4.7 Wetted Footprint (or Area)
4 Critical Design Decisions
The design of irrigation systems that have the potential to be operated effectively and efficiently involves a large number of decisions related to management and engineering design. The following list contains the most critical design decisions with respect to achieving high levels of performance, as it is defined in Section 2:
- Effectiveness (business risk, crop, soil, and climate)
- Adequacy (production/economics, efficiency, and climate)
- Mode of Operation (crop, climate, efficiency, farm management, irrigation system options, and water supply)
- Uniformity (irrigation system options, soil, crop, climate)
- Application rate (soil, irrigation system options)
- Application depth range (soil, crop, adequacy, efficiency, mode of operation, and irrigation system options)
- Wetted footprint (application rate, application depth range, irrigation rate, irrigation system options)
In summary, the first two decisions are essentially management decisions concerning what are acceptable levels of business risk and income. The remaining decisions are the engineering decisions that must be made in order to achieve the goals implicit in the decisions about acceptable levels of effectiveness and adequacy.
The purpose of this section is to highlight the issues that must be addressed in making these critical decisions.
4.1 EFFECTIVENESS
For the purpose of this section it will be assumed that the measure of effectiveness is the proportion of the growing season that the soil water content equals or exceeds the critical soil water content for the crops grown on the farm.
The decision is "What is an acceptable degree of risk to farm production and income?".
The issue is business risk. Climate is the physical driving force of agricultural production and so in many areas of New Zealand there exists a significant production risk, and therefore business risk, to do with water supply and demand at the field level. Irrigation is about managing the business risk of farming by augmenting the supply of water to crops.
In most circumstances, minimum business risk does not coincide with zero production risk with respect to water supply, because of the costs of achieving zero production risk and the imbalance that would exist between water related and other sources of production risk.
Assessing the level of risk associated with a particular irrigation design scenario involves integration of factors and dependencies to do with the crop, market, soil, climate, water supply, and irrigation system. Computer simulation is probably required to achieve the level of integration that is necessary if a meaningful relationship between risk and system design decisions is to be developed.
4.2 ADEQUACY
The decision is "Over what proportion of the area will we endeavour to meet the effectiveness performance target?".
In making this decision, two issues must be addressed. These are an economics issue and an environmental effects issue.
The larger the proportion of the area that is adequately watered, i.e. meets the effectiveness target, the larger and more reliable will be the income. However for real (non-uniform) systems the cost increases as the adequacy increases because the mean application rate must increase. This increases both capital and operating costs. At some point the marginal benefit of increasing the adequacy will equal the marginal cost of achieving the increase.
As the mean application rate increases to achieve higher adequacy the application efficiency decreases. This means more water is required from the water source and more water drains to downstream receiving waters. Generally this means increased adverse effects on both environments.
4.3 MODE OF OPERATION
The design of the application system (and associated water allocations) must be based on a workable irrigation management strategy.
The decision is "What will the management strategy be?".
The issue is irrigating at the right time and applying the right amount of water.
The two ends of the operating spectrum are:
- Apply a constant mean application depth. This implies a variable return period.
- Irrigate on a fixed return period basis. This implies a variable mean application depth.
Somewhere within this spectrum is a strategy that is to irrigate when it is optimal to do so. This will probably involve varying the mean application depth and the return period.
Where a farm operates within this spectrum should be determined by the crops grown, the water supply characteristics, and in consideration of other farm management issues.
The return period is simply the time between successive irrigations of a field. It is a function of the soil water content immediately before irrigation, the mean application depth, the critical soil water content, application uniformity, the required adequacy, and the actual evapotranspiration during the period following irrigation.
Crops with a constant root depth could be irrigated equally well under either a constant mean application depth strategy, or a constant return period strategy – providing the water supply characteristics and the application system permit the flexibility needed to modify either the timing or the depth, respectively.
Effective and efficient irrigation of annual crops requires flexibility in both the timing and amount of irrigation. The return period should vary during the season because the critical soil water content will increase as the rooting system develops, and as the evapotranspiration rate changes. In principle the mean application depth could be held constant at a value suited to early season needs. However in practice the return period would become unacceptably short as the actual evapotranspiration rate increases.
4.4 UNIFORMITY
The decision is "What uniformity should the irrigation system be designed to achieve?".
There are many issues involved in making this decision because of the pivotal role uniformity plays in the performance of the irrigation system. The effects of uniformity on aspects of irrigation performance are clearly illustrated in Figures 2-2 and 2-3. The benefits of achieving high uniformity, with respect to high adequacy and application efficiency, are obvious. The benefits in terms of irrigation rate and cost per unit area are illustrated in the following example.
Suppose that throughout a field the soil water content has reached the critical soil water content, and that it is desired to return the soil water content to field capacity. Assume that this requires an application depth of 50 mm, and that there are no losses due to evaporation or drift off the target area. The only losses are due to non-uniform applications. The purpose of the exercise is to design a travelling irrigator that will achieve specified levels of adequacy, 80 percent, 90 percent, and 100 percent. The design must examine the effects of different levels of application uniformity on the irrigation rate and the maximum area that can be irrigated by one machine. It is assumed that the evapotranspiration rate is 5 mm/day and that the return period is 10 days. A mean application rate of up to 30 mm/hr is permissible and the wetted foot print of the irrigator has been selected so that the full use is made of the water supply of 60 cubic metres per hour. The calculations are summarised in the following three tables.
