- FLOOD IRRIGATION
When compared with overhead irrigation, flood or surface irrigation has a number of disadvantages, the main ones being:
- More water is used than with a sprinkler system;
- More water is wasted through porous ditches, deep percolation into the soil, and evaporation;
- Irrigation efficiency is 25% ‐ 70% compared to 60% ‐ 80% for sprinklers;
- Due to the patchy application of the water, germination of the seeds is not as adequate. A sprinkler system gives a good, even germination;
- There are serious limitations on the use of flood irrigation on some farms due to soil texture, steep slopes and topography of the land. Sprinkler irrigation can be used to irrigate any farm;
- Due to permanent ditches, water ways, etc., good land is wasted and tractor operations are made difficult. The corrugations, ridges and furrows and land leveling all require farm machinery;
- In some cases labour requirements are much higher for flood irrigation which makes the operation more costly;
- Soil erosion and the leaching of nutrients from the soil can also be a problem with flood irrigation. A well designed sprinkler system can eliminate erosion, compaction, sealing and leaching of the soil; and
- Flood irrigation can create serious drainage problems when too much water is applied.
The main advantages of flood systems are:
- When plenty of water is available irrigation can be completed very quickly;
- The capital cost can be less than for overhead irrigation;
- Provided that water is running down from a high point on the farm, no pumping is required and;
- Wind has no effect on surface irrigation, whereas it can be a problem with overhead irrigation.
LAYOUT OF A SCHEME
The diagramme below shows the general layout of a flood irrigation scheme. The general requirements for such a scheme are:
- Night storage or a permanent dam is needed to supply the water to the scheme;
- A permanent canal to carry the water to the lands;
- An off‐take furrow to carry the water down the lands and;
- Measuring devices are required to measure the flow of water down the canal.
Figure 1: Layout of a flood irrigation scheme
Figure 2: Large canals carrying water to irrigate
These can be constructed from a variety of materials and the size of the canal will depend on the amount of water required from the irrigation scheme.
Materials that can be used are:
- Concrete laid in a trench;
- Brick and slab lining; and
- A pre‐cast concrete slab lining.
The illustrations below shows the various sizes and shapes of irrigation canals in cross section. The table gives the rates of flow of water and the number of bricks per 100 metres to construct the canals.
Figure 3: Shows various sizes and shapes of irrigation canals
Table 1: Rates of flow of water and number of bricks
These are required to measure the flow of water down a canal so that the irrigator knows how much water is being delivered to his lands. Three devices which are used are:
- A 90° Notch Weir;
- A Rectangular Notch Weir; and
- A 6 inch Marshall Flume.
Each of these has its own table showing the rate of flow of water in relation to the height or head of water. The table for the 90° Notch Weir is shown below as an example.
FLOW OVER A 90° NOTCH WEIR
The rate of the flow of water is measured by the head above the bottom of the 90° ‘V’ subject to the following conditions:
- The channel approach must be so wide that the velocity of approach is negligible; and
- The head must be measured 1.3 metres upstream of the notch.
Table 2: Flow over a 90° Notch weir
These are needed to take the water from the main canal to the lands as there usually are a number of furrows leading from the main canal. A trench lined with concrete or asbestos can be made, or a flat plastic pipe which is a long tube of thin plastic can be laid. Short lengths of plastic tube serve to take the water from the off‐take furrow onto the land.
LAYOUT ON THE LANDS
There are a number of different layouts that can be used to run the water from the off‐take furrows and through the crops. These are:
- Border Strips which are long beds divided by small ridges to direct the water flow down the bed;
- Corrugations – small furrows which the water runs down;
- Contour Flooding allowing the water to run between the contours on a land; and
- Ridge and Furrow which means directing the water down furrows, drawn down‐land while the crop is growing on the ridges.
Table 3: The following table gives the Agritex (Zimbabwe) recommendations for these different layouts.
|BORDER STRIPS||CORRUGATIONS||CONTOUR FLOODING||RIDGE & FURROW|
|DIMENSIONS||Width: 3 ‐ 20m Length: 50‐ ‐ 200m Border Ridges: 500 ‐ 750mm wide x 250mm high||Furrows: 150mm wide x 100mm deep Centre: 0,4 ‐1m Length run: 50 – 150m||Furrows: 450mm wide x150mm deep Width of 10 – 15mm between furrows||Ridges 50 ‐ I50mm Maximum length run Light soils ‐ 100m Heavy soils ‐ 200m|
|GRADIENTS||Light soils: 1:100 ‐ 200 Heavy soils: 1 :3 00 ‐ 600||Max. 1:50 Min. 1:250||Furrow Gradients of 1:200 ‐ 1:300||Light sods 1: 100 ‐ 300 Heavy soils 1: 300 – 600|
|STREAM FLOW||About 7 litres/sec. for 1m width/100m length||Start with 1 ‐2 litre’s/s. Cut back to 0.5 l/s||7‐ 30 Litres/s||7 litres//s|
|ADVANTAGES||Row and Broadcast. Large streams||Row and Broadcast crops. Slopes up to 2%. Depth of||Slopes over 2%. Cheap & quick.||Supplementary irrigation for row crops. Layout|
|with minimum labor. Best on flat land with low permeability.||irrigation easily controlled.||easily changed.|
|LIMITATIONS||14 litres/sec. minimum.||Minimum slope 1:250. Requires skilled labor. Necessary to level.||Inefficient||1 labourer to 3 streams. Needs large labour force.|
|GENERAL||Easily adapted to small‐scale farming and ox drawn machinery.||Syphon tubes required to feed water into the corrugations.||Furrows dammed with plastic bags.||Land leveling need not be too accurate.|
- GATED PIPE IRRIGATION
One of the problems with flood irrigation is that the permanent canals are costly to build and interfere with the movement of tractors and implements across the lands. These problems can be overcome by using a system of piping that is portable and can be moved and removed around the lands to allow tractor operations to take place. Furthermore, the length and direction of run of the water can be altered and the pipes can be moved to other parts of the farm if required. This is particularly useful for supplementary irrigation where low‐costs and flexibility are important.
