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A Strategic Approach to Enhance the System Efficiency of Village Tanks a demonstration from the Kapiriggama cascade system

IUCN programme on Restoring Traditional Cascading Tank Systems Technical Note # 5 A Strategic Approach to Enhance the System Efficiency of Village Tanks a demonstration from the Kapiriggama cascade system

IUCN programme on Restoring Traditional Cascading Tank Systems Technical Note # 5 The designation of geographical entities in this book, and the presentation of the material, do not imply the expression of any opinion whatsoever on the part of IUCN concerning the legal status of any country, territory, or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries.

The views expressed in this publication do not necessarily reflect those of IUCN

Published by: IUCN, Sri Lanka Country Office

Copyright: © 2016 IUCN, International Union for Conservation of Nature and Natural Resources

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Citation: IUCN (2016). A Strategic Approach to Enhance the System Efficiency of Village Tanks: a demonstration from the Kapiriggama cascade system. IUCN programme on Restoring Traditional Cascading Tank Systems Technical Note # 5. Colombo: IUCN, International Union for Conservation of Nature, Colombo, Sri Lanka & Government of Sri Lanka. iii + 20 pp.

ISBN: 978-955-0205-35-6

Lead contributor: Dr. P. B. Dharmasena, Consultant, IUCN

Cover photo: mound formed with the soil dredged during partial desiltation at Maha Kadiyawa tank, Kumudu Herath © IUCN;

Layout by: Padmi Meegoda

Produced by: IUCN, Sri Lanka Country Office

Available from: IUCN, Sri Lanka Country Office 53, Horton Place Colombo 7, Sri Lanka Phone: ++94-011-2694094, 2682418, Fax: 2682470 http:// iucn.org/

i Table of Contents

Table of Contents ...... ii List of Figures ...... iii List of Tables ...... iii 1. Introduction ...... 4 2. The concept of partial desiltation ...... 7 3. Benefits of partial desiltation ...... 8 3. 1 Increased extent for cultivation ...... 8 3.2 More agriculturally productive lands ...... 8 3.3 Increased cropping intensity ...... 9 3.4 Opportunities for cottage industries ...... 9 3.5 Freshwater fishery ...... 9 4. Methodology for partial desiltation ...... 9 4.1 Tank bed level survey ...... 9 4.2 Tank bed survey ...... 10 4.3 Partial desiltation design ...... 10 4.4 Implementation ...... 12 5. Impact assessment ...... 12 5.1 Before partial desiltation ...... 12 5.2 At partial desiltation ...... 12 5.3 After partial desiltation ...... 12 5.4 Progress indicators ...... 12 6. Project achievements ...... 13 7. Recommendations for the Future ...... 15

ii List of Figures

Figure 1. Cropping intensity and rainfall ...... 4 Figure 2. Probability of cropping intensity under minor irrigation in Anuradhapura ...... 5 Figure 3. Effect of tank bed geometry on water loss ...... 6 Figure 4. Design of the Partial Desilting method ...... 7 Figure 5. Formation of soil mounds ...... 8 Figure 6. Tank bed contour map ...... 10 Figure 7. Part of the tank bed is now free of water...... 11 Figure 8. Partial desiltation of Puliyankulama tank ...... 11 Figure 9. Partially desilted tanks ...... 14 Figure 10. Upstream earthen ridge formed with the soil dredged during partial desiltation Maha Kadiyawa tank (Kumudu Herath © IUCN) ...... 15

List of Tables

Table 1. Quantity of removed from various tanks ...... 13

iii

1. Introduction

Minor tanks in the dry and intermediate zones of Sri Lanka are gradually becoming inefficient in providing an assured supply of irrigation water because of changes occurring in the size and shape of the tank bed. Water bodies become shallower as a consequence of sedimentation.

An enormous amount of foreign aid and loans has been diverted, in the last few decades, to tank rehabilitation and restoration. In these programmes, the standard actions were to strengthen the tank bund, repair or replace structures, and regain the capacity lost due to sedimentation by raising the spill and the tank bund, thereby increasing the water holding capacity of the tank. However, because the tank is still silted, the water spread also increased, resulting in the formation of larger, shallower water bodies. Shallower, larger water bodies lose more water through evaporation and percolation. When a comparison was made between amount of rainfall in maha season and annual cropping intensity during the period from 1970 - 2003 in Anuradhapura district, it showed that the response of cropping intensity of minor irrigation schemes to rainfall has not varied with time (Figure 1). The cropping intensity was as low as 0.2 - 0.3 in low rainfall periods, and it has gone up to 0.8 in high rainfall period irrespective of the rehabilitation activities.