Assume that the goal is to irrigate to ensure that 80% |
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Mean application rate (mm/hr) 30
Field capacity (mm) 100 |
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Cu |
Required Application Depth |
Efficiency |
Irrigation Time |
Irrigation |
Travel |
Maximum Irrigated Area |
100 |
50 |
100 |
1.7 |
2.88 |
12.0 |
29 |
90 |
56 |
89 |
1.9 |
2.57 |
10.7 |
26 |
80 |
63 |
72 |
2.1 |
2.29 |
9.5 |
23 |
70 |
72 |
35 |
2.4 |
2.00 |
8.3 |
20 |
Assume that the goal is to irrigate to ensure that 90% |
||||||
| Mean application rate
(mm/hr) 30 Field capacity (mm) 100 Wetted width (m) 20 Critical soil water content (mm) 50 Wetted length (m) 100 Design depletion (mm) 50 Wetted footprint (m2) 2000 Design ET rate (mm/day) 5 Flow rate (m3/hr) 60 Return period (day) 10 |
||||||
Cu |
Required Application Depth |
Efficiency |
Irrigation
Time |
Irrigation |
Travel |
Maximum
Irrigated Area |
100 |
50 |
100 |
1.7 |
2.88 |
12.0 |
29 |
90 |
60 |
83 |
2.0 |
2.40 |
10.0 |
24 |
80 |
73 |
67 |
2.4 |
1.97 |
8.2 |
20 |
70 |
95 |
51 |
3.2 |
1.52 |
6.3 |
15 |
Assume that the goal
is to irrigate to ensure that 100% |
||||||
| Mean application rate
(mm/hr) 30 Field capacity (mm) 100 Wetted width (m) 20 Critical soil water content (mm) 50 Wetted length (m) 100 Design depletion (mm) 50 Wetted footprint (m2) 2000 Design ET rate (mm/day) 5 Flow rate (m3/hr) 60 Return period (day) 10 |
||||||
Cu |
Required Application Depth |
Efficiency |
Irrigation
Time |
Irrigation |
Travel |
Maximum
Irrigated Area |
100 |
50 |
100 |
1.7 |
2.88 |
12.0 |
29 |
90 |
75 |
65 |
2.5 |
1.92 |
8.0 |
19 |
80 |
150 |
32 |
5.0 |
0.96 |
4.0 |
10 |
70 |
300 |
Very Low |
10.0 |
0.48 |
2.0 |
5 |
The tables show that the lower the application uniformity the higher the mean application depth must be to ensure that a given proportion of the area is fully irrigated. The application efficiency decreases significantly.
The effect of the decrease in application uniformity is to increase the irrigation time, because more water must be applied, and decrease the total area that can be irrigated with a given irrigation machine. The tables show that reducing uniformity from 100 percent to 70 percent reduces the area able to be irrigated by one third. The capital cost per hectare would therefore increase by 33 percent, assuming it costs no more to buy a machine that irrigates at 100 percent uniformity, compared to a 70 percent uniformity machine.
The uniformity of application depth for a representative range of the travelling irrigators available in NZ has been reported as generally being in the 69 percent to 96 percent (John et al., 1985). The uniformity of application depth for the majority of irrigators in use today is probably at the lower end of this range, but there is no field data to substantiate this.
In summary the uniformity of application depth significantly affects the design application depth, the application efficiency, and the cost per irrigated hectare. From both economic and environmental points of view it is a significant issue.
4.5 APPLICATION RATE
The decision is "What application rate should be used?".
The issues are the resulting irrigation rate and the uniformity of infiltrated depth.
Generally the application rate is chosen to be as high as possible in order to achieve high irrigation rates (average area irrigated per day).
However, it is critical that surface redistribution of the applied water does not occur because of the importance of maintaining high uniformity within the soil. To achieve this, it is necessary that over the expected range of initial soil water contents and mean application depths, the depth of ponding must be less than the surface roughness of the field.
It is also necessary that the product of the average application rate and the wetted footprint is no greater than the farm water supply rate.
4.6 APPLICATION DEPTH RANGE
The decision is "What range of mean application depths should the irrigation system be capable of providing?".
The issue is irrigation adequacy and operational flexibility.
Many irrigation systems are not able to apply water in depths small enough to match soil water storage capacity at the time of irrigation, resulting in drainage through the soil profile. Application efficiency is therefore lower than it could be. However it should be born in mind that the lower the mean application depth the more critical it is that the uniformity be high. If it is not, the adequately watered area will be unacceptably low, and economic objectives will not be met, even though the application efficiency will be very high.
4.7 WETTED FOOTPRINT (OR AREA)
The decision is "What should the wetted width (in the direction of irrigator travel) of the irrigator be?".
The issue is the application rate and travel speed required to apply a given depth of water, and therefore the irrigation rate and cost per hectare irrigated. The greater the wetted width, the lower the application rate required to apply a given depth of water for a given travel speed. However, it is very difficult to maintain a high uniformity coefficient while increasing the wetted width to reduce the application rate to an acceptable level. This issue remains largely unresolved, as evidenced by the
characteristics of most travelling irrigators currently available in New Zealand.
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