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 Reproduction of this publication for educational or other non- commercial purposes is authorized without prior written permission from the copyright holder provided the source is fully acknowledged. Reproduction of this publication for resale or other commercial purposes is prohibited without prior written permission of the copyright holder. 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: Soil 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 sediment 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 silt 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 4 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,