5 Planning the Season’s Irrigation
This is probably the most important aspect of irrigation after design, and one area that many irrigators give insufficient time to.
In addition to setting goals, farmers should prepare an irrigation strategy or management plan that describes how they are going to manage their irrigation systems on a daily basis to achieve the targets or goals they have set out. The irrigation strategy should include a method which will be used to determine where to irrigate each day, and how much water to apply at each irrigation, taking all factors into account.
Targets for indicator values should be set, so that they can be monitored during the irrigation season to determine progress, and reviewed either during the season or at the end of the season.
The plan must be written down so that it can be referred to during the season, and reviewed at the end of the season.
The strategy used for every farm will be different, but there are some basic principles that all irrigation farmers should be aware of. To help you plan an irrigation strategy and set targets for indicators, some of these principles are described below.
5.1 MAXIMISING APPLICATION EFFICIENCY
No irrigation system applies water uniformly over an area, except perhaps small basin irrigation where basins are filled rapidly. Borderdyke irrigation tends to apply more water at the top of the border than the bottom. Sprinkler irrigation, because of uneven sprinkler distribution patterns, tends to apply more water at some locations than others. Drip irrigation applies water in a small area.
Regardless of how uniformly the water is applied to the soil, varying infiltration rates, surface redistribution of water, variation in soil water holding capacities and crop root depths all contribute to some unevenness of water storage in the soil and water use by the crop.
In theory, the most efficient time to irrigate is when the soil moisture is at the stress point, and the correct depth to apply is the depth that returns the soil moisture to field capacity.
However, this approach does not take into account the non-uniform nature of irrigation applications, plant water use, and soil moisture holding capacities of the soil. If the theoretical depth is applied, some areas get over-watered, which results in losses to drainage. Likewise, some areas get underwatered, and at the end of the irrigation cycle, just before the next irrigation, these areas may be below the stress point and suffer some yield loss.
The most efficient practice, therefore, is to irrigate when soil moistures are a few percent above the stress point, and to apply enough water to bring the soil not back to field capacity, but a few percent below this level.
For example, if a soil holds 100 mm of water between the stress point and field capacity, that is the soil in the root zone of the crop has a readily available soil moisture of 100 mm, then irrigating when the deficit is at say 85 mm and applying 70 mm of water to finish with a soil moisture deficit of 15 mm, will result in more efficient use of water than irrigating at a deficit of 100 mm and applying 100 mm. The irrigation trigger level in this example is 85 mm, not 100 mm as would often be used.
What happens with this approach is that the areas that would normally have been over-watered end up with soil moistures near field capacity. Because of the higher trigger level, areas that would normally suffer some moisture stress remain above the stress point.
This simple example illustrates the advantage of irrigating with smaller applications, but more often. However, there is a cost. Systems have to be designed to operate on shorter cycles, and for non-automatic systems, this may cost more. This should be addressed at the design stage of the process.
5.2 OPTIMISING THE USE OF RAINFALL
An irrigation strategy that applies less water more often also has a major advantage in that it allows rainfall to be used more effectively.
The closer that soil moistures are to field capacity, the more likely that rainfall will be lost to deep drainage. Conversely, the drier soils are, the less likely that rainfall will drain below the root zone of the plants. Given that the total water used by a crop in a season, i.e. the sum of irrigation and rainfall, does not vary much, the more rainfall that can be used, the less irrigation that will be required. Using rainfall efficiently reduces irrigation costs.
In theory, the most efficient use of rainfall would occur if soil moistures were retained at a little above the stress point by applying very small depths of water. This would leave the soil as dry as possible without causing any crop yield loss. This is only possible with some automatically controlled horticultural irrigation systems, as it requires daily irrigation and a system capacity to meet the demand of the highest ET days. Irrigation systems are not normally designed to this level of capability.
For large field sprinkler irrigation systems, it is not realistic to design or operate the system at this level, and a compromise must be reached. There is a trade-off between irrigation efficiency and optimum use of rainfall with irrigation system capacity and cycle time. For well-designed systems, the approach of using a trigger level a little above the stress point and irrigating to just under field capacity is a reasonable compromise.
