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Rainwater Harvesting for Restoration of Degraded Soils in Drylands

Water shortage in drylands is a recurrent crisis, and rainwater harvesting is an effective and economical means of addressing the issue. Rainwater harvesting (RWH) can be defined as the process of concentrating rainwater over catchments through runoff to be stored and beneficially used. Rainwater harvesting has been practiced successfully for millennia in parts of the world, and some recent interventions have also had significant local impact. Water harvesting offers under-exploited opportunities for the predominantly rain-fed farming systems of the drylands in the developing world. It works best in those areas where rural poverty is worst. When practiced well, its impact is to simultaneously reduce hunger and alleviate poverty, as well as to improve the resilience of the environment.

Overview
According to UN (2011), drylands cover approximately 40 % of the world’s land area and support more than two billion people, 90 % of whom live in developing countries, with relatively low amounts of precipitation mostly in the form of rainfall. In these regions, precipitation is insufficient to support crop production throughout the year. The non-uniform distribution of precipitation, in arid and semiarid regions, usually results in frequent drought periods during crop growth which severely stresses growing crops thereby reducing yields, sometimes leading to crop failure. Today, rain is the cheapest and often the only source of water for agricultural purposes, although not always reliable. In many dry regions of the world, there is no alternative but better and more effective use of rain to increase and secure food production. This is the essence and potential significance of harvesting rainwater for farming in a hungry world.

Concepts of Rainwater Harvesting (RWH)
There are three biophysical components of all RWH systems (Figure 1 (Insert diagram of Components of a rainwater harvesting system);

(i)  The catchment is the area that receives rainfall and processes runoff downstream. It can be either a micro-catchment, with a relatively small area of natural or treated surface (to induce runoff), or a macro-catchment, usually of large surface area, such as natural watersheds with larger amounts of runoff flowing downstream.

(ii) The storage medium, where runoff is stored temporally before being utilized. It can be a pond, a cistern, an aquifer where water is stored for later use, or the soil profile where water is used directly by plants as ‘green’ water,

(iii) The target, where stored water is being used for domestic, industrial, environmental, and agricultural purposes.

Rainwater Harvesting Methods and Relevance to Drylands
RWH methods are classified in several ways, based on one or more of the system components and uses (size of the catchment, means of water conveyance, type of storage, and water use).

Micro-catchments Rainwater Harvesting (MIRWH)
These are also called in-situ water harvesting, and they have a relatively small surface runoff catchment (from a few square meters to a few thousand square meters), where overland flow travels short distances. Runoff water from the catchment is usually applied to an adjacent agricultural area, to be stored in the soil profile to be used directly by plants or in a small reservoir, cistern, or tank, such as rooftop systems, for later use by humans or animals. According to Oweis et al., (2012), MIRWH includes two major systems;

(a) The rooftop systems are usually runoff-induced catchments; house roofs, greenhouse covers, or courtyards paved with materials having very high runoff coefficients. Runoff water is usually stored in tanks, cisterns, jars, or similar devices. Water is used domestically or for livestock watering, but it also can be used inside greenhouses for irrigating cash crops. Water quality issues are of a concern, especially in domestic use.

(b) On-farm systems are usually earthen catchments, but also runoff-induced small catchments are common in some areas, especially in sandy soils serving cash crops. Runoff inducement may be achieved by smoothing or compacting soil surface, by adding impermeable membranes, or treating the surface with paraffin wax or sodium chloride to reduce infiltration. Usually, runoff water is stored in the root zone of a target area adjacent to the catchment area and used directly by plants. In some cases, water is collected in ponds and used later for supplemental irrigation of the crops during drought spells. Among the most common on-farm RWH techniques are contour ridges, terraces, bunds, runoff strips, and basins.

Macro-catchments Rainwater Harvesting (MARWH)
Macro-catchments, sometimes called medium-sized catchments, are characterized by large flood zones that are situated outside the cropping area. Often farmers must use structures such as dams or bunds to divert, transfer, collect and store the runoff. Such systems are often difficult to differentiate from conventional irrigation systems. Examples of macro-catchments include stone dams, large semi-circular hoops, trapezoidal bunds, hillside conduit systems and cultivated reservoirs, which from 1000 m2 to 200 ha.

Enhancing soil moisture and vegetation growth
The major portion of the rainfall in drylands runs off the land with limited opportunity and time to infiltrate and be stored in the soil profile (especially in degraded soils with low infiltration rates). This leaves seeds/plants with little moisture to germinate, emerge, and grow. Macro- and micro-catchment RWH systems have a variable but positive impact on soil moisture regimes and crop yields. Tian et al., (2003) found that macro-catchment water harvesting of ridges and furrows with plastic mulch increased tuber yields of potatoes by 158–175 % for two years and corn yield by 1.9 times due to higher water use efficiency. The plastic used to mulch the ridges poses environmental problems so biodegradable plastic film should be used.

Controlling soil erosion and combating desertification
In arid environments, wind and water erosion is very much influenced by anthropogenic pressures including the overgrazing and cultivation exacerbated by climatic changes, urbanization, and management factors. Micro-catchment rainwater harvesting (MIRWH) practices attempt to control soil erosion within small areas and deposit it in the target areas. The amount of soil erosion captured by contour ridges and bunds vary greatly with the spacing, slope, soil type, and cover, and rainfall intensity and amount. Therefore, MIRWH can largely prevent water erosion from leaving a field or a farm by redistributing it at small target locations. Eroded top soil is usually more fertile and contains higher organic matter. When deposited at the target areas, it improves soil fertility and enhances soil-water conservation processes such as infiltration and water holding capacity.

Conjunctive use of water resources
Rainwater can be used directly by plants but can also be stored in surface reservoirs or in groundwater aquifers to be used later. Harvesting rainwater in reservoirs/ponds, cisterns, and other containers can provide supplemental irrigation during the dry season and dry spells. For instance, cultivating feed crops on gentle slopes and in runoff strips can reduce the risk of crop failure and increase the yields of feed crops. When the crop is cultivated on a strip of 0.5 to 1 m wide, leaving an adjacent upstream parallel strip as a runoff catchment of about 1 to 2 m wide, will allow more runoff water to be stored and used by the crop. Furthermore, the available harvested water in small surface reservoirs can provide one or two doses of irrigation to alleviate drought spells during the growing season. Also, the harvested water can be used conjunctively with groundwater resources, when available, to provide drinking water for people and livestock.