A farmer will never have too much water. There is always a place on his farm where he can utilise it, and it will always be directly beneficial to his yields. We have a particular problem during our long dry season, and every drop counts after the rains stops.

Saving and retaining water for agriculture offers essential benefits:

• Drinking water for humans and animals making animal husbandry possible
• Vegetable production for improved domestic food security
• Increased production and reduced risks
• Prevention of erosion by run-off reduction

The main component of rainfall harvesting consists of a run-off area that allows little or no infiltration and a catchment or collection area where the accumulated water can be stored and used. The storage volume must be sufficient, so that the water can be used for a reasonably long time. If water is collected in earth dams, the volume of water must at least be able to tide the user over the dry season and the volume of water in tanks must be sufficient for at least two months.

The following factors must be kept in mind with rainfall harvesting systems:

• Rainfall distribution
• Rainfall intensity
• Run-off characteristics of the catchment area
• Water storage capacity
• Evaporation

Benefits of rainfall harvesting:

• Improved yields compared to crops raised with normal rainwater is free
• A better living standard can be maintained

Disadvantages that rainfall harvesting might cause:

• High risk that too little rain may occur at critical stages
• Costs associated with catchment areas and containers for storage
• Cost to distribute water to the usage points

Rainfall harvesting can be done in various ways. The following methods are discussed in this article:

• Harvesting water from roof surfaces and storage in tanks or reservoirs
• Harvesting water from prepared ground surfaces or stone slabs and storage in reservoirs
• Harvesting water between widely spread tree rows and storage in a soil profile where roots are present
• Harvesting of rainwater from micro catchment areas and storage in reservoirs or a soil profile where roots are present

Collection of water from roofs

Tin roofs (corrugated iron)

Large roof surfaces are usually fitted with gutters and down-pipes and only storage capacity needs to be erected. Small roof surfaces do not usually have gutters and the following method can be used (see Figure 1). Cut a slit into a 50mm diameter polythene pipe. Bind the pipe firmly to the roof or rafters with wire, so that it can hold the water flowing into it. The end of the pipe, which is not tied, is kept long and is not cut, so that it can be used as a down-pipe to the container.

Thatched roofs

Use short pieces of corrugated iron sheets, secure these under the edge of the thatched roof, so that it juts out ±300mm and in such a way that it allows the water to run onto the zinc sheets from the roof. The entire roof edge is elongated with these corrugated sheets and the same method is then followed as with the tin roof and polythene pipes. (See Figure 2).

Water harvesting from ground surface or stone slabs

Ground surfaces that are prepared should be on a reasonable slope to ensure fast run-off. If the soil is not of a hard type, the following practices may be followed to ensure a better run-off from the run-off area:

• Remove vegetation, level the surface and compact the soil with a roller.
• Use a soil and cement mixture method and compact firmly. In all cases, the regrowth of vegetation must be prevented and weirs must be erected to direct water to a reservoir for storage.
• Stone slabs or large granite boulders give a good run-off and water must be collected with weirs and barriers and transported to a reservoir by means of a pipe. If natural run-off patterns appear on the rocks, the water can be stored by means of a weir. (Figure 3).

The apparatus can be mounted behind any implement and used during each tilling between widely-spaced plant rows.

The apparatus consists of a V-blade scraper and an oval wheel, which lifts up and drops the blade to form little dams. A chip in the wheel prevents the dams from being trampled. During thunderstorms, these weirs form little dams that collect water.

Collection of water on exposed areas under field conditions

Three implements generally used by farmers to prevent rainwater from running off from exposed fields, are:

Spotting plough

This plough is used by letting the hydraulic system of the tractor move up and down to draw short oblong trenches. During rain showers the water is collected in these trenches and infiltration is improved. As a result of the better infiltration, the vegetation increases, which prevents further run-off of water.

Ripper tine plough (sub-soiler)

The ripper tine plough (or sub-soiler), also constructs little dams – as does the spotting plough – but its advantage is that the ripper tine remains permanently in the soil and an oval wheel lifts the plough to make little oblong dams. The operator therefore does not have to operate the hydraulic system of the tractor and the tractor keeps working at constant traction. The ripper tine loosens the soil between the little dams and more water infiltrates the soil. A better seedbed for the establishment of vegetation is prepared in this way.

Disk ripper plough

A disk ripper plough is used in the same way as the two abovementioned ploughs and in some soils, re-establishment of vegetation is very successful. The advantage of this plough is that it can easily be made from old used ploughs. The greatest disadvantage of this plough is that, in certain soils, it smears the side of the little dams and the water then takes too long to penetrate the soil. This may cause the seedlings to become waterlogged.

Harvesting of water from micro catchment areas

The same principle applies as in the section about water harvesting between widely spread tree rows, except that the run-off area can be much greater and that it has to serve a very small catchment area (see Figure 4). The treated area must be free of vegetation and must be kept as sealed as possible.

Area required for water collection

Much water can be harvested by means of the aforementioned rainfall harvesting techniques. This water can be used for different purposes. Plants need approximately 5mm of irrigation per day over a period of six months. A rough size of the run-off area should be for irrigating one square meter if the run-off is 100% and the storage space is large enough. Use the following method:

Therefore, for 1 ha irrigation under these conditions, 0,58 x 10 000 = 5 800 m³ storage volume and 1,45 ha collection area is required. A very rough rule of thumb that can be used is 2 m² collection area necessary for each m² irrigation. Factors such as run-off potential and storage capacity must be considered when using this calculation.

Where specific long-term rain data is available, more accurate calculations can be done.

Harvesting of water between widely spread tree rows

This method consists of a run-off area and a collection area (see Figure 4). The run-off area must be large enough to supply the root zone with enough water on a regular basis. The storage area must be able to easily absorb and hold water. Evaporation can be reduced by covering the collection area. Small dams around the trees shaped in such a way that the one runs into the other, increases water penetration. The height of the weir must be designed to handle a 1-in-10-year rainfall run-off.

Harvesting of water between widely-spaced crop rows

An apparatus which makes dams for collection of water and prevents run-off, is being used with great success in sandy and clayey soils.

References

• Boers, Th. M. 1994. Rainwater harvesting in arid and semi-arid zones.
• Prins, D. 1994. Water harvesting and sustainable agriculture in arid and semi-arid regions.
• Practical evaluations and experiments. (Praktiese evaluerings en eksperimentering) 1997. ARC Institute for Agricultural Engineering (ARC-IAE), Silverton.