5.3 WATER SUPPLY RESTRICTIONS
Because of the way water for irrigation tends to be allocated and controlled by Regional Councils in New Zealand, whether restrictions will be imposed and if they are when that will occur, is often unknown at the start of the irrigation season.
For groundwater takes, restrictions are usually applied when groundwater levels reach predetermined minimum levels. When these levels are reached depends on many factors, such as whether there is any natural recharge, the amount of abstractions, the status of levels at the start of the season, etc. Some councils may be able to give an indication of whether restrictions are likely to be imposed, and perhaps when.
For takes from rivers or streams, abstractions are normally controlled according to river or stream flows, and when that is likely to occur depends on a number of factors, but is primarily dependent on rainfall.
The need for irrigation varies significantly between seasons because of rainfall variations. Some indication of the requirements for the coming season can be obtained from long-term weather forecasts, and in particular, the likely effects of El Nino or La Nina weather patterns should be noted.
Your irrigation strategy for the season is essentially risk management, and should take these factors into account. If a very dry year is forecast, you may decide at the start of the season to irrigate less area, or irrigate a larger area at the start of the season with the expectation that you will drop some areas out of the irrigation cycle later on.
If appropriate, you should allow for the worst case scenario of having no water at all during some parts of the season.
5.4 WHERE TO IRRIGATE
A strategy of where to irrigate during the season should be planned at the start of the season.
Whether a paddock needs irrigating or not depends on soil moisture levels in the paddock, the vulnerability of crops to soil moisture deficits and the time until the next irrigation of that paddock. A crop does not need to be irrigated if there is sufficient water stored in the soil to meet ET requirements and to keep moisture levels above the stress point until the next time irrigation is available. The stress point will differ between crops and will be higher for certain stages of crop development where plants are vulnerable to water stress.
The frequency at which a paddock can be irrigated is determined by water availability and irrigation system design, and must be taken into account.
As an example, you could use a very simple rule such as irrigating each crop as it comes up in the irrigation rotation unless moisture levels are above 70 percent. In other words, you only skip watering a paddock if the soil is still quite moist.
If system capacity or water supply is limited, you may set a priority system such as irrigating grain crops during head forming if moisture levels drop below 90 percent as a first priority. Next priority might be non-pasture crops if moisture levels could drop below 80 percent.
There are many alternatives and best use of the available water should be planned so that the main objective of maximising profit can be obtained.
5.5 WHEN TO START IRRIGATING EACH SEASON
One of the main decisions you have to make is when to start irrigating each season.
To make this decision, you should have a preseason plan, know your crops stress point, keep in mind your irrigation system’s capacity and ability to keep up or catch up, and most importantly, measure soil moisture.
Soil moistures, because of winter rainfall, are normally at field capacity at the beginning of the growing season. ET rates are lower in the early part of the season, but soils can become dry without farmers realising it. When they do notice, it is often too late to catch up, and crop yields suffer. However, delaying irrigation for as long as possible is desirable, provided that it doesn’t cause significant yield reductions.
There are a number of ways that you can decide when to start irrigating, some better than others. Possibilities include carrying out water budgets, inspecting crops, checking the soil, watch for neighbours irrigating, using weather forecasts, or using soil moisture measurements.
Using soil moisture measurement combined with ET predictions is the best method because it is more certain than any of the other methods.
The decision as to what depth to apply should be based on soil moisture levels in each paddock. After the irrigation, there needs to be sufficient water stored in the soil to meet plant ET requirements and keep moisture levels above the stress point until the next time irrigation can be applied. Some allowance (i.e. a soil moisture deficit remaining after irrigation) could be made to accommodate any rain that occurred in the interval between irrigations, and to improve efficiency.
As an example, assume that it is the 15th September, and you wish to calculate when to start irrigating your pasture.
Your soil has readily available soil moisture of 80 mm, and your irrigation system has the capacity to apply 5 mm/day, which is close to the maximum ET expected. The farm is divided into 10 paddocks and the irrigation system can apply 50 mm every 10 days. You have been told that the efficiency of your system is about 80 percent. Your soil moisture meter indicates that the soil currently has a 25 mm water deficit.
