne
J. M. A. R.
2016
Country Report: Democratic Socialist Republic of Sri Lanka
.
Ministry of Disaster Managment
,
Sri Lanka
.
Rekha
Nianthi K. W. G.
2014
Farmers’ Responses to Drought: Dry Zone of Sri Lanka: (Case Study in Medirigiriya). Int. Journal of Case Studies 5(6)
.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).