1.1 The basis of water harvesting: History and perspectives
1.2 Definitions and classification
1.3 Basic categories of water harvesting systems for plant production
1.4 Overview of main WH systems
1.1.1 Historical perspectives
1.1.2 Recent developments
1.1.3 Future directions
As land pressure rises, more and more marginal areas in the world are being used for agriculture. Much of this land is located in the arid or semi-arid belts where rain falls irregularly and much of the precious water is soon lost as surface runoff. Recent droughts have highlighted the risks to human beings and livestock, which occur when rains falter or fail.
While irrigation may be the most obvious response to drought, it has proved costly and can only benefit a fortunate few. There is now increasing interest in a low cost alternative - generally referred to as "water harvesting".
Water harvesting is the collection of runoff for productive purposes. Instead of runoff being left to cause erosion, it is harvested and utilized. In the semi-arid drought-prone areas where it is already practised, water harvesting is a directly productive form of soil and water conservation. Both yields and reliability of production can be significantly improved with this method.
Water harvesting (WH) can be considered as a rudimentary form of irrigation. The difference is that with WH the farmer (or more usually, the agro-pastoralist) has no control over timing. Runoff can only be harvested when it rains. In regions where crops are entirely rainfed, a reduction of 50% in the seasonal rainfall, for example, may result in a total crop failure. If, however, the available rain can be concentrated on a smaller area, reasonable yields will still be received. Of course in a year of severe drought there may be no runoff to collect, but an efficient water harvesting system will improve plant growth in the majority of years.
Figure 1 The principle of water harvesting
Various forms of water harvesting (WH) have been used traditionally throughout the centuries. Some of the very earliest agriculture, in the Middle East, was based on techniques such as diversion of "wadi" flow (spate flow from normally dry watercourses) onto agricultural fields. In the Negev Desert of Israel, WH systems dating back 4000 years or more have been discovered (Evanari et al. 1971). These schemes involved the clearing of hillsides from vegetation to increase runoff, which was then directed to fields on the plains.
Floodwater farming has been practised in the desert areas of Arizona and northwest New Mexico for at least the last 1000 years (Zaunderer and Hutchinson 1988). The Hopi Indians on the Colorado Plateau, cultivate fields situated at the mouth of ephemeral streams. Where the streams fan out, these fields are called "Akchin". Pacey and Cullis (1986) describe microcatchment techniques for tree growing, used in southern Tunisia, which were discovered in the nineteenth century by travelers. In the "Khadin" system of India, floodwater is impounded behind earth bunds, and crops then planted into the residual moisture when the water infiltrates.
The importance of traditional, small scale systems of WH in Sub-Saharan Africa is just beginning to be recognized (Critchley and Reij 1989). Simple stone lines are used, for example, in some West African countries, notably Burkina Faso, and earth bunding systems are found in Eastern Sudan and the Central Rangelands of Somalia.
A growing awareness of the potential of water harvesting for improved crop production arose in the 1970s and 1980s, with the widespread droughts in Africa leaving a trail of crop failures. The stimulus was the well-documented work on WH in the Negev Desert of Israel (Evanari et al. 1971).
However much of the experience with WH gained in countries such as Israel, USA and Australia has limited relevance to resource-poor areas in the semi-arid regions of Africa and Asia. In Israel, research emphasis is on the hydrological aspects of microcatchments for fruit trees such as almonds and pistachio nuts. In the USA and Australia WH techniques are mainly applied for domestic and livestock water supply, and research is directed towards improving runoff yields from treated catchment surfaces.
A number of WH projects have been set up in Sub-Saharan Africa during the past decade. Their objectives have been to combat the effects of drought by improving plant production (usually annual food crops), and in certain areas rehabilitating abandoned and degraded land (Critchley and Reij 1989). However few of the projects have succeeded in combining technical efficiency with low cost and acceptability to the local farmers or agropastoralists. This is partially due to the lack of technical "know how" but also often due to the selection of an inappropriate approach with regard to the prevailing socio-economic conditions.
Appropriate systems should ideally evolve from the experience of traditional techniques - where these exist. They should also be based on lessons learned from the shortcomings of previous projects. Above all it is necessary that the systems are appreciated by the communities where they are introduced. Without popular participation and support, projects are unlikely to succeed.
Water harvesting technology is especially relevant to the semi-arid and arid areas where the problems of environmental degradation, drought and population pressures are most evident. It is an important component of the package of remedies for these problem zones, and there is no doubt that implementation of WH techniques will expand.
Figure 2 Classification of water harvesting techniques
Notes:
* Water supply systems (i.e. ponded water) used for a variety of purposes, mainly domestic and stock water but also some supplementary irrigation.** The term "farming" (as in "Runoff Farming") is used in its broadest sense - to include trees, agroforestry, rangeland rehabilitation, etc.
Water harvesting in its broadest sense will be defined as the
"collection of runoff for its productive use".
Runoff may be harvested from roofs and ground surfaces as well as from intermittent or ephemeral watercourses.
Water harvesting techniques which harvest runoff from roofs or ground surfaces fall under the term:
RAINWATER HARVESTING
while all systems which collect discharges from watercourses are grouped under the term:
FLOODWATER HARVESTING
A wide variety of water harvesting techniques for many different applications are known. Productive uses include provision of domestic and stock water, concentration of runoff for crops, fodder and tree production and less frequently water supply for fish and duck ponds.
