Field irrigation systems
1. Basin irrigation
Basin irrigation is one of the oldest methods of irrigating and is widely practiced where rice is irrigated. As explained in Section 6, rice (unlike most crops and most weeds) can grow when the soil is completely saturated. The basin is formed by leveling the area completely and enclosing it with berms, or levees, Figure 10-1. The side berms will run essentially on the contour. If the land is very sloping, the berms become terraces and a large amount of earth must be moved from the upper side to the lower side. On very steep slopes, the basin will be fairly narrow to reduce the amount of leveling required, Figure 10-2. Drop structures are required to lower water from one level to another.
2. Border Method
With the border method, land is laid out with side berms running downhill on a slight slope. The land is levelled between side berms to make the irrigation water run in a narrow sheet from the upper to the lower end of the field, Figure 10-3. When irrigation starts, the infiltration rate is high at the upper end of the border but, as the soil becomes saturated, the leading edge of the water continues to move downhill. Its rate of forward movement depends on soil type, slope, and quantity of water released. To provide enough water at the lower end of the field without over watering the upper end, a high berm is constructed at the lower end to hold back a pool of water to irrigate the lower end after the supply is cut off.
Determining the correct length and slope of a border system is by trial-and-error, depending upon the factors listed above; however a good starting point can be made as follows.
Lay out the border strip so the lower end is lower than the upper end by about the average amount of irrigation water to be applied in one irrigation.
When the irrigation water has progressed to about 80 percent of the length of the border, cut off the irrigation water and let the residue pond to the lower end. The water that ponds should irrigate the lower end of the border.
Figure 10-3 shows a border type system and water distributed in an almost level system with the pond formed about the time flow is cut off. Figure 10-3 also shows infiltration both during the run and from ponding. The two infiltration rates should provide water over the full length of the border strip.
Figure 10-1. Basin type irrigation system on relatively level land
Figure 10-2. A basin type system on a steeply sloping land
Figure 10-3. Views of a border irrigation system
Distributing water uniformly across the width of the border strip requires that the level of the soil be very flat (level) across the width of the border. If the border is very wide, water should be supplied at more than one point from the distribution channel.
Obviously the level of water in the distribution channel must be above the level of the land at the upper end of the border. That is, the channel must not be in an excavated area but must be contained between berms. The berms may be formed from excess earth taken from the channel between the water source and the field or from some area not to be irrigated.
The water’s level may be raised at the head of the border by placing a small dam across the distribution channel just downstream from the border. This temporary dam may be earthen, a sheet metal dam inserted into the bottom and sides of the channel, or a plastic sheet dam.
The plastic sheet dam is made by rolling several turns of plastic around a wooden pole. The pole is then laid across the channel berms and the sheet laid upstream along the bottom and sides of the channel for a meter or two. The pressure of the water holds the sheet against the bottom and sides tightly enough to prevent leakage.
Leveling across the border will usually be required. It may be done with shovels or other hand tools. An animal-drawn scraper, Figure 10-4, may be convenient for moving earth over short distances. The tail board is provided so the operator can stand on it and provide added weight for cutting soil. The pipe handle is used to provide more, or less, cutting angle.
Water may be discharged from the supply channel to the border by gated pipes through the channel berm or by siphons over the berm. Figure 10-5 shows a wooden pipe with control device and a round pipe turn out. Note now the head “h” is measured depending upon whether the pipe outlet is submerged. Table 10-1 shows the capacities of various sizes of wooden pipes of square cross sections. Table 10-2 shows the capacity of round pipes. Table 10-3 shows typical dimensions for border strips.
3. Furrow irrigation
The border system is well adapted to watering forage crops or other crops that cover the ground entirely. Crops normally grown in rows, such as grain or vegetable crops, are more frequently irrigated with furrow systems–a series of furrows and ridges with about 75 to 100 cm between furrows and 15 to 20 cm deep, Figures 10-6 and 10-7. The furrows run downhill, as with borders. Where the furrows are constructed 15 to 20 cm deep, it is possible to irrigate a field with a significant amount of side slope.
