There is no one-size-fits-all method of irrigating fields that differ in soil, crop size, or crop types. Any location’s best system is the one that can provide appropriate irrigation without water wastage, is easy for the farmer to comprehend and put to use; and is dependable and easy to fix in the event that something goes wrong.
Many irrigation schemes around the world will never recover the cost of their implementation. It is best to start small and cheap for the inexperienced novice farmer so that information and experience can be obtained before making a large commitment.
Choosing a location
Aside from soil fertility, depth, and location relative to the market and homestead, there are more factors to take into account:
Water source: Irrigation is made easier when the field is located near a water source.
Topography: If the farmer intends to irrigate with a surface approach, a gently sloping area is the best choice. This means that it takes less time and effort to build channels, beds, and other things.
Drainage: When deciding on a location, make sure that it has adequate subsurface drainage. Large areas of good farmland have been ruined by the installation of irrigation projects without adequate drainage.
The reason for this is that water tends to rise in the soil in hot climates with high evaporation rates, and then vanish into thin air. This can cause large amounts of salt to be deposited in the topsoil, which inhibits plant growth. The only way to get rid of the salts in the soil is to ensure that there is a net downward movement of water through the soil. In areas with poor drainage, adding more water will cause the soil to become saturated, making it unusable for farming.
The watering can and hosepipe method is best suited for small basins and beds since it ensures that the water is evenly distributed across the area. If the crops are widely separated, a small basin can be prepared around each plant. However, manually watering a wide area is incredibly time-consuming and thus ineffective.
Flood or surface irrigation systems are cheap and easy to construct, but if you have sandy soil, you will need a lot of water.
Basins or beds
A basin or a bed is a flat area surrounded by a bank designed to hold back water. To ensure that the water is uniformly distributed, the area must be absolutely level. Filling the basin with water before planting is an easy way to confirm that it is level. Because of the water’s distribution, no area should be deeper or shallower than another.
Tractors and ploughing equipment or oxen can quickly and efficiently prepare a ridge-and-furrow system. Irrigating crops in rows on a gently sloping plot of land is an efficient method that saves time on field preparation.
Water flows from the top of the slope to the bottom of a furrow. In order to prevent water from overflowing at the furrow’s bottom, the flow of water must be controlled.
During irrigation, crops that are planted in rows along the tops or sides of ridges will get water seeping into the soil in a downwards and sideways fashion. Furrows can be spaced according to the crop’s needs.
In order to accomplish good water distribution, surface irrigation basins are limited to 50 m² (for example, 5 m wide by 10 m long) and furrows to 100 metres in length.
Due to the slower application rate than the rate of water infiltration into the soil, sprinklers don’t necessitate as much particular land formation. In order for sprinklers to work, they must be supplied with pressurised water from either a pumped supply or an elevated reservoir. Investments in larger pumps and stronger pipelines are required here, which is more expensive than in other systems. This is more efficient than surface irrigation systems, but for the more open-minded farmer, it would be well worth considering.
Irrigation through drip
Currently, drip irrigation is the most cutting-edge irrigation technique available. There are a variety of options available. They are constructed from a series of thin plastic pipes with incredibly small holes spaced out along the pipe’s length (30 centimetres to 1 metre apart). One or two plants are irrigated at a time by water that drips from each of the holes at pre-calculated rates.
Drip irrigation systems can save as much as 30% of the water used by other systems. In Namibia, drip irrigation should always be advised to farmers who can afford it. Using this method, a small business can learn a lot and then grow after they have some success.
Irrigation frequency and quantities
Water requirements of crops
Crop type, age, temperature, humidity, sunlight, and wind speed all play a role in how much water a plant needs. The quantity of water utilised by the plant and the evaporated amount in the soil around the plant are included in the depth of water indicated in millimetres per day. Crop water use can be estimated for any site by looking at climate records.
However, the following scenarios must be taken into account:
- The crop will consume more water if the weather is exceptionally hot, windy, or dry.
- The crop will consume less water if the weather is unusually humid or foggy.
- Young plants can only take half of this water, but mature plants can use 1,25 times as much.
Using the following formula, you could figure out how much water a field needs:
Volume (litres) = Depth (mm) x Area (m²).
For example: in March, a field of tomatoes 10 metres by 5 metres growing near Rundu needs 4,4 mm per day, so the volume needed = 4,4 mm x 50 m² = 220 litres.
A little overestimation from the figures given above is recommended when using this method to estimate water use. This can make it hard to figure out what “normal” weather is, and it can also cause irrigation water to be lost because of leaks or ineffective application methods.
The roots of a plant require both water and air to thrive, and they can only thrive if both are present. If the topsoil is always wet, most plants prefer to extend their roots horizontally rather than produce deep roots in order to take advantage of this water. Water is scarce in the topsoil, and roots will grow deeper to find it.
Characteristics of Namibia’s best-performing dry-land crops and trees include:
- They grow deep roots in order to find water in the soil.
- They are very efficient at absorbing water from the soil.
It is important for vegetables to have deep roots so they can draw water from the soil’s lower layers. Table 2 shows some typical rooting depths for crops grown in optimum conditions.
Crops that are grown in good, deep soil but do not establish fine roots to the depths mentioned above are likely signs that too much water is being provided.
The “sausage test” can be used to determine if watering is necessary before it is done. Dig all the way down to the roots. In your palms, squeeze a little dirt from the bottom of the hole to get a better grip on the earth. If, after squeezing, the soil forms a sausagelike mass, it is still moist and does not require more watering (see illustration). If the soil falls apart, it is time for a good watering. After watering, dig another hole and do it again to make sure the plants got enough water.
In order to avoid evaporation losses, the water will be pumped into the deeper layers of the soil once a week, rather than once a day. On the other hand, the soil loses very little water in unreachable, deep layers.
Deep watering has the additional benefit of allowing the topsoil to dry between irrigations, which can delay many insects’ reproduction cycles. However, daily irrigation of seedlings is required.
When should you irrigate
Generally speaking, it is best to irrigate in the morning or at night. Because of this, there are two possibilities:
- When the entire plant is wet with water, even the smallest drops can linger on the leaves for many hours. If there is a lot of sunlight, these drips can intensify the sun’s rays and cause leaf damage. This is most commonly found on cabbage plants.
- In the middle of the day, evaporation rates are at their peak, so more of the water applied at this time will evaporate. Wet surface area and evaporative loss increase dramatically when water is applied to the entire plant. There is one exception: the leaves of tomatoes can be cooled by water sprayed on them during the peak hours of daytime heat.
The information in this article is credited to the Namibia Agricultural Union and Namibia National Farmers Union who published the Crop Production Manual in 2008.