In the context of this manual, the end use is plant production, including fodder and trees.
Classification of water harvesting techniques is as varied as the terminology (Reij et al. 1988). Different authors use different names and often disagree about definitions.
It is not the intention of this manual to introduce new terms but instead it was considered appropriate to make use of the terminology which has been established within the context of the "Sub-Saharan Water Harvesting Study," undertaken by the World Bank in 1986-1989. The general and practical classification is presented in Figure 2.
1.3.1 Microcatchments (rainwater harvesting)
1.3.2 External catchment systems (rainwater harvesting)
1.3.3 Floodwater farming (floodwater harvesting)
The water harvesting techniques described in this manual fall under three basic categories whose main characteristics are summarized as follows:
(sometimes referred to as "Within-Field Catchment System")
Main characteristics:
- overland flow harvested from short catchment length
- catchment length usually between 1 and 30 metres
- runoff stored in soil profile
- ratio catchment: cultivated area usually 1:1 to 3:1
- normally no provision for overflow
- plant growth is even
Typical Examples:
Negarim Microcatchments (for trees)
Contour Bunds (for trees)
Contour Ridges (for crops)
Semi-Circular Bunds (for range and fodder)
(Long Slope Catchment Technique)
Main Characteristics:
- overland flow or rill flow harvested
- runoff stored in soil profile
- catchment usually 30 - 200 metres in length
- ratio catchment: cultivated area usually 2:1 to 10:1
- provision for overflow of excess water
- uneven plant growth unless land levelled
Figure 3 Microcatchment system: Negarim microcatchment for trees
Typical Examples:
Trapezoidal Bunds (for crops)
Contour Stone Bunds (for crops)
Figure 4 External catchment system: trapezoidal bunds for crops (Source: Critchley and Reij 1989)
(often referred to as "Water Spreading" and sometimes "Spate Irrigation")
Main Characteristics:
- turbulent channel flow harvested either (a) by diversion or (b) by spreading within channel bed/valley floor- runoff stored in soil profile
- catchment long (may be several kilometres)
- ratio catchment: cultivated area above 10:1
- provision for overflow of excess water
Typical Examples:
Permeable Rock Dams (for crops)
Water Spreading Bunds (for crops)
Figure 5 Floodwater farming systems: (a) spreading within channel bed; (b) diversion system
An overview of the main Water Harvesting systems which are described in detail in Section 5 is given in Table 1. This summary will be useful as a quick reference.
The eight techniques presented and explained in the manual are not the only water harvesting systems known but they do represent the major range of techniques for different situations and productive uses. In a number of cases, the system which is described here is the most typical example of a technique for which a number of variations exist - trapezoidal bunds are a case in point.
Table 1 - Summary chart of main WH techniques
|
|
Classification |
Main Uses |
Description |
Where Appropriate |
Limitations |
|
|
negarim microcatchments |
microcatchment (short slope catchment) technique |
trees & grass |
Closed grid of diamond shapes or open-ended "V" s formed by small earth ridges, with infiltration pits |
For tree planting in situations where land is uneven or only a few tree are planted |
Not easily mechanised therefore limited to small scale. Not easy to cultivate between tree lines |
|
|
contour bunds |
micro catchment (short slope catchment) technique |
trees & grass |
Earth bunds on contour spaced at 5-10 metres apart with furrow upslope and cross-ties |
For tree planting on a large scale especially when mechanised |
Not suitable for uneven terrain |
|
|
semi circular bunds |
micro catchment (short slope catchment) technique |
rangeland & fodder(also trees) |
Semi-circular shaped earth bunds with tips on contour. In a series with bunds in staggered formation |
Useful for grass reseeding, fodder or tree planting in degraded rangeland |
Cannot be mechanised therefore limited to areas with available hand labour |
|
|
contour ridges |
microcatchment (short slope catchment) technique |
crops |
Small earth ridges on contour at 1.5m -5m apart with furrow upslope and cross-ties Uncultivated catchment between ridges |
For crop production in semi-arid areas especially where soil fertile and easy to work |
Requires new technique of land preparation and planting, therefore may be problem with acceptance |
|
|
trapezoidal bunds |
external catchment (long slope catchment) technique |
crops |
Trapezoidal shaped earth bunds capturing runoff from external catchment and overflowing around wingtips |
Widely suitable (in a variety of designs) for crop production in arid and semi-arid areas |
Labour-intensive and uneven depth of runoff within plot. |
|
|
contour stone bunds |
external catchment (long slope catchment) technique |
crops |
Small stone bunds constructed on the contour at spacing of 15-35 metres apart slowing and filtering runoff |
Versatile system for crop production in a wide variety of situations. Easily constructed by resouce-poor farmers |
Only possible where abundant loose stone available |
|
|
permeable rock dams |
floodwater farming technique |
crops |
Long low rock dams across valleys slowing and spreading floodwater as well as healing gullies |
Suitable for situation where gently sloping valleys are becoming gullies and better water spreading is required |
Very site-specific and needs considerable stone as well as provision of transport |
|
|
water spreading bunds |
floodwater farming technique |
crops & rangeland |
Earth bunds set at a gradient, with a "dogleg" shape, spreading diverted floodwater |
For arid areas where water is diverted from watercourse onto crop or fodder block |
Does not impound much water and maintenance high in early stages after construction |
|