The ET rate in your area for the second half of September is 2.8 mm/day, and for the first half of October is 3.1 mm/day.
Gross depth applied 50 mm (A)
Net application depth (80%) 40 mm (B)
Current soil moisture deficit 25 mm (C)
Deficit at stress point 80 mm (D)
Current available soil moisture 65 mm (E) = (D) – (C)
If irrigation does not take place, the soil moisture would be at the stress point in 22 days.
ET for September 15 days x 2.8 mm/day 42 mm (F)
Remaining water at end of September 23 mm (G) = (E) – (F)
Days to use water up in October 23/3.1 7 days
Total days until stress point is reached 22 days (7 October)
If you started irrigating in 22 days time, all paddocks would be at the stress point at the same time. Because your system takes 10 days to cover the farm, you need to start irrigating 10 days before the stress point is reached.
Starting date for irrigation, therefore, should be in 12 days time, i.e. on 29 September.
On this date, the remaining soil moisture in the first paddock will be 31 mm. If 40 mm net is applied, the soil moisture deficit remaining after irrigation will be 9 mm (80-31-40). This is because your soil can hold 80 mm of water. (If your soil could only hold 40 mm of water, some of the applied water will drain through the soil profile. Already 10 mm has been allowed for due to inefficiencies in the system, so potentially more will be lost. If your irrigation system can only apply a fixed depth of 50 mm gross, you will have to accept these losses. If, however, your system can apply smaller depths, for example by running the irrigator at a higher speed, the gross depth should be calculated and applied for each paddock until the soil is able to accept the full application.)
In this example, none of the paddocks will have been irrigated to field capacity. The first paddock will have a deficit of 9 mm after irrigation while the last paddock in the rotation will have a deficit of 40 mm after irrigation on 7 October.
There has been no allowance for rainfall in these calculations. In such a short time period (12 days), there is a high chance that rainfall will not occur. If it does, soil moistures should be measured again, and the calculations redone.
The strategy used in this example assumes that you wish to start irrigation as late as possible in the season, and have sufficient system capacity to keep up with the high ET rates later in the season. This strategy is often used for horticultural irrigation to maximise the use of rainfall and to maximise efficiency. If your system has a limited capacity, a different strategy should be applied, as described later.
5.6
IRRIGATING MID SEASONOnce you have completed one cycle, you have to decide when to start the next cycle. Assuming a strategy of irrigating as late as possible, the same process is applied again. This time, however, the soil moistures of the paddocks will all be different.
Using the above example again, assume that ET rates for the second half of October are 3.6 mm/day.
Paddock 1
Soil moisture deficit on 29 September after irrigation 9 mm
ET for remaining September 2 days x 2.8 mm 6 mm
ET in 1st half of October 15 days x 3.1 mm/day 47 mm
Deficit at stress point 80 mm
Remaining water on 15 October 18 mm
Days to use water up in October 18/3.6 5 days
Total days until stress point is reached 22 days
This means that Paddock 1 does not have to be irrigated for 22 days after 7 October, i.e. on 30 October.
Now, carry out the same calculations for Paddock 10, which was irrigated on 7 October.
Soil moisture deficit on 7 October 40 mm
Deficit at stress point 80 mm
Remaining soil moisture on 7 October 40 mm
ET in 1st half of October 8 days x 3.1 mm/day 25 mm
Remaining water on 15 October 15 mm
Days to use water up in October 15/3.6 4 days
Total days until stress point is reached 12 days
This means that the last paddock will have to be irrigated again on the 19 October. Again, because it takes 10 days from irrigating the first paddock until you can irrigate the last paddock, irrigation has to start 10 days before the last paddock will be at the stress point, assuming that no rain falls.
This means that irrigation has to start in Paddock 1 on 9 October, even though it has enough soil moisture to wait until the end of October to be irrigated again.
The same process should be repeated to determine the date for the next irrigation.
If soil moisture measurements are being taken, actual soil moisture deficits should be used in the calculations, rather than calculated deficits.
