The importance of rainwater harvesting:

Water resources constitute the most significant natural resource for the survival of humankind and the socio-economic development of nations. The relevant knowledge of water resources is consequently a prerequisite condition for the elaboration of development projects and to appraise sustainable resources. However, this knowledge of water resources is intricate, taking into account its multidimensional character and the varied utilisations.

Given population growth in arid and semi-arid regions, the erratic and variable rainfall, poor soil fertility, and limited possibilities to increase cultivated area, the agricultural priority across all farming systems is to increase biological and economic yield per unit of water. In rainfed fields, improvement can come only from conserving rainfall water in the rooting zone of crops, and from managing the field and the crops to use water more efficiently. In limited cases, supplementation of water collected from off-site water harvesting can be used to bridge small periods of water deficit. Actual water use efficiency in current farming systems in the drought prone countries of West Africa is often very low.

In the populated arid and semi-arid regions, people are by definition exposed to irregularity or insufficiency of rains. These effects are sometimes catastrophic for agricultural production, and peoples’ very survival is often threatened because of the recurring drought and desertification. Under such conditions, irrigation could have been the ideal solution if the basic infrastructure were put in place.

It is for this reason that during the last 50 years, a number of governments and institutions in West Africa especially in the Sahel region have launched programmes and projects aiming at the conservation of water resources and improving rainfed agriculture.

 

The Concept of Surface Water Harvesting

According to FAO, water harvesting in its broadest sense is defined as “the collection of runoff for its productive use”. Water harvesting is the collection for productive purposes. Instead of runoff being left to cause erosion, it is harvested and utilised. In the 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.

Runoff may be harvested from roofs and ground surface as well as from intermittent or ephemeral watercourses. A wide variety of water harvesting techniques for many different applications is known. Productive uses include the 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. Water harvesting can be considered as a rudimentary form of irrigation. The difference is that with water harvesting the farmer has no control over the timing. Runoff can only be harvested when it rains. In regions where crops are entirely rainfed, a reduction of 50% in the seasonal rainfall, 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. 

 

Typology of RWH Systems

Rainwater harvesting (RWH) systems can be divided into  two runoff farming types, i.e. micro-catchment and  macro-catchment runoff farming. Micro-catchment runoff farming is a method of collecting surface runoff from a small area and storing it in the root zone of an adjacent infiltration area. It is also sometimes described as traditional or indigenous soil and water conservation technique or in situ water harvesting technique. Macro-catchment runoff farming, on the other hand, refers to runoff farming from long slopes, as medium size catchment water harvesting or as harvesting from external catchments systems stored and usually used for supplementary irrigation or other productive purposes. A wide variety of these water harvesting techniques are known and have been implemented during the past three decades to combat the effect of drought and to rehabilitate degraded land in Sub Saharan Africa (SSA). In West Africa, the most common water harvesting techniques are micro-dams, stonewalls, stone bunds, improved lowlands, and earthen dams. Concurrently, other techniques are applied in certain countries, which notably concern trenches, little walls, earthen bunds, half-moons, ‘zai’ or ‘tasa’, ridges, stone dikes, bench terraces, scratching techniques and impluvium, etc. Many techniques are easily reproducible and can be carried out manually and at low cost. Similarly, maintenance of the retention and collection devices can be done by hand - even though they may require considerable labour input. The cost of transportation of the material (stones) and the limited lifespan of certain types of structure constitute the principal constraints.

It has been reported that the adoption of water harvesting techniques and soil conservation practices did indeed improve food security in some arid and semi-arid regions of West Africa e.g. in Burkina Faso, Mali and Niger. Additional advantages of water harvesting are erosion control as well as the replenishment of aquifers, as in other inter-tropical African countries, insufficient, irregular and spatially uneven rainfall limits crop production. Many efforts have been made by research and development organizations to improve crops. However, it must be recognized that  classical intensification techniques (such as fertilization, animal traction, improved crop variety) have not always guaranteed good agricultural production in the harsh, risky environment of the Sahel and the Sudanian zones.

 

Advantages of Rainwater Harvesting Systems

One of the main benefits of using the RWH techniques is the considerable increase in production it can generate when the appropriate technique or set of techniques are applied. Other benefits can be seen, such as improved spring flow, and more fodder and firewood, although most projects do not keep statistics on these side benefits.

Small-scale, low input and long-lasting technologies are the most beneficial ones for farmers and the most promising ones in achieving sustainable outcomes. This is especially true for small and medium stone constructions, for terraces, for vegetation barriers and for soil pits and half-moons, the former being more low cost and sustainable and the latter easier to individually setup and manageable. In the Bam Province (Burkina Faso), ameliorated plots under millet and sorghum show an average increase of yield in comparison to non-ameliorated plot. The region’s most widespread types of construction are permeable rock bunds which vary from small stone lines to larger dikes and dam.

Constraints

There are two main constraints to the development of RWH systems in many regions of SSA – labour shortages and land tenure. In general, soil and water conservation work suffers from labour constraints because it is usually conducted between January and June, a period which coincides on the one hand with intensive vegetable cultivation and harvesting and, on the other hand, with the out-migration of the most able-bodied villagers. Labour is often the most important economic factors if local material is used. The construction of contour ridges of 0.2 m height with an horizontal interval of 1.5 m needs 90 man days (MD) per ha in the first year and 50 MD in the second year.

 

Socioeconomic Considerations

The socio-economic impacts on revenue generation, land use, labour allocation and rural migration presented in recent surveys in the Sahel, indicate that the increase in food production and incomes in zones where there are few alternatives to water harvesting (ex: Sahelian zone in Burkina Faso, Plateau Dogon in Mali, Bassin Arachidier in Senegal, Hamdallaye watershed in Niger, etc.) constitutes the largest socio-economic impact of these techniques. It was also noted that there are effects set off, such as the reduction of rural outmigration and a positive impact on the organization of the farmers, because large operations require solidarity and cooperation. For these socioeconomic effects, the best results are obtained using water harvesting techniques such as stone dikes, terraces, zaï and boulis.

Access and use by gender

The accessibility to all these technologies varies greatly according to the type of producer, social class, the production objective and technical nature of production. For small, low-income producers, access to innovations that require a substantial allocation of additional resources, poses many problems. This category of producers often turns to technologies that enable them to manage risk and survive: their aim is not to obtain maximum yield. In the case of women’s groups, their access to certain natural resource management technologies is equally problematic due to difficulties they face regarding land ownership and acquisition for farming. By contrast, these womens groups have  easy access to innovations in processing and preservation of agricultural products. Generally, technology access and usage depend on the socio-economic conditions of each social class.

A lot of studies have proven that all social classes in a given community do not necessarily react in the same way to the same technology proposed to them. This situation is due to the fact that all these socio-cultural classes (women, men, the old, the young, rich, poor, literate, illiterate, etc) can present different profiles at the level of activities or tasks, as well as at the level of access to and control of resources. For example, women encounter difficult problems in making use of certain agro-forestry technologies (involving tree planting or water and soil conservation) because the traditional land tenure system in force does not give them any right to land ownership. Generally, men are more involved than women in decisions relating to water used for food production throughout water harvesting and irrigation schemes. Women’s lack of decision-making power follows from the position of men and women in relation to ownership of land.

 

BY: Pôle Eau de Dakar / Dakar Water Hub