Figure 1. Cropping intensity and rainfall

In addition, the following environmental issues in the tank ecosystem were also observed:.  Disappearance of the gasgommana (see technical note # 1 for description), enhancing water loss;  Development of salinity in the upstream area around full supply level, especially if the

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tank is second or third in the cascade;  Flooding of upstream rice fields. This has caused conflict among village communities; and  Disappearance of some of the fish species, which cannot survive in shallow waters.

Clearly, a new methodology for the restoration of small tanks is needed. If current rehabilitation strategies are adopted without revising the methodology, the probability of achieving a cropping intensity of 0.8 would be only about 10 percent (Figure 2). Scientists and engineers must adopt a new methodology for small tank restoration, if they are to prevent the disappearance of minor tanks from the landscape of the dry zone during next few decades.

100 90 80 70 60 50 40 30 Probability (%) 20 10 0 0 0.2 0.4 0.6 0.8 1 1.2 Cropping Intensity Fig. 2. Probability of Cropping Intensity under minor irrigation in Anuradhapura

Figure 2. Probability of cropping intensity under minor irrigation in Anuradhapura

The Walagambahuwa concept was aimed at using maha rainfall as possible by advancing maha rice cultivation and conserve tank water for a second crop in yala season. It seems reasonable to postulate that following facts might have hindered the success of extrapolating the adoption of concept to other areas. 1. For most of the tanks effective catchment per unit area of the irrigable land is smaller than that of Walagambahuwa case. 2. Tank water losses are higher than anticipated therefore, storing water for few months may be wasteful. 3. Advancing maha cultivation in lowland is not always possible as cultivation activities in rain-fed upland should take place prior to heavy rains occur in November.

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4. Early investigations on tank water losses revealed that the changes in tank bed geometry due to of sediment remarkably increased water loss. Water losses due to evaporation, seepage and percolation increase with increasing storage in an exponential form1. Losses due to evaporation and other losses follow a similar pattern of variation with the storage, and it indicates that the governing factor of water loss must be the water spread area of that particular tank. This assumption can be proved true when the water loss is related to the tank bed geometry. Figure 3 shows how the rate of tank water loss varies with the storage/ water spread area ratio in eight minor tanks. This shows that that the geometry of tank bed must be with high capacity/ water spread area ratio in order to reduce losses. It further cautions that raising the tank bund to accommodate the lost capacity of tank in rehabilitation programmes would lead to increase losses making less economic return to the investment.

100.0

90.0 y = 59.471x-1.3351 80.0 R2 = 0.786

70.0

60.0

50.0

40.0

Percent water loss water Percent 30.0 0.7 0.9 1.1 1.3 1.5 Capacity/ water spread area (m)

Figure 3. Effect of tank bed geometry on water loss

Desiltation of minor tanks should aim not only at increasing storage potential and reducing tank water loss but also at protecting the tank ecosystem. As desiltation is an expensive task, it is important to develop a technological concept, which generates a low- cost and effective desiltation process. The partial desiltation concept is introduced in this context, on the basis of findings from hydrological research studies conducted by the Department of Agriculture.

1 Dharmasena, P.B (1985). System loss studies of village tanks. Tropical Agriculturist. Dept. of Agriculture, Peradeniya, Sri Lanka 141:95-108.

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2. The concept of partial desiltation

The process of desiltation in this concept is not essentially aimed at expanding the present capacity of tank. The main objective of the concept is to reduce tank water losses by manipulating the tank bed geometry through desiltation. It is clear that the said objective cannot be successfully achieved by a complete desiltation, which would not alter the capacity/ water spread area ratio of the tank storage.

Studies of sedimentation indicate that the amount of sediment deposited in minor tanks is between 20-35%, and half of this sediment is found within one-third of the tank bed area close to the bund2. The same capacity can be maintained by removing sediment in this area and heaping it in the upstream tank bed. Such partial desilting decreases the spread of the water and keeps more than 50% of the tank bed free of water. An illustration of this desilting technique is given in Figure 4. In order to prevent sediment moving back to the desilted area there is a need to stabilise the soil mounds with vegetative cover (Figure 5).