If heavy rainfall occurs, and all soil moistures are returned to field capacity, the process for timing the first irrigation should be used to determine when to start irrigation again.
If soils with different water holding capacities exist, or if crop water use patterns vary between paddocks, the calculations should be made for each paddock. The order in which paddocks are irrigated may need to change.
5.7 WHEN TO APPLY THE LAST IRRIGATION
In New Zealand, winter rainfall generally eliminates moisture deficits that may remain at the end of the irrigation season. Usually soil moistures are returned to field capacity by the beginning of the next season.
It makes sense therefore, to leave the soil as dry as possible at the end of the irrigation season. The difficulty is that you don’t know when rain will fall and how much there will be.
The need for additional irrigation at the end of the season is best determined using a water balance worksheet that takes into account the predicted ET and an estimate of likely rainfall over the rest of the growing season, rather than just over the next irrigation cycle. In mid season, it is normally wise to assume that no rain will fall. This is not the case at the end of the season, because of the longer times involved.
As an example, consider the situation where it is the 1st of April on your irrigated dairy farm, and you need to decide whether any more irrigation is required this season, i.e. before the end of May.
Example calculation:
Current date 1 April
End of season date 31 May
Crop pasture
Allowable soil moisture deficit 80 mm
Current deficit 20 mm
Water available 60 mm
Expected useful rainfall 50 mm
Total available water 110 mm
Crop needs based on average ET
April 30 days at 2.5 mm/day 75 mm
May 31 days at 2.0 mm 62 mm
Total crop needs 137 mm
Net irrigation required 27 mm
Gross irrigation required 32 mm
In this case, one further irrigation is recommended, because it is likely that soil moistures will be too low by the end of May.
If this was a horticultural crop, and the crop required watering until the end of May, more than one irrigation may be needed, depending on the depth applied at each irrigation. If the system applied 10 mm per application, then perhaps three irrigations would be required.
This calculation should be made for each paddock in the rotation or each block in the orchard.
The current soil moisture depletion must be measured using soil moisture measuring equipment, or estimated using a water balance work sheet. There is some risk involved in estimated rainfall, but this can be adjusted according to the value of the crop, and the consequences of getting it wrong.
For high value crops, a rainfall amount that has a 90 percent chance of being equalled or exceeded could be used. It is extremely rare for no rain at all to fall in April and May. For lower valued crops, average rainfall could be used.
If an unusually dry situation did occur, irrigation can be restarted, but on systems with long cycles, catching up in all paddocks may be difficult.
5.8 SYSTEMS THAT APPLY LARGE FIXED DEPTHS
Flood irrigation systems such as borderdyke systems typically apply large amounts of water, and scheduling irrigation for these systems can be difficult. Many of these systems run on fixed rosters, and very little flexibility is available to farmers. When irrigation water is available, farmers tend to take it, because if they don’t, the time until the next irrigation may cause soil moistures to drop to the point where serious production losses could occur.
For systems with some flexibility in watering time, irrigation scheduling is a trade-off between minimising crop water stress and minimising leaching or drainage. On high water holding capacity soils, losses to drainage could be minimal if irrigation is able to take place when soil moistures are at or near the stress point. On low water holding capacity soils, losses to drainage can be quite high, and difficult to avoid.
At the start of the season and after heavy rainfall, irrigation managers often irrigate early to prevent stress at the end of the cycle and accept more drainage at the start of the cycle soil moisture deficits are quite low. For subsequent irrigations, soil moistures tend to even themselves out.
There is no perfect solution to managing flood irrigation systems on light soils. System changes that reduce the depth of application need to be considered for significant improvements to be made.
5.9 WHAT TO DO WHEN IT RAINS
Irrigation gives a farmer control of the watering for a crop. Scheduling irrigation is easiest when it does not rain.
If rain falls during the middle of an irrigation cycle, and the amount of rain is enough to return the most recently irrigated paddocks back to field capacity, but not in the areas yet to be irrigated, the soil moistures will vary across the farm.
The approach that should be taken depends on the capacity of the irrigation system and whether or not the system can apply variable depths of water.
