Use of Groundwater for Agriculture

FAO Staff Training for EU - SDDP May 2014

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IMPORTANCE

Domestic water supply – water security Agricultural production and crop

security Environmental protection – surface

vegetation, wetland ecosystem, river aquatic ecosystem, coastal water ecosystem

Commercial ventures – industries, hotels etc.

Effect on surface water resources – river water, reservoirs, drainage process

Vavuniya

Mannar

Batticaloa

Ampara

Mulaithivu

Anuradhapura

Jaffna

Deep confined miocenelimestone aquifers

Shallow coastal sand aquifers

Alluvial aquifers

Laterite aquifers

Regolith aquifers

Shallow karstic aquifer

Shallow Karstic aquifers – Jaffna miocene limestone

Shallow coastal sand aquifer (kalpitiya, Mannar, Kalkuda, Batticaloa, Pothuwil,

Chilaw, Negombo, Puttalam, Katunayake

Rainy season

Dry Season

GROUNDWATER RESOURCES IN SRI LANKA

Deep confined miocene limestone aquifers – North West Sri Lanka – Puttalam, Mannar, Mulaitivu districts

Geologic cross section across the Murunkan basin

GROUNDWATER RESOURCES IN SRI LANKA Shallow groundwater in the hard rock basement –

Dry zone undulating landscape – ‘Regolith aquifers’

Groundwater table behaviour – Maha Illuppallama

LITHOLOGY OF REGOLITH AQUIFERS

Soil (0 – 2m)

Highly weathered rock (2 – 4.5m)

Moderately weathered rock (4.5 – 8.8m)

Sap rock (8.8 – 10m)Fresh rock

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Laterite aquifer –South West Sri Lanka – Gampaha, Colombo, Kaluthara, part of Galle and Mathara

Deep hard rock aquifers – fractures and fissures

GROUNDWATER USE AND LIMITATIONS

Limited reserves Low specific yield Low

transmissivity Sea water

intrusion High fluctuation –

temporal and spatial

Poor quality water

WATER

Rainwater

Surfacewater

Groundwater

Water Resources Use in Villages of the Dry and Intermediate Zones of Sri Lanka

Drinking and domestic water

Rain-fed Farming

Agro-well farming

Lowland farming

Fishery

Homestead farming

Livestock

Integrated Water

Resources Management

Water is essential for

environmental existence

Desirable levels of drinking water

Acidity pH: 7.0 – 8.5 Electrical Conductivity EC: 750

µS/cm Hardness – 250 (CaCo3mg/l) Fluoride F – 0.6 mg/l Phosphate PO4

-3 – 2.0 mg/l Nitrate NO3

-1 – 10 mg/l Arsenic As – 0.01 mg/l

Distribution of EC in Vavuniya

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Distribution of F values in Vavuniya

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Shallow metamorphic hard rock aquifers

Well drained shallow

Moderately drained

moderately deep

Imperfectly

drained deep

Poorly drained deep

Four location categories identified for agro-wells

Cultivable extent (ha)

Required volume of water (m)3

Required water

depth (m)

Category

> 0.4 > 55 >1.9 I

0.2 – 0.4 35 – 55 1.3 – 1.9 II

0.1 – 0.2 20 – 35 0.65 – 1.3 III

< 0.1 < 20 < 0.65 IV

Location categories for agro-well construction

Catenary position

Drainage Min. Storage(m)3

Min. water depth(m)

Category

Depression PD > 55 > 1.9 I

Mid low slopes

ID/PD 35 – 55 1.3– 1.9 II

Mid slope WD/ID 20 – 35 0.65 – 1.3 II

Crest and upper slope

WD < 20 < 0.65 IV

PD – Poorly drained, ID – Imperfectly drained, WD – Well drained

Pumping Test (simplified)

Well specific capacity (K)– Volume of water recovered per unit time

per unit depression head– K = 0.54 D2/ T1/2

– m3/ hr per metre drawdown– T1/2 Half recovery time (the time taken to

recuperate half the depth pumped out

Application of the Pumping Test Extent 0.4 ha (1 ac.) Water requirement = 1,500 m3

Recommended well diameter

Water depth in July (m)

Observed K> 3 1.5 – 3.0 < 1.5

2 5.5 7.0 9.53 4.5 6.5 8.04 4.0 5.5 7.05 3.5 5.0 6.0

Recommendations for groundwater use1. Conjunctive use of rainwater, groundwater and surface water

for agriculture2. In construction of agro-wells control the spatial spread and

density to prevent any environmental destruction1. Spacing between two agro-wells should be hagher than 100 m2. Maintain the density at a rate of less than 30 acres of catchment

for one well3. Make aware the public on consequence of over-abstraction of

groundwater4. Annual contribution of rainfall to the groundwater is only 6 %,

thus, use must be less than that5. In order to assure the sustainable use of groundwater, wells

should be located on the imperfectly drained land class of the land catena

6. Construct an observation well first to test the water quality 7. In well construction excavate down to shattered rock layer to

obtain the highest recuperation rate.8. Cropping intensity of paddy fields can be increased by

constrcuting common agro-wells closer to the paddy field.