DESIGN OF PARTIAL DESILTING CONCEPT

Designed curve Area (ha) Area

Sill level

Elevation (m) FSL FSL

Figure 4. Design of the Partial Desilting method

2 Dharmasena, P.B (1992). Magnitude of sedimentation in village tanks. Trop. Agric. Dept. of Agric., Peradeniya, Sri Lanka. 148:97:110

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Figure 5. Formation of soil mounds (© P. B. Dharmasena)

3. Benefits of partial desiltation

Partial desiltation of a tank would provide various benefits to the community some of which cannot be assessed by a direct economic analysis. The economic analysis should therefore, be based on consideration of following benefits in order to determine the return to investment of partial desiltation.

3. 1 Increased extent for cultivation Even though the asweddumized lands are available for cultivation in most of the command areas, water availability in tanks limits the cultivable extent. Reduction of tank water losses from partial desiltation leads to improved water availability, in turn, increasing opportunities for cultivating a larger extent. The project has already noted a 50% increase in field harvest in more than 60% of the restored tanks.

3.2 More agriculturally productive lands Partial desiltation reduces the water spread area. More than half the land inundated with tank water will be free of surface water after a successful partial desiltation and the water body is confined close to tank bund. The land area freed from such water spread can be used for agricultural purposes, if measures for preventing are followed. This soil is fertile with nutrients and high level of organic matter (5-8%). There is also easy access to groundwater. This area can be transformed to paddy land or any perennial crop, which is tolerant to shallow groundwater.

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3.3 Increased cropping intensity Because the water storage efficiency of the tank is increased by partial desiltation, any water remaining in the tank after maha cultivation can be kept without much loss for a yala cultivation.

Further, this tank storage can raise the groundwater in the command area and yala cultivation can be supplemented by well water. This, in turn, leads to an increase in the cropping intensity of the command area.

3.4 Opportunities for cottage industries

The new area emerged at the upper portion of the tank bed and the soil mounds formed from partial desiltation are more suitable for perennial crops because and sedimentation must be prevented. For a cottage industry type programme, this land could be used to grow plants — such as Bambusa spp. (Sinhala: Una; Tamil: Mungil), Calamus spp. (Sinhala: Wewal; Tamil: Pirambu), Cyperus pangorei (Sinhala and Tamil: Pan,) Pandanus spp. (Sinhala: Vetakeiya; Tamil: Talai), Hibiscus tiliaceus (Sinhala: Beli pata; Tamil: Artia); Borassus flabellifer (Sinhala: Thal; Tamil: Pannai marum), Caryota urens (Sinhala: Kitul; Tamil: Tippillipanai) — all of which provide various raw materials for cottage industries.

3.5 Freshwater fishery Minor tanks are mostly seasonal reservoirs. They can be used for raising fish species for a short duration or harvesting half-matured fish stock. An adequate dry season storage of a tank with favourable geometry can improve this situation for rearing even fish species that need a longer time for maturation. However, the choice of species used for such a fishery must be ecologically sound, ensuring that only non-invasive species are selected. The other advantage of having a good dead storage during dry periods is that these tanks can be used for raising fingerlings in protected areas.

4. Methodology for partial desiltation

The partial desiltation technique consists of preliminary field surveys, preparation of plans, designs and estimates, establishment of the upstream reservation (gasgommana) including formation of soil mounds, renovation of tank bund and sluices, establishment of downstream reservations (kattakaduwa and kiul ela). The process must consist of all these activities, without which the impact of partial desiltation will not be very effective.

However, before commencement of technical planning a Participatory Rural Appraisal must be carried out to obtain farmers' views on tank rehabilitation and to consider their suggestions for incorporating in the subsequent planning and implementation programme.

Listed below are the steps in the process of partial desiltation.

4.1 Tank bed level survey A tank bed engineering survey is carried out to understand the tank bed geometry, storage capacity and area-capacity-elevation relationship. The grid size is at least 20 m x 20 m in order to obtain adequate information for desiltation work. The survey map is prepared at 1:4000 scale with 0.25 m contour intervals.

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Figure 6. Tank bed contour map

4.2 Tank bed sediment survey A sediment depth survey is then carried out to prepare an original (prior to sedimentation) contour map and area-capacity-elevation curves, which is later super-imposed to the existing tank bed survey map. The same grid network used for tank bed level survey can be followed up for the sediment survey too. The depth to original tank bed can be determined by field experience. It is identified as the depth at which the /(silt+) ratio shows a sudden contrasting higher value3.

4.3 Partial desiltation design The design is based on above two surveys along with following criteria. 1. The required live and dead storage after completing the desiltation. 2. The minimum requirement of sediment layer to avoid excessive percolation. 3. The volume of soil required to repair the upstream side of the tank bund and ‘isweti’.