If the system capacity is limited and there is some difficulty in satisfying the soil moisture deficits, the rainfall may have been a bonus. Irrigation may be able to be turned off for a limited time, but soil moistures should be retained at or near to field capacity, so that when evapotranspiration exceeds the capacity of the system to apply water, the buffer in the soil keeps soil moistures above the stress point for as long as possible.
If the irrigation system can apply variable depths, irrigation can continue, and the depth applied is adjusted to the deficits remaining after the rainfall. Irrigation on each successive area will require progressively more water. Within one cycle, irrigation applications will return to the depths applied before the rainfall.
If the system cannot apply variable depths but is not limited in capacity, irrigation can be delayed until ET has used up the effective rainfall, and irrigation must then start again, with the full depth being applied. If 25 mm of rain fell, and average ET was 5 mm per day, irrigation could be delayed for 5 days and no more, even though the soil moistures in some paddocks would still be above the trigger level.
If the rain was sufficient to completely bring all areas back to field capacity, irrigation should proceed exactly as for the first irrigation of the season.
5.10 IRRIGATING IN WINDY CONDITIONS
Surface irrigation systems and drip or sub-surface irrigation systems are generally not affected by wind. However, sprinkler or microsprinkler systems can be seriously affected.
Wind affects irrigation in three main ways.
The first is by distorting the sprinkler application pattern so that water is applied very unevenly. Some areas receive more water than they should and some less, resulting in less efficient irrigation. The second is through evaporation of water in the air, on the crop surface, and on the ground surface. This water is lost and not available to the crop. The third is by physically blowing water away from the irrigated area.
The best solution is not to irrigate during windy conditions. Often it is less windy at night, so irrigation should be carried out at night. Irrigating at night also reduces losses due to evaporation. Some days, it may be better to turn the irrigation system off altogether. However, turning the system off reduces your effective irrigation system capacity. If your system has a limited system capacity, turning off may have serious consequences through crops being water stressed, and you may need to keep irrigating, even if it is very inefficiently.
If you are irrigating in an area that tends to be quite windy, it is preferable to design the system with some additional capacity so that you can turn off during windy periods and use the extra capacity to catch up during calmer conditions.
Operating sprinklers at lower pressure can reduce losses in the air and water blown off paddocks by keeping droplet sizes up. Using systems that place water close to the point where it is required, such as drop tubes on centre-pivots or travelling irrigators, will also reduce distortion and losses.
5.11 HOW TO HANDLE SOIL VARIATIONS
The likelihood of obtaining optimum production is greater on uniform soils than on soils that have large variations in water holding capacity. Variable soils create difficulties with matching irrigation applications to soil moisture deficits.
If irrigation applications are based on the smaller depletions of the lower water holding capacity soils, the parts of the farm with the high water holding capacity soils become wetter than needed, and water can be wasted. If the applications are based on the high water holding capacity soils, the moisture content in the low water holding capacity soils could drop below the stress point, and production losses will occur.
Unfortunately, irrigation systems that are able to precisely match irrigation applications to variable soil moisture deficits are extremely expensive. If possible, it is best to irrigate different soil types separately, but this may not be a realistic option on many farms.
Generally, the paddock is scheduled based on the low water holding capacity soils because the potential irrigation savings are lower than the potential yield losses. However, because evapotranspiration tends to be lower on the poorer soils and higher on the better soils, the amount of water to apply is based on the depletion on the high water holding capacity soils.
The rate at which water should be applied may also vary because of varying soil infiltration rates or areas with steep slopes. Either of these factors may change the application efficiency and alter the depth requirement. The irrigator should select the most representative situation in the paddock and irrigate to that.
5.12 IRRIGATING WITH LIMITED SYSTEM CAPACITY
When irrigation system capacity will not satisfy crop water requirements during the peak of the irrigation season, the period of the season where limitations will occur should be identified before the season starts, and a strategy developed to accommodate the limitation.
The situation is quite different to using the latest irrigation date strategy that is recommended for systems with adequate system capacity to maximise the use of rainfall.
