3.1 When to Use Furrow Irrigation
3.2 Furrow Layout
3.3 Furrow Construction
3.4 Irrigating Furrows
3.5 Planting Techniques
3.6 Maintenance of Furrows
Furrows are small, parallel channels, made to carry water in order to irrigate the crop. The crop is usually grown on the ridges between the furrows (Figures 23 and 24).
3.1.1 Suitable crops
3.1.2 Suitable slopes
3.1.3 Suitable soils
Furrow irrigation is suitable for a wide range of soil types, crops and land slopes, as indicated below. Under which circumstances to choose furrow irrigation is further discussed in Chapter 7.
Furrow irrigation is suitable for many crops, especially row crops. Crops that would be damaged if water covered their stem or crown should be irrigated by furrows.
Figure 24 Top view and cross-section of furrows and ridges
Furrow irrigation is also suited to the growing of tree crops. In the early stages of tree planting, one furrow alongside the tree row may be sufficient but as the trees develop then two or more furrows can be constructed to provide sufficient water. Sometimes a special zig-zag system is used to improve the spread of water (Figure 25).
Corrugation irrigation, frequently mentioned in literature, is a special type of furrow irrigation, used for broadcast crops. Corrugations are small hills pressed into the soil surface. The application of this method is limited and is not included in this manual.
In summary, the following crops can be Irrigated by furrow irrigation:
- row crops such as maize, sunflower, sugarcane, soybean;
- crops that would be damaged by inundation, such as tomatoes, vegetables, potatoes, beans;
- fruit trees such as citrus, grape;
- broadcast crops (corrugation method) such as wheat.
Figure 25 Zig-zag furrows - A: Zig-zag furrows used for irrigating trees on land with a moderate slope (0.5-1.5%)
Figure 25 Zig-zag furrows - B: Another zig-zag pattern for furrow irrigation on fairly flat slopes (under 0.5%)
Uniform flat or gentle slopes are preferred for furrow irrigation. These should not exceed 0.5%. Usually a gentle furrow slope is provided up to 0.05% to assist drainage following irrigation or excessive rainfall with high intensity.
On undulating land furrows should follow the land contours (see Figure 26). However, this can be a difficult operation requiring very careful setting out of the contours before cutting the furrows (see section 3.3 Furrow Construction).
Furrows can be used on most soil types. However, as with all surface irrigation methods, very coarse sands are not recommended as percolation losses can be high. Soils that crust easily are especially suited to furrow irrigation because the water does not flow over the ridge, and so the soil in which the plants grow remains friable.
This section deals with the shape, length and spacing of furrows. Generally, the shape, length and spacing are determined by the natural circumstances, i.e. slope, soil type and available stream size. However, other factors may influence the design of a furrow system, such as the irrigation depth, farming practice and the field length.
Furrows must be on consonance with the slope, the soil type, the stream size, the irrigation depth, the cultivation practice and the field length. The impact of these factors on the furrow length is discussed below.
Slope
Although furrows can be longer when the land slope is steeper, the maximum recommended furrow slope is 0.5% to avoid soil erosion. Furrows can also be level and are thus very similar to long narrow basins. However a minimum grade of 0.05% is recommended so that effective drainage can occur following irrigation or excessive rainfall. If the land slope is steeper than 0.5% then furrows can be set at an angle to the main slope or even along the contour to keep furrow slopes within the recommended limits. Furrows can be set in this way when the main land slope does not exceed 3%. Beyond this there is a major risk of soil erosion following a breach in the furrow system. On steep land, terraces can also be constructed (see Basin Irrigation) and furrows cultivated along the terraces.
Soil type
In sandy soils water infiltrates rapidly. Furrows should be short (less than 110 a), so that water will reach the downstream end without excessive percolation losses.
In clay soils, the infiltration rate is much lower than in sandy soils. Furrows can be much longer on clayey than on sandy soils. The determination of the infiltration rate is explained in Annex 2.