Rows of tall-growing crops like maize are planted on the ridges. Two rows of low-growing crops like onions may be planted on each ridge.
Figure 10-4. Buck scrapper
Figure 10-5. Turn-outs to carry water through a berm to a border or furrow.
Table 10-1. Flow through rectangular submerged orifices
Table 10-2. Capacities of short pipes in liters/sec
Table 10-3. Suitable dimensions for border strips
|
Type |
Infiltration rate |
Dimensions |
|||
|
of soil (mm/hr) |
Slope (%) |
Width (m) |
Length (m) |
Flow (liters/sec) |
|
|
Sands |
25 and over |
0.2 |
15-30 |
60-90 |
220-450 |
|
0.4 |
10-12 |
60-90 |
100-120 |
||
|
0.8 |
5-10 |
75 |
30-70 |
||
|
Loams |
7 to 25 |
0.2 |
15-30 |
250-300 |
70-140 |
|
0.4 |
10-12 |
90-180 |
40-50 |
||
|
0.8 |
5-10 |
90 |
12-25 |
||
|
Clays |
2.5 to 7 |
0.2 |
15-30 |
350-800 |
45-90 |
|
0.4 |
10-12 |
180-300 |
30-40 |
||
One problem that may affect row placement on the ridge is having enough soil moisture to germinate seed.
In areas that normally have sufficient rain during the planting season, rainfall should provide moisture for seed germination. In drier areas, the field may be very heavily irrigated just before or after seeding so enough moisture moves side ways and up by capillary action to germinate the seed. But it is usually best to place seeds into moist soil. In severe problem cases, such as a sandy soil and low rainfall, seed may be planted on the side of the ridge so they are closer to the wetted area. Once the seedling root system develops a few inches, there should be no further problems.
Figure 10-7. Cross section of irrigation furrows showing plant locations
As with border systems, the slope along the furrow in furrow systems must be flat enough to prevent erosion but steep enough to allow water to reach the end of the furrow. That is so infiltration is relatively uniform the full length of the furrow. The more permeable the soil, the steeper and/or shorter the furrows must be.
In general, furrow slopes should range from 0.1 to 2 percent. The slope must not be steep enough to erode the furrow severely and can generally be greater than the slope of the distribution channel which has a much greater hydraulic-radius value. If the distribution channel is run on a very slight grade, essentially on the contour, then the furrows can be supplied and laid out on the downhill side of the channel although they need not run perpendicular to the channel.
If the field’s topography varies widely, it may not be possible to run all furrows parallel and maintain the desired slope. At intervals, it may be necessary to leave an unirrigated a strip of variable width between one set of furrows and another.
Table 10-4 shows some typical lengths and slopes for furrow systems. Because of the many variables involved, a good operating rule is that water should reach the end of the furrow within 25 percent of the total time for one irrigation. That will provide about 25 percent more irrigation at the top of the field than at the lower end. Water to irrigate a furrow can be applied at a high rate at the beginning of the period and then reduced as the soil becomes wetted. That reduces the time required for water to reach the end of the furrow and prevents excessive loss later from the end of the furrow..Also, a dam may be placed at the end of the furrow to pond water and increase infiltration rate.