If the irrigation is delayed to use rainfall effectively and to maximise water use efficiency, soils could dry out to below the stress point as the season develops because the system will not be capable of keeping up.
The irrigation manager must store as much water as possible in the soil by maintaining soil moisture levels up at field capacity at the beginning of the season, and holding them at that level for as long as possible. This requires looking forward to the maximum ET periods, i.e. mid-summer rather than just looking at the current irrigation cycle. It means starting irrigation early enough with a trigger level that returns the soil to field capacity.
If the practice of storing water in the soil is not able to prevent crop stress in mid summer, the response to stress of various crops should be considered, and subject to economic value, the crops with the highest sensitivity to stress should be irrigated first.
With many well-watered crops, the consequences of stressing the crop in mid-season could be more severe than uniformly stressing the crop throughout the season. Stress conditioning, and controlled deficit irrigation, that is deliberately stressing crops at various stages, can be planned at the start of the irrigation season.
The key point with irrigation under a limited system capacity is to develop an irrigation management plan at the beginning of the season, so that the plan can be implemented, and the best results possible obtained from the limited water.
5.13 WASTEWATER IRRIGATION
5.13.1 Nitrogen Loading
Nitrogen loading in the wastewater can place pressure on the environment due to nitrates leaching into groundwater or surface runoff discharging into waterways. If the wastewater application area is too small the soil will become overloaded and nitrogen may leach into groundwater.
Application of wastewater at rates that can be utilised by the pasture or crop pose less threat to groundwater or surface water. Application at higher rates will result in larger leaching losses and consequently higher nitrate concentrations in groundwater.
The following points regarding the leaching of nitrates following wastewater application to the land should be noted:
- Leaching losses can be high from soils ploughed, subject to wastewater application and then left fallow over an extended period;
- Leaching losses are generally low with a pasture cut for hay and/or silage; and
- With all crops, leaching loss depends on the application rate relative to plant requirement.
- The nitrogen loading rate (kg/ha/year) should not exceed that allowed for in the permit covering this activity.
There is unlikely to be a need for nitrogen fertiliser to be applied to soils that receive wastewater.
5.13.2 Operational Guidelines
The following operational guidelines should be applied to areas irrigated with wastewater:
The maximum depth of any application should not exceed 50 percent of the water holding capacity of the soil. The wastewater application system to be managed so that ponding of wastewater does not occur. The wastewater application system to be managed so that no runoff into any surface water body or neighbouring property occurs. Most Regional Councils and Unitary Authorities have specific regulations regarding the distance of the irrigation of wastewater from any water race, river, stream, creek, lake, wetland or other surface water, as well as the distance to any well used for drinking water supply. The wastewater should be applied as evenly as possible over the area set aside for wastewater irrigation. Wastewater should be applied to short pasture, which is immediately after grazing. Allow at least a 7-day withholding period prior to grazing after applying wastewater. For hay and silage, wastewater should not be applied within 14 days prior to harvesting.
- For cropping land, wastewater should be worked into the topsoil before sowing or planting.
- Where freezing of the wastewater in the delivery lines can occur gravity drain the line after use.
Generally the highest odour emissions occur while the application of wastewater is taking place. The most practical way of reducing aerosoling effects is to use low pressure spray nozzles (i.e. 100 kPa to 300 kPa) with large orifices (i.e. between 8 mm and 16 mm).
The following management strategies should be followed to minimise odour nuisance and hygiene problems:
- Avoid spraying at night. Stable atmospheric conditions, which slow micro-organism die off and odour dispersion, usually occur at night or during calm mornings when temperature inversions exist. Further, more any faults in the system are unlikely to be detected at night and permanent system damage, soil overloading and subsequent accidental breaches of regulations may result.
- Avoid spraying at weekends, on public holidays and in the evenings when people are around.
- Check wind direction in relation to nearby dwellings before application. Spraying up wind of nearby dwellings should be avoided.
- The best conditions for application are those causing odours to disperse quickly, typically sunny windy days followed by cloudy windy nights. The worst weather conditions for odour dispersion are typically high relative humidity and very light winds or clear still nights.
