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Assessment of Rainwater Harvesting Potential in Zanzibar May 2007

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Assessment of Rainwater Harvesting Potential in Zanzibar May 2007 ZERO DRAFT FOR DISCUSSION ONLY The Revolutionary Government of Zanzibar In Collaboration with Millennium Development Goals - MDG Centre, Nairobi Kenya United Nations Development Programme (UNDP), Tanzania and World Agroforestry Centre – ICRAF, Nairobi, Kenya An Assessment of Rainwater harvesting Potential in Zanzibar May 2007 Table of Contents List of Figures ------------------------------------------------------------------------------ 5 List of Tables------------------------------------------------------------------------------- 6 List of Plates-------------------------------------------------------------------------------- 6 Executive Summary------------------------------------------------------------------------ 8 Introduction -------------------------------------------------------------------------------10 Background -----------------------------------------------------------------------------11 Objectives of the study -----------------------------------------------------------------12 Overview of the problem---------------------------------------------------------------13 Definition of rainwater harvesting------------------------------------------------------14 Rationale of RWH----------------------------------------------------------------------14 Methodology ------------------------------------------------------------------------------15 Literature review -----------------------------------------------------------------------15 Interviews with Senior Government Officials, Farmers and Traders ------------------15 Field Visits------------------------------------------------------------------------------15 Random inspections --------------------------------------------------------------------15 GIS Mapping of Rainwater Harvesting potential in Zanzibar--------------------------16 Literature review --------------------------------------------------------------------------16 Historical Background ------------------------------------------------------------------16 Growing Demand for Water in Zanzibar -----------------------------------------------16 Government Policy on Rainwater Harvesting------------------------------------------17 Sectoral efforts to promote RWH ------------------------------------------------------17 Rainwater Harvesting and Domestic Water Supply------------------------------------23 Rainwater harvesting and the Millennium Development Goals------------------------23 Rainwater harvesting for poverty alleviation----------------------------------------23 Rainwater harvesting and gender ----------------------------------------------------23 Rainwater harvesting for water accessibility ----------------------------------------24 Rainwater Harvesting and Integrated Water Resources Management -----------------24 Private sector support to RWH ---------------------------------------------------------26 Community participation in RWH -----------------------------------------------------27 Results-------------------------------------------------------------------------------------29 2 Water Resources Potential in Zanzibar-------------------------------------------------29 Rainfall-------------------------------------------------------------------------------29 GIS mapping of rainfall--------------------------------------------------------------29 Microscopic Water Balance in Zanzibar---------------------------------------------31 Verification of Water Balance using GIS Mapping Methods -----------------------32 Water requirements to meet Humans needs -----------------------------------------35 Zanzibar’s Opportunities and Challenges in Water Management----------------------38 Past and ongoing RWH investments in Zanzibar --------------------------------------39 Level bunds --------------------------------------------------------------------------40 River diversions and surface irrigation ----------------------------------------------40 Groundwater -------------------------------------------------------------------------41 Spring water--------------------------------------------------------------------------42 Observed Challenges in the field -------------------------------------------------------43 Impacts of RWH Practices -------------------------------------------------------------44 Approaches and Technological Options --------------------------------------------------45 Approaches -----------------------------------------------------------------------------45 Technological Options------------------------------------------------------------------46 Artificial ground water recharge for borehole and well rehabilitation --------------47 Runoff collection using in-stream structures ----------------------------------------54 Runoff collection using surface structures-------------------------------------------56 Rooftop rainwater harvesting --------------------------------------------------------60 In-situ Water Harvesting for agriculture---------------------------------------------65 Zai Pits -------------------------------------------------------------------------------65 Earthen bunds ------------------------------------------------------------------------66 Water application technologies ------------------------------------------------------70 Investment Cost for initiating RWH in Zanzibar -----------------------------------------73 In-situ RWH Systems ------------------------------------------------------------------74 Roof Catchment Systems---------------------------------------------------------------74 Runoff Catchment Systems-------------------------------------------------------------74 Artificial Groundwater Recharge-------------------------------------------------------75 Conclusions and Recommendations ------------------------------------------------------76 3 Base maps ---------------------------------------------------------------------------------79 References---------------------------------------------------------------------------------84 Appendix ----------------------------------------------------------------------------------86 4 List of Figures Figure 1 Rivers of Zanzibar. Source: ICRAF GIS Unit 2007.......................................... 20 Figure 2 Zanzibar Mean Annual Rainfall (Source: ICRAF GIS Mapping Unit) ............. 30 Figure 3 Unguja Microscopic Water Balance Adapted from ZWRDP, 1994 .................. 31 Figure 4 Pemba Microscopic Water Balance, Adapted from ZWRDP 1994. .................. 32 Figure 5 Mean Rainwater Availability by Population density in Zanzibar ...................... 33 Figure 6 Rainwater portioning and water utilisation in Zanzibar ..................................... 34 Figure 7 Global Rainwater Partitioning............................................................................ 35 Figure 8 Water requirements for diets versus GDP/capita/year (Source SEI, 2006)........ 36 Figure 9 Approaches for instituting RWH in Zanzibar .................................................... 46 Figure 10 Technological RWH Options Applicable to Zanzibar ..................................... 47 Figure 11 Recharge well for shallow water tables............................................................ 49 Figure 12 Groundwater recharge well with Roof catchment ............................................ 49 Figure 13 Groundwater recharge using trench filled with boulders ................................. 50 Figure 14 Dugs out wells used as recharge facility.......................................................... 50 Figure 15 Recharging grounwater using existing Hand pump ......................................... 51 Figure 16 Filtering medium for Groundwater recharge structures ................................... 52 Figure 17 Roof Catchment with underground storage and overflow for groundwater recharge ............................................................................................................................. 53 Figure 18 Saltwater Intrusion............................................................................................ 54 Figure 19 Lay out of Check dams..................................................................................... 56 Figure 20 Charco dam....................................................................................................... 57 Figure 21 Development Domains for Runoff Catchment Systems .................................. 59 Figure 22Rooftop rainwater harvesting ............................................................................ 60 Figure 23 Plan and pictorial view of an estate storage and groundwater recharge facility ........................................................................................................................................... 62 Figure 24 Factory with Underground storage and recharge facility................................. 62 Figure 25 Estate with central storage tank and groundwater recharge facility................. 63 Figure 26 Estate with independent underground storage tanks and recharge facilities.... 63 Figure 27 Development domains for Roof Catchment Systems....................................... 64 Figure 28: Zai Pit .............................................................................................................. 66 5 Figure 29: Earthen Bunds (contour, circular, trapezoidal)................................................ 67 Figure 30 Development domains for in-situ RWH in Zanzibar ....................................... 69 Figure 31 Rope washer pump ........................................................................................... 71
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