groundwater for irrigation in cambodia

www.iwmi.org

Water for a food-secure world

Photo:cc: N

estle

Groundwater for irrigation in

Cambodia

Robyn JohnstonMarch 20, 2013IWMI-ACIAR workshopPhnom Penh

www.iwmi.org


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

www.iwmi.org


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

www.iwmi.org


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

www.iwmi.org


www.iwmi.org


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

www.iwmi.org


www.iwmi.org


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)

www.iwmi.org


www.iwmi.org


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

www.iwmi.org


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)

www.iwmi.org


MRC

Young alluvium

Exposed Old Alluvium

www.iwmi.org


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)

www.iwmi.org


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

www.iwmi.org


If recharge

< withdrawal

If recharge

> withdrawalLimit for hand pumping

www.iwmi.org


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

www.iwmi.org


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

Arsenic risk

www.iwmi.org


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?

www.iwmi.org


• 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)

What has already been done?

www.iwmi.org


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?

www.iwmi.org


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?

www.iwmi.org


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)

• •

groundwater for irrigation in cambodia

Documents