Groundwater for irrigation in Cambodia


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Groundwater for irrigation in Cambodia

  1. 1. Groundwater for irrigation in CambodiaRobyn JohnstonMarch 20, 2013 Photo:cc: NestleIWMI-ACIAR workshopPhnom Penh Water for a food-secure world
  2. 2. Objective of the session• Discuss / validate reasons why ground water resource assessment (GWRA) for Cambodia is needed• Outline how GWRA might be structured – Priority areas – Who should be involved Water for a food-secure world
  3. 3. Why groundwater?• Widely used for domestic water supply in Cambodia, and increasingly for small-scale irrigation in some areas.• Small scale pump irrigation from groundwater does not need large infrastructure; and gives farmers direct control over water access.• BUT - potential problems with overpumping and depletion, water quality and pumping costs• In India, Pakistan and China, rapid expansion of groundwater irrigation, supplementing or even replacing large scale public systems – similar pattern in south of Cambodia Water for a food-secure world
  4. 4. Opportunity or risk?• Groundwater could be an opportunity for Cambodia to “leap-frog” to efficient small-scale irrigation, with minimal infrastructure and maintenance costs• …..or could be unsustainable, suitable only for limited applications?• The only way to know is to understand the nature, extent and sustainability of the groundwater resource.• Need to assess the costs of developing and supplying groundwater compared to surface water Water for a food-secure world
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  6. 6. Groundwater for irrigation• Shallow alluvial aquifers in the Mekong lowlands accessed via shallow dug and tube wells to produce second crop – Mechanised and treadle pumps (not hand pumps) – In Prey Veng, the number of tube-wells used for irrigation grew from 1600 in 1996 to 25,000 by 2005 (IDE 2005).• Partial irrigation either for an early or late wet season crop, supplementary irrigation of the wet season rice crop and late-season recession rice.• Access to groundwater increases adoption of double cropping Water for a food-secure world
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  8. 8. Groundwater for domestic use• >50% households use groundwater in dry season from hand-dug wells or shallow tube-wells – more than 270,000 tube-wells for drinking water in use – mostly used with simple hand pumps, draw from < 6m.• Urban supplies in larger towns, using mechanised pumping to access deeper aquifers• Industrial use in Phnom Penh area - many industries drill their own wells• Known problem with arsenic contamination in some areas (UNICEF) Water for a food-secure world
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  11. 11. Groundwater for ecosystems• High connectivity between surface and ground water in floodplains• Aquifers store water during the wet season and release it during the dry season to provide base-flow for rivers, streams and wetlands – over-extraction can reduce dry season discharge – surface water bodies may lose water to the aquifer Water for a food-secure world
  12. 12. Hydrogeology of Cambodia• Lowlands underlain by thick pile of alluvial deposits (>160m in some areas)• “Young Alluvium”- surface deposit up to 10m thick, silty, poor aquifer, poor water quality (arsenic, iron)• “Old Alluvium” – multiple aquifers, high quality but spatially variable – Sub-artesian - pressurised by overlying Young Alluvium• Potential aquifers upland areas: – Tertiary basalts (east and central Cambodia) – Permian limestones (Battambang, Kampot) Water for a food-secure world
  13. 13. Exposed Old Alluvium Young alluviumMRC Water for a food-secure world
  14. 14. Water availability and sustainability• Wells in the quaternary aquifers (mainly Old Alluvium) of Svay Rieng, Prey Veng and southern Kandal provinces can yield 500 to 800 m³ per day (JICA 2002, Roberts 1998) – sufficient to irrigate four to five hectares of rice per well.• Yields from the Young Alluvium and the basement aquifers were much lower, and generally unsuitable for irrigation (1.5 - 150 m3/day) Water for a food-secure world
  15. 15. Resource sustainability• Sustainability depends on balance between withdrawal and recharge• If there is full annual recharge, withdrawals may be sustainable over the long term even if annually pumped out at local scales – BUT seasonal drawdown jeopardises domestic supply and late DS crops• We can only answer questions about sustainability by groundwater resource assessment Water for a food-secure world