TYPE OF PIPE
In any surface flow system, large quantities of water must be carried across the lands, the only pressure coming from the force of gravity. Flow rates can range from 50 ‐ 300 cubic metres an hour (m3/h), therefore, large diameter pipes must be used. The two types of pipe are:
Rigid PVC Pipe: This pipe is hard‐wearing and can withstand fairly high pressures but is awkward to handle and expensive.
Lay‐flat Plastic Tubing: This is a flexible thin walled plastic tube which lies flat when empty of water. When filled with water at a pressure head of 0.5 metres, it is round, and at lower pressures oval in shape. This tube can be bought with a wall thicknesses from 0.1mm ‐ 0.5mm and when fully expanded from 160mm ‐ 300mm. Although Lay‐flat tube can
be damaged more easily than rigid PVC it is much cheaper and handled easier.
One man can fold up and carry a 50 metre length of tube, 250mm in diameter with a wall thickness of 0.4 mm. This type of tube is used for many irrigation schemes in the USA.
The rates of flow of water in various sizes of Rigid PVC pipe are given below. Under general farm conditions and allowing for kinks in the pipe and rough soil surfaces beneath the pipe, the rate of flow in Lay‐flat tube will be 20 ‐ 25% below the flow in Rigid PVC pipe.
Table 4: Rates of Flow through Rigid P.V.C. Pipes (Figures are cubic metres/hour)
Both Rigid PVC pipe and Lay‐flat tube can be bought with outlets or gates built into the pipe. These outlets can be adjusted to control the amount of water which flows from the pipe.
Figure 4: PVC piping used for irrigation
Figure 5: Lay‐flat pipe being used to irrigate
- DRIP OR TRICKLE IRRIGATION
Drip irrigation is used to apply a water and fertilizer solution to the base of the plant frequently. The object is to keep the root zone at, or just below, field capacity which eliminates water stress and irregular growth of the plant. The system wets the ground along the rows of plants and will leave the ground dry between the rows. Advantages of the system are:
- The prevention of soil crusting or capping because the soil around the plant is always wet;
- Reduced growth of weeds between the rows because this ground is always dry;
- Fungicides and insecticides remain on the plants during irrigation;
- Direct application of fertilizer and nutrients to the root zone of the plant, with little wastage;
- Due to the absence of water stress and subsequently the small amounts of fertilizer can be fed through the system at frequent interval which helps the crop or ripen earlier and therefore gives higher yields;
- Water savings of up to 30% are possible, because the spaces between rows are not watered;
- The system requires very little labour. Once the pipes are laid out for the crop they remain until after the crop has been harvested. In the case of orchards they are not moved at all; and
- Very high system efficiency because very little water is wasted through leakage or evaporation.
Disadvantages of the system are:
- The capital cost can be high depending on the plant population of the crop, the soil wetting patterns required and whether the system is fixed or portable.
- Due to the very small diameter nozzles used water has to be filtered thoroughly. Algae, colloids and chemicals in the water pass through any filter system and can block the nozzles. The system has to be inspected and maintained frequently to make sure that the water is spread evenly.
Layout of the System
Small diameter polythene pipes with drip nozzles or other outlets are laid on the soil surface along each plant or tree row. These lateral pipes are usually 12‐16mm in diameter and are fed by field mains usually 50mm in diameter. The main pipes are buried underground and pressure controls and filters are included in the design.
There are various designs of outlets or nozzles and they supply water at very low rates of 2 ‐ 10 litres per hour. One type of nozzle is a micro‐tube with an internal diameter up to 1mm. They are supplied in different lengths which together with diameter of the tube regulates the flow of water. Another type of nozzle is made of a coiled plastic tube, which increases friction in the tube and reduces the water flow. The nozzles are attached directly to the lateral pipes at specific intervals, usually 0.6m apart for vegetable crops and 0.5m ‐ 1.25m for trees in an orchard.
The long labyrinth dripper slows down the rate of water flow by passing the water through a series of small bends and curves inside the outlet; litreally a labyrinth. The pressure compensating dripper is a small flat plastic nozzle about the size of a 50 cent coin containing a valve which equalises the water pressure along the whole length of pipe. This means that a pipe can be laid up a slope and the water flow will be the same from all the outlets along the pipe. The use of this valve has significantly simplified the design of drip irrigation systems for hilly areas.
Figure 6: Drip irrigation outlet