Stream size
Normally stream sizes up to 0.5 l/sec will provide an adequate irrigation provided the furrows are not too long. When larger stream sizes are available, water will move rapidly down the furrows and so generally furrows can be longer. The maximum stream size that will not cause erosion will obviously depend on the furrow slope; in any case, it is advised not to use stream sizes larger than 3.0 l/sec (see Table 3).
Irrigation depth
Applying larger irrigation depths usually means that furrows can be longer as there is more time available for water to flow down the furrows and infiltrate.
Cultivation practice
When the farming is mechanized, furrows should be made as long as possible to facilitate the work. Short furrows require a lot of attention as the flow must be changed frequently from one furrow to the next. However, short furrows can usually be irrigated more efficiently than long ones as it is much easier to keep the percolation losses low.
Field length
It may be more practical to make the furrow length equal to the length of the field, instead of the ideal length, when this would result In a small piece of land left over (Figure 27). Equally the length of field may be much less than the maximum furrow length. This is not usually a problem and furrow lengths are made to fit the field boundaries.
Figure 27 Field length and furrow length
Table 3 gives some practical values of maximum furrow lengths under small-scale irrigation conditions. The values shown in Table 3 are lower than those generally given in irrigation handbooks. These higher values are appropriate under larger scale, fully mechanized conditions.
Table 3 PRACTICAL VALUES OF MAXIMUM FURROW LENGTHS (m) DEPENDING ON SLOPE, SOIL TYPE, STREAM SIZE AND NET IRRIGATION DEPTH
|
Furrow slope (%) |
Maximum stream size (l/s) per furrow |
Clay |
Loam |
Sand |
|||
|
Net irrigation depth (mm) |
|||||||
|
50 |
75 |
50 |
75 |
50 |
75 |
||
|
0.0 |
3.0 |
100 |
150 |
60 |
90 |
30 |
45 |
|
0.1 |
3.0 |
120 |
170 |
90 |
125 |
45 |
60 |
|
0.2 |
2.5 |
130 |
180 |
110 |
150 |
60 |
95 |
|
0.3 |
2.0 |
150 |
200 |
130 |
170 |
75 |
110 |
|
0.5 |
1.2 |
150 |
200 |
130 |
170 |
75 |
110 |
Important:
This table only provides approximate Information relating furrow slope, soil type, stream size and irrigation depth to furrow lengths. This should only be used as a guide as the data are based primarily on field experience and not on any scientific relationships. Maximum values of furrow length are given for reasonably efficient irrigation. However, furrow lengths can be even shorter than those given in the table and in general this will help to improve irrigation efficiency. Only by Installing a furrow system, following the guidelines, and then evaluating its performance can an appropriate system be developed for a given locality.
The shape of furrows is influenced by the soil type and the stream size.
Soil type
In sandy soils, water moves faster vertically than sideways (= lateral). Narrow, deep V-shaped furrows are desirable to reduce the soil area through which water percolates (Figure 28). However, sandy soils are less stable, and tend to collapse, which may reduce the irrigation efficiency.
In clay soils, there is much more lateral movement of water and the infiltration rate is much less than for sandy soils. Thus a wide, shallow furrow is desirable to obtain a large wetted area (Figure 29) to encourage infiltration.
Figure 28 A deep, narrow furrow on a sandy soil
Figure 29 A wide, shallow furrow on a clay soil
Stream size
In general, the larger the stream size the larger the furrow must be to contain the flow.
The spacing of furrows is influenced by the soil type and the cultivation practice.
Soil type
As a rule, for sandy soils the spacing should be between 30 and 60 cm, i.e. 30 cm for coarse sand and 60 cm for fine sand.
On clay soils, the spacing between two adjacent furrows should be 75-150 cm. On clay soils, double-ridged furrows - sometimes called beds - can also be used. Their advantage is that more plant rows are possible on each ridge, facilitating manual weeding. The ridge can be slightly rounded at the top to drain off water that would otherwise tend to pond on the ridge surface during heavy rainfall (Figure 30).