  16. 16. If recharge < withdrawal If recharge >Limit for hand pumping withdrawal Water for a food-secure world
  17. 17. Resource sustainability• Recharge patterns and rates mostly unknown, estimates vary widely – Detailed studies only in south (IDE, JICA)• Annual recharge by flood? – Well levels vary with river height for up to 30 km from river• Young Alluvium acts as blanket to restrict recharge? (IDE) – concerns about drawdown affecting domestic supplies – steady fall in gw levels in wells in Prey Veng 1996-2008 (monitoring was discontinued)• We can only answer questions about sustainability by groundwater resource assessment Water for a food-secure world
  18. 18. • Generally suitable for irrigation use, but high levels of arsenic, iron, manganese, fluoride and salinity Water quality are observed in some areas• Over 15% of wells tested nationally had arsenic above the provisional national limit of 50ug/l – strong geological control, with high arsenic almost always in Young Alluvium (UNICEF) – As less of risk for irrigation than drinking water, but needs to be considered• Poor quality GW can reduced crop yields and in extreme cases harm Arsenic risk soil chemistry and structure Water for a food-secure world
  19. 19. Qs. – resource assessment• Where is there greatest potential for groundwater development?• What are the sustainable limits to extraction from the major known aquifers?• What are the main recharge processes for each aquifer, and how do they operate spatially?• What are the interactions between groundwater and surface water systems, particularly in the floodplain? How does groundwater serve ecosystems?• Where are water quality issues likely to limit groundwater irrigation? Can these be mitigated or managed? Water for a food-secure world
  20. 20. What has already been done?• On-going geologic mapping (MIME)• Well database with >60,000 records by MRD• Groundwater study in 7 provinces (MRD)• National hydrogeological reconnaissance study by USGS (Rasmussen and Bradford 1977) including 1100 test wells• Detailed studies in Prey Veng and Svay Rieng (IDE 2009 Roberts 1998), including monitoring of 49 wells for 13 years up to 2008, and development of groundwater flow model (MODFLOW)• Detailed study of hydrogelogy in southern Cambodia, Kampong Cham and Kampong Chhnang (JICA/CMRD 2002• Extensive studies of arsenic in groundwater by UNICEF and others Arsenic database (UNICEF / MRD / Ministry of Health?)• Ongoing MRC initiatives: – Rapid appraisal of agricultural water use, including safe yield map using MODFLOW (MRC 2012) – USGS – comprehensive groundwater monitoring program proposed for Water for a food-secure world LMB (Landon 2011)
  21. 21. Qs. – resource use• How has the existing level of groundwater use impacted poverty and the environment?• Why is groundwater used for irrigation in some areas and not others – is it availability, or other constraints?• What are the costs of supplying groundwater compared to surface water in different areas?• How do patterns of surface water use affect groundwater demand (and recharge)?• What are the opportunities for conjunctive use? Can „infilling‟ with groundwater within large irrigation systems overcome head-tail inequalities?• What are the key challenges to management by groundwater use? What institutional arrangements are needed to support sustainable groundwater use? Water for a food-secure world
  22. 22. Questions for discussion• How could a GWRA best be structured?• Are there priority areas, or is a full national assessment needed?• Who should be involved? How do we involved them? Water for a food-secure world
  23. 23. What still needs to be done?• Research to understand spatial and temporal dynamics of groundwater recharge (and use)• Groundwater monitoring systems• Groundwater management strategies for each area (ecological / GW zones; different social / use contexts)• Regulatory environment and institutions to ensure acceptable level of exploitation – centralized control or by farmer-managed systems or both?• Water-energy implications – sources and costs of energy for pumping (diesel, electricity perhaps solar or wind pumps)• Water for a food-secure world