Groundwater for irrigation in Cambodia

Robyn Johnston

March 20, 2013

IWMI-ACIAR workshop

Phnom Penh

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Photo:cc: Nestle

Objective of the session

• Discuss / validate reasons why ground water resource assessment (GWRA) for Cambodia is needed

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• Outline how GWRA might be structured
• Priority areas
• Who should be involved

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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

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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.

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• Need to assess the costs of developing and supplying groundwater compared to surface water

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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

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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)

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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

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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)

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MRC

Young alluvium

Exposed Old Alluvium

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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)

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Resource sustainability

• Sustainability depends on balance between withdrawal and recharge

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• 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

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If

recharge

< withdrawal

If

recharge

> withdrawal

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Limit for hand pumping

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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

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Water quality

• Generally suitable for irrigation use, but high levels of arsenic, iron, manganese, fluoride and salinity 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 soil chemistry and structure

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Arsenic risk

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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?

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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 LMB (Landon 2011)

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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?

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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?

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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)
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