Figure 30 A double-ridged furrow
Cultivation practice
In mechanized farming a compromise is required between the machinery available to cut furrows and the ideal spacings for crops. Mechanical equipment will result in less work if a standard width between the furrows is maintained, even when the crops grown normally require a different planting distance. This way the spacing of the tool attachment does not need to be changed when the equipment is moved from one crop to another. However, care is needed to ensure that the standard spacings provide adequate lateral wetting on all soil types.
The most common way to construct furrows is with a ridger. Figure 31 shows animal- and hand-drawn ridgers.
Figure 31 Ridger plough: (a) wooden body, animal-drawn
Figure 31 Ridger plough: (b) iron type, animal-drawn
Figure 31 Ridger plough: (c) hand-drawn version
CONSTRUCTION OF FURROWS ON FLAT OR MILDLY SLOPING LAND
The following steps are taken to construct furrows: setting out; forming one (or more) ridge(s); forming one (or more) parallel ridge(s).
Step 1
A straight line is set out in the field along the proposed line of furrows. This can be done by setting up ranging poles or marking a line on the ground with chalk powder or small mounds of earth. An experienced ploughman should be able to plough along the line by aligning the poles or earth mounds by eye (Figure 32).
Figure 32 Markers are put along a straight line
Step 2
The ridger is moved along the line. The resulting furrow should be straight. If not, the area should be ploughed again and the procedure repeated.
Step 3
About every five (5) metres, a new straight line should be set out.
If a ridger-drawbar connected with a tractor is used, four furrows can be drawn simultaneously. On the track back the left ridger is put in the last furrow track to make sure the new furrows arc parallel to the previous ones (Figure 33). Also here it should be checked that straight lines are followed: for every track a centre line is set out (see Figure 33).
Attention: It should always be kept in mind that a new straight line has to be set out before a new furrow track is made.
Figure 33 A ridger-drawbar behind a tractor makes four ridges simultaneously
CONSTRUCTION OF FURROWS ON SLOPING OR UNDULATING LAND
Special care is needed to construct furrows along the contour on sloping or undulating land. The following steps are taken to construct furrows along the contour:
Step 1
A guide furrow must first be set out along the upper edge of the field close to the farm channel using a levelling device to locate the contour line. Further guide furrows are set out every 5 metres on undulating ground and every 10 metres on uniformly sloping land (Figure 34).
Figure 34 Making guide furrows
Step 2
Working from each guide furrow, furrows are made to halfway along the next guide furrow (Figure 35).
Water is supplied to each furrow from the field canal, using siphons or spiles (see Annex 1). Sometimes, instead of the field canal with siphons or spiles, a gated pipe is used (Figure 36).
Depending on the available flow in the farm channel, several furrows can be irrigated at the same time.
When there is a water shortage, it is possible to limit the amount of irrigation water applied by using 'alternate furrow irrigation'. This involves irrigating alternate furrows rather than every furrow. Figure 37 is an example of this procedure. Instead of irrigating every furrow after 10 days, furrows 1, 3, 5, etc. are irrigated after 5 days and furrows 2, 4 and 6, etc. are irrigated after 10 days. Thus the crop receives some water every 5 days instead of a large amount every 10 days. Small amounts applied frequently in this way are usually better for the crop than large amounts applied after longer intervals of time.
Figure 37 Alternate furrow irrigation
Runoff at the ends of furrows can be a problem on sloping land. This can be as much as 30 percent of the inflow, even under good conditions. Therefore a shallow drain should always be made at the end of the field, to remove excess water. When no drain is made, plants may be damaged by waterlogging. Light vegetation allowed to grown in the drain can prevent erosion. Excessive runoff can be prevented by reducing the inflow once the irrigation water has reached the end of the furrows. This is called cut-back irrigation. It may also be possible to reuse runoff water further down the farm.
In order to obtain a uniformly wetted rootzone, furrows should be properly spaced, have a uniform slope and the irrigation water should be applied rapidly.
As the root zone in the ridge must be wetted from the furrows, the downward movement of water in the soil is less important than the lateral (or sideways) water movement. Both lateral and downward movement of water depends on soil type as can be seen in Figure 38.
