Water Harvesting • An umbrella term for techniques which collect temporary surface and subsurface flows and store them for use later. • Short cuts the hydrological cycle by preventing losses to the sea and groundwater • Much in common with soil conservation structures • Other names – Water spreading, runoff farming, spate irrigation also many others

Why harvest water? • As land pressure rises, more and more marginal areas in arid or semi-arid areas are being used for agriculture. • In these areas rain falls irregularly and much of the precious water is soon lost as surface runoff. • Irrigation has proved costly and can only benefit a fortunate few. • Prevents runoff from causing erosion • Can improve reliability and yields of rain fed agriculture • Water harvesting (WH) can be considered as a rudimentary form of irrigation with no control over timing.

Water Harvesting water sources • • • •

Runoff Subsurface flow River or flood diversion Mist

Ideal soil conservation situation

Ideal water harvesting catchment

P P Et Litter layer

Et

R R

Sub soil

S

Impermeable bed rock

↑ Vegetation, ↑ interception, ↑subsurface storage and evaporation ↑ litter layer , ↑ infiltration ↑ subsoil porosity, ↑ throughflow

S

Water harvesting can: • Smooth out variations in water availability, providing water when other sources fail • Increase crop yields • Collect and store water near to the place of use thereby saving time and labour • Provide good quality water where the groundwater is polluted or saline • Provide water supplies for drinking in areas with as little as 50mm of rainfall (>200mm better)

Water harvesting restrictions • SLOPE – not recommended for areas where slopes are greater than 5% due to:• uneven distribution of run-off and • large quantities of earthwork required which is not economical.

• SOILS – they should be deep > 1m, 2m ideal, and underlain by impermeable layer – Should not be:• saline or sodic and ideally possess inherent fertility. • sandy – difficult to generate runoff

Considerations • Appropriateness – – – –

Social (roof use, land uses, preferred taste of water) Environmental (EIA,-landscape, ecosystems) Technically feasible (access, materials, skills, budget) Economic (comparison with alternative sources)

• In relation to : – Climate • Rainfall, amount, intensity, pattern • Evaporation

– Topography, soils, geology, vegetation

Appropriateness • Appropriate Technologies – – – – – –

PRA and RRA techniques Employs local materials and methods Sustainable Village level maintenance Small capital investment Minimize environmental impact

• Inappropriate Technologies – Capital intensive – Use modern materials and methods requiring special skills and equipment – Little consultation

Types of system • Agricultural Recharge systems – Rely on predominantly natural catchments – Collected water is stored in the soil – Soil depth >1m, with impermeable layer below Soil depth > 1m Soil moisture stored here Impermeable Layer

• Water supply systems – Water collected by natural and artificial catchments – Collected water is stored in tanks, ponds, reservoirs, underground.

Types of Agricultural systems External catchments

Internal systems

• • • • • •

• •

Runoff areas distinct, Usually community owned overland flow or rill flow harvested runoff stored in soil profile catchment 30 - 200 m in length ratio catchment: cultivated area usually 2:1 to 10:1 • provision for overflow of excess water • uneven plant growth unless land levelled

•

• • • •

Runoff areas not distinct from cropping areas, usually in the same field Easy to construct- less ownership problems overland flow harvested from short catchment length catchment length between 1 and 30 m runoff stored in soil profile ratio catchment: cultivated area usually 1:1 to 3:1 normally no provision for overflow plant growth is even

Types of Internal systems • • • • •

Contour bunds Run-on terraces Tied ridging Microcatchments Stone lines

Effect of runoff areas in internal cropping systems

Contour ridges

Runoff Strips

Effectiveness of different terrace types Conservation bench (run on) terraces increases soil moisture storage under the terrace

Contour bunds also increase soil moisture under the bund

Level bench terraces do not magnify rainfall

Contour ridging

Contour ridges with ties

Contour ridges with cross ties

Example of graded contour ridges with cross ties lower than the main ridges to retain water between the cross ties, but allow excess rainwater to flow between the ridges rather than spill over or break the main ridges