The design is initially worked out on the area-elevation diagram and transferred with relevant information to the tank bed contour map. This design map shows the locations of soil mounds and the depth of sediment to be removed on a 20 m x 20 m grid basis. In each grid square, the volume of soil to be removed is marked, and the cost of desiltation worked out accordingly. The following designs and plans can be prepared after completing the initial surveys.

3 Dharmasena, P.B (1992). Magnitude of sedimentation in village tanks. Trop. Agric. Dept. of Agric., Peradeniya, Sri Lanka. 148:97:110

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1. A diagram showing the cross-sectional view of tank bund including the sill level and dimensions of kattakaduwa. 2. Cross-sectional view of soil mounds and isweti. 3. Plan of kattakaduwa with improvements. 4. Plan of gasgommana area with proposed vegetation.

Figure 7. Part of the tank bed is now free of water.

Figure 8. Partial desiltation of Puliyankulama tank (Kumudu Herath © IUCN)

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4.4 Implementation

The Department of Agrarian Development (DAD) planned the above activities in collaboration with Project staff. Although excavation of soil needs the support of machinery, other activities — such as stabilisation of bunds and mounds with vegetative cover, upstream drains, establishment of the gasgommana and kattakaduwa were carried out with support from the community. Data collection and required technical supervision were the responsibility of the Department of Agrarian Development staff. Farmers were made aware at the inception of the programme of how they were supposed contribute in this programme. All work was undertaken by Farmer Organisations. However, data collection and required technical supervision was the responsibility of DAD staff.

5. Impact assessment

The monitoring and evaluation programme commenced just after the site selection. The following assessments evaluated the success and performance of partial desiltation:

5.1 Before partial desiltation  Basic tank data — capacity, tank bed area, full supply level;  Water balance study; and  Cropping intensity of the command area.

5.2 At partial desiltation  Cost of planning;  Cost of desiltation, repairing of tank bund and construction of upstream ridges;  Development cost of kattakaduwa and gasgommana; and  Farmer participation in implementation.

5.3 After partial desiltation  Basic tank data — capacity, tank bed area, full supply level;  Water balance study (hydrological modelling);  Cropping intensity of the command area; and  Cost-benefit analysis of the development programme.

5.4 Progress indicators  Increase in annual income of farmers from paddy fields;  Opportunities available for cottage industries;  Tank water storing efficiency;  Cropping intensity;

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 Other agricultural opportunities generated; and  Plant diversity index.

6. Project achievements

 Preliminary investigations were carried out on 21 tanks to identify what needed to be done. Combining the needs of the community and the results of these preliminary investigations, work was identified and carried out in 18 tanks. This work included restoration of sluices, bunds, spills, irrigation structures, distribution canals, retaining walls, drainage canals, and service paths associated with irrigation systems. (Figure 9.)  Tank bed surveys were carried out on 20 tanks to understand the shape and size of the tank bed — the existing tank geometry.  Based on the results and in consultation with the Department of Agrarian Development and community, partial desiltation was carried out in five tanks. (Figure 9.)  Surveys to identify the depth of accumulated sediment were carried out in these identified tanks and partial desiltation plans were then developed.  During partial desiltation, the following amounts of silt were removed from the tanks as shown in Table 1.

Table 1. Quantity of silt removed from various tanks

Tank Total amount of silt removed (in cubic meters) Konakumbukwewa 6,345 Peenagama 3,100 Massalawa 17,687 Puliyankulama 4,765 Maha Kadiyawa 6,000

 In over 60% of the tanks restored, field harvest has increased by 50%.  Shareholders (ranging 15-80 persons per tank) now engage in water management programmes for each rehabilitated tank.

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Figure 9. Partially desilted tanks

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Figure 10. Upstream earthen ridge formed with the soil dredged during partial desiltation Maha Kadiyawa tank (Kumudu Herath © IUCN)

7. Recommendations for the Future

 Strengthen community participation to obtain labour and thereby reduce the cost of partial desiltation work;  Refine the partial desiltation technology to reduce costs;  Ensure ecological stabilisation, with replanting, after hard improvement — such as soil movement;  Conduct technical studies to determine the tank bed area which can be freed for ecological restoration, and obtain the consent of relevant authorities and the community;  Establish a legal provision to reduce the spill and free the tank bed for such ecological restoration;  Link the above improvement to a study on water budgets and provide technical support to irrigation water management in order to increase the cropping intensity; and  Adopt soil conservation measures to minimise .

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January 2016