Table 10-4. Typical furrow lengths for various soil types and slopes Furrow lengths (meters)
Table 10-4. Typical furrow lengths for various soil types and slopes
|
Furrow lengths (meters) |
||||||||
|
Slope (%) |
0.25 |
0.50 |
1.00 |
1.50 |
2.00 |
3.00 |
||
|
Soil type |
Application depth (mm)* |
Discharge (l/min) |
180 |
90 |
45 |
30 |
22 |
15 |
|
Coarse |
50 |
150 |
120 |
70 |
60 |
50 |
25 |
|
|
100 |
210 |
150 |
110 |
90 |
70 |
60 |
||
|
150 |
260 |
180 |
120 |
120 |
90 |
70 |
||
|
Medium |
50 |
250 |
170 |
130 |
100 |
90 |
70 |
|
|
100 |
375 |
240 |
180 |
140 |
120 |
100 |
||
|
150 |
420 |
290 |
220 |
170 |
150 |
120 |
||
|
Fine |
50 |
300 |
220 |
170 |
130 |
120 |
90 |
|
|
100 |
450 |
310 |
250 |
190 |
160 |
130 |
||
|
150 |
530 |
380 |
280 |
250 |
200 |
160 |
||
Source: Witkers and Vipond. Irrigation: Design and Practice. B. T. Batsford Ltd. London, 1974
4. Sprinkler system
Evaporation is extremely high. Efficient use of irrigation water and minimum land leveling are characteristics of sprinkler systems. But operating and investment costs are higher than for gravity flow systems. Pressures must be matched to sprinkler size and manufacturers’ representatives should be consulted to design the systems.
5. Drip irrigation
Drip irrigation is a relatively new development. With it water is piped under pressure, and small outlets are located at each plant to be watered. The system is usually applied to trees but large plants like tomatoes may be irrigated. The system is designed to apply water very slowly at a rate a specific plant needs. Other areas are not watered.
Major disadvantages of the pressure system are its cost and small holes plugging up with foreign material.
6. Wild flooding
In this system, water is released from a distribution channel at the top of a field that has had little if any leveling. Water distribution will be very nonuniform. The system should be used only where there is a permanent ground cover such as alfalfa or grass to prevent erosion.
Example problem
A furrow irrigation system is to be designed to supply irrigation water to a crop of maize (corn). The furrows will be approximately 100 m long and the soil type is a clay loam. The furrows are to be placed 1 m apart.
From Table 2-1, the infiltration rate is 5-10 mm/hr. From Table 4-2, 110 mm is required to restore the root zone to field capacity when soil moisture has dropped to 50 percent of field capacity.
With an infiltration rate of 10 mm per hour and 110 mm to be applied, the duration of the irrigation will be:
Time = 110/10 = 11 hours.
The amount of water to be applied per furrow is:
Quantity = 1 m x 100 m x 0.1 m/fur = 10 m³/hr
or
10 m³/3,600 =.0003 m³/sec
or
.0003 x 1,000 = 0.3 liters/sec.
From Table 10-1, a wooden-box field turn-out of 15 x 15 cm with 3 cm head would have more than adequate capacity. It could be closed down after water reached the end of the furrow. From Table 102, a 2.5 cm diameter pipe with 5 cm head would have about the correct capacity. Figure 5-1 shows that maize in Kansas requires about 8 mm of soil moisture per day. In a similar climate, with an application of 110 mm, the irrigation would have to be repeated about each 14 days.
Construction and maintenance
Channels to and within a field require regular routine maintenance to remove weeds that reduce water velocity and cause additional evaporative losses. Some erosion will occur along channels and furrows and some silt deposits will have to be removed to maintain channel cross-section area.
With time berms will erode and require some maintenance to maintain their height.
Originally and with time some levelling of basins and border systems will be required. High and low points should be marked when water covers the surface. Using a large plane of water is a more rapid way to locate high and low spots than using a surveying instrument.
Levelling may be done with shovels and rakes. If animal power is available, a simple float or drag (Figure 10-8) may be efficient and save labor. The wider and longer the drag (or float), the more effective it will be.
Irrigation systems should be checked both before they are needed and during use. Most maintenance probably will require no more than a hoe and shovel. Small leaks, particularly through or over berms should be repaired promptly before water erodes them severely. Watch for holes made by animals through berms.
Erosion during a rainy season can cause serious damage unless the area is well protected with drainage ditches or terraces that divert surface flood-type flow. Drainage is discussed extensively in the next section.
Inspect drop structures frequently to plug leaks around the sides.
Figure 10-8. Animal powered float for land leveling. (Source: http://www.nzdl.org)