1. Groundwater
for irrigation
in Cambodia
Robyn Johnston
March 20, 2013
Photo:cc: Nestle
IWMI-ACIAR workshop
Phnom Penh
Water for a food-secure world
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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
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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
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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
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5. Water for a food-secure world
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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
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7. Water for a food-secure world
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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)
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9. Water for a food-secure world
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10. Water for a food-secure world
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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
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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)
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13. Exposed Old Alluvium
Young alluvium
MRC
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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)
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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
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16. If
recharge
<
withdrawal
If
recharge
>
Limit for hand pumping
withdrawal
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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
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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
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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?
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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
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LMB (Landon 2011) www.iwmi.org

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?
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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?
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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
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