Figure 38 Different wetting patterns in furrows, depending on the soil type (A - SAND)
Figure 38 Different wetting patterns in furrows, depending on the soil type (B - LOAM)
Figure 38 Different wetting patterns in furrows, depending on the soil type (C - CLAY)
Ideal wetting pattern
In an ideal situation adjacent wetting patterns overlap each other, and there is an upward movement of water (capillary rise) that wets the entire ridge (see Figure 39), thus supplying the root zone with water.
Figure 39 Ideal wetting pattern
To obtain a uniform water distribution along the furrow length, it is very important to have a uniform slope and a large enough stream size so that water advances rapidly down the furrow. In this way large percolation losses at the head of the furrow can be avoided. The quarter time rule is used to determine the time required for water to travel from the farm channel to the end of the furrow, in order to minimize precolation losses. The quarter time rule is further discussed in Annex 3.
Poor wetting patterns
Poor wetting patterns can be caused by:
- unfavourable natural conditions, e.g. a compacted layer, different soil types, uneven slope;- poor layout, e.g. a furrow spacing too wide;
- poor management: supplying a stream size that is too large or too small, stopping the Inflow too soon.
i. Unfavourable natural conditions
Compacted soil layers or different soil types have the same effect on furrow irrigation as they have on basin irrigation - see section 2.4.1. The solution to the problem is also similar.
An uneven slope can result in uneven wetting along the furrow. Water flows fast down the steep slopes and slowly down the flatter slopes. This affects the time available for infiltration and results in poor water distribution. The problem can be overcome by regrading the land to a uniform slope.
ii. Poor layout
If the furrow spacing is too wide (Figure 40) then the root zone will not be adequately wetted. The spacing of furrows needs careful selection to ensure adequate wetting of the entire root zone (Figure 40).
Figure 40 The spacing between two adjacent furrows is too wide
iii. Poor management
A stream size that is too small (Figure 41) will result in inadequate wetting of the ridges. Even if the plants are located at the sides of the ridge, not enough water will be available. A small stream size will also result in poor water distribution along the length of the furrow. The advance will be slow and too much water will be lost through deep percolation at the head of the furrow.
Figure 41 Stream size is too small to wet the ridge
If the stream size is too large on flat slopes, overtopping of the ridge may occur (Figure 42). On steeper slopes with too large a stream size, erosion of the bed and sides of the furrow may take place (Figure 42).
Figure 42 Stream size too large causing overtopping or erosion
A common management fault is to stop the inflow too soon. This is usually done to reduce runoff, but it results in a poor water distribution and the plants in particular at the end of the furrow do not get enough water. If the Inflow of irrigation water is not stopped soon enough, the runoff is excessive and plants at the end of the furrow may drown when an adequate drainage system to evacuate excess water is not provided (see also Annex 3).
The location of plants In a furrow system is not fixed but depends on the natural circumstances. A few examples will be mentioned.
- In areas with heavy rainfall, the plants should stand on top of the ridge in order to prevent damage as a result of waterlogging (Figure 43).- If water is scarce, the plants may he put in the furrow itself, to benefit more from the limited water (Figure 44).
- As salts tend to accumulate in the highest point, a crop on saline soils should be planted away from the top of the ridge. Usually it is planted in two rows at the sides (Figure 45). However, it is important to make sure there is no danger of waterlogging.
- For winter and early spring crops in colder areas, the seeds may be planted on the sunny side of the ridge (Figure 46). In hotter areas, seeds may be planted on the shady side of the ridge, to protect them from the sun.
Figure 43 Protection against waterlogging
Figure 44 Protection against water scarcity
Figure 45 Protection against accumulation of salt
Figure 46 Winter and early spring crops: seeds planted on the sunny side of the ridge
After construction the furrow system should be maintained regularly; during irrigation it should be checked if water reaches the downstream end of all furrows. There should be no dry spots or places where water stays ponding. Overtopping of ridges should not occur. The field channels and drains should be kept free from weeds.