Tied contour bunds with pits

Tied contour bunds for gently sloping land (5%) (dimensions in m)

http://www.fao.org/ag/agl/aglw/wharv/wh07/sld010.htm

Microcatchments

Gentle slopes

Flat areas

More microcatchments

http://www.fao.org/ag/agl/aglw/wharv/wh10/sld006.htm

http://www.fao.org/ag/agl/aglw/wharv/wh10/sld007.htm

http://www.fao.org/ag/agl/aglw/wharv/wh10/sld008.htm

Planting Trees Appropriately

Casuarina, olive

Drought tolerance

Fig, Acacia Pistachio Citrus

Building microcatchments in Turkana Kenya before

after

Microcatchments

Tied Furrows

Tie maker

Stone lines Pioneered by Oxfam in Burkino Faso

External Systems • • • • •

Ownership problems Higher costs (but smaller costs per unit) Need spillways Need supervision May also have water conveyance components and storage components (apart from soil storage)

• Types – – – – –

Swales Trapezoid bunds Semi circular bunds Water spreading Storage systems

Some examples of external catchments

Spot the internal system here!

Tied Contour Ditch (Ethiopia)

Semi circular bunds

Macro-catchment water harvesting in Niger (Critchley et al., 1992).

Groundwater recharge structure (Ethiopia)

Trapezoid Bunds

(for 1% slope)

Off contour Bunds – used for water spreading

Typical external catchment system (Macrocatchment)

Design of Spillways • Water retaining bunds should have spillways every 20m • Size of spillway depends on max volume of water after typical rainfall event • Spillways should be built of stone, timber or concrete • Downstream slope as gentle as possible. • Encourage grasses to stabilize the structure • Typical spillway dimensions: Height 0.1-0.15m, width 0.8m, length 1-2.5m • Total length of spillway (m) = 0.5 x catchment area (ha)

Spill way design

Ethiopian Spillway

Design of Bunds • For 0.1 ha plot on 1% slope bund might be: – 0.4m high and 0.5-1m wide. – One man could build 10m/day of bund. – Bunds at 15-20m intervals, few at first and if volumes of water allow then more later.

3 stages in the development of a runoff farm, Baringo, Kenya (Critchley,1984) a) simple runoff farming as practiced before project began, runoff concentrating naturally : b) improved collecting spillways:

version bunds

with and

c) farmers adaptation which directed flow to the lower basin.

Negev Runoff Farms Everani, Shanan and Tadmor (1971)

Storage systems • Collected water is stored and then distributed to its end use such as: – Irrigation – Drinking/domestic – Livestock – Industrial • Storage system catchment types – Roofs – Surface catchments – Subsurface

Surface Catchments • Tend to be: large communal and have poor water quality, similar to external systems • Types – Natural – rock catchments, hollows and valleys – Artificial- Roads, airports, modified landscapes

• Need a funnelling shape, or diversion bunds/ channels.

Hafir – Livestock, Sudan

Water harvesting In Ethiopia

Roof Harvesting systems • Communal and individual households supplies including garden irrigation • Tank storage (0.5-200m3) • Supplies next to the point of use • Unlikely to meet all household demands • Traditional materials inefficient and can have water quality problems • Modern materials efficient but expensive • Need guttering or a spigot • First flush system will improve water quality • Roofs may have other purposes

Roof harvesting for garden irrigation

First flush systems

More first flush systems

More first flush systems

Downpipe rain water collector

Storage Structures • Tanks • Open reservoirs • Subsurface reservoirs

Example of roof system

Construction of cement water storage jar

Construction of part buried tank

Construction of an excavated tank

Construction of a surface tank

Construction of a surface tank

Tanks can also be built under buildings

Surface system for domestic water

Examples of sub surface storage tanks

Collect water from roofs and yard areas. Similar structures in Jerusalem enabled Israelis to with stand 5 day war

Nabatean surface system using cave storage

Nabatean surface system using cave storage

Nabatean surface system using cave storage

Open reservoirs – Evaporation losses – Seepage losses- fixed by linings, clay, grouting – Mosquito and snail breeding grounds • Can be addressed by surface coverings, floats,oil films.

– Sedimentation – Can be vulnerable to pollution

Water harvesting reservoir Tigray

Unusual Example of an Open Reservoir

Subsurface dams • Built in river channels and dry river beds • Requires digging and inserting of an impermeable layer or using gabions to trap coarse sediments • Reduces Evaporation and disease vector breeding • Storage capacity < dependant on porosity (