- Any odour complaints received are logged in a diary and all complaints are investigated and corrective action taken.
5.13.3 Nuisances and Odour
Wastewater must be applied to land in such a way that it does not cause a nuisance to the public or endanger human health.
The land application of wastewater should not:
- promote physical nuisances such as flies;
- generate odour nuisance; and
- create health risks and hygiene problems.
These problems can be overcome by the use of buffer zones and sensible application management.
Most Regional Councils and Unitary Authorities have specific regulations regarding the proximity of the land receiving wastewater to neighbouring properties, public roads, neighbouring dwellings and surface water.
5.14 ENERGY MANAGEMENT PRACTICES
Irrigators can limit the cost of energy by employing energy-saving practices into their irrigation management strategies.
5.14.1 Reducing Pumping Hours
Reducing pumping hours has an immediate effect on electricity use. This can be achieved by reducing the seasonal depths of irrigation applied, or by improving irrigation efficiency, and is possible without physically changing the irrigation system. Separating out the effects of irrigation efficiency and application depth is difficult. However, efficient irrigation scheduling can significantly reduce the amount of irrigation water pumped, and through that, energy use.
The biggest opportunity in New Zealand for reduced pumping is through better use of rainfall. Managing the irrigation system to use as much of the rainfall that occurs in a season as possible has direct benefits, because that water does not have to be pumped. Effective use of rainfall has been described in Section 5.9.
5.14.2 Improving Irrigation Efficiency
Improving application efficiency reduces the gross water applied and reduces energy use.
The net water used by the crop is climatically driven, and is independent of the irrigation system. The gross irrigation depth applied that is necessary to maintain plant growth depends on irrigation system type and the irrigation management practices used. However, except perhaps for systems that apply large fixed depths of water, irrigation efficiency depends more on management than on the irrigation system.
Irrigation efficiency changes during the irrigation season. For systems with limited system capacity, efficiencies may be lower at the beginning of the season because of the need to totally fill the soil profile. As the soils dry out as the season develops, irrigation efficiency increases, because the system does not have the capacity to fill the soil profile completely.
Poor irrigation efficiency depends primarily on four inter-related factors. These are: irrigation timing, which is a function of soil moistures before irrigation; the depth applied, which is a function of soil moistures after irrigation; application uniformity, which affects how much of the water will be lost to deep drainage; and application rate, which if excessive, causes poor uniformity because of surface redistribution.
Opportunities for improving irrigation efficiency are specific to each site. These can range from minor improvements to the irrigation system through to major equipment changes.
Simple changes to management such as adjusting application depths through nozzle size changes and covering the area more quickly (if equipment allows), may result in significant efficiency improvements. Close monitoring of irrigation applications through soil moisture measurement can result in significant reductions in water pumped. Proper maintenance of the system, for example by replacing worn sprinklers and nozzles, or blocked emitters, will improve efficiency.
Major changes, such as changing the type of irrigation, for example from flood irrigation to pressurised sprinkler irrigation, could result in significant improvements in irrigation efficiency. However, the benefits in terms of energy use may not be realised if the flood irrigation system was previously a gravity supply system that did not involve pumping.
5.14.2 Reducing Pump Pressure
There are a number of ways of reducing the pump duty required for an irrigation system, but most require large physical changes to the system.
Changing to low pressure spray nozzles will reduce energy consumption, but may not result in significant savings because of the high application rates that occur, causing surface redistribution and run-off. Whether reducing the operating pressure of sprinklers is worthwhile depends on what percentage of the total pump head the sprinkler requirement makes up. For deep well pumps, most of the energy is used to lift water to the surface, and modifying the system at the surface will change little percentage-wise.
Replacement of pressurised systems with surface systems will certainly save energy, but if more water has to be pumped to make up for lower efficiency, the benefits may not be significant.
5.14.4 Improving Pump Efficiency
An efficiently designed pumping plant is necessary to minimise energy required for pumping.
Different pump brands and types of pumps can have quite different efficiencies even though they are pumping the same amount of water. This efficiency depends on the design of the pump. Likewise, different motor brands have different efficiencies. The energy cost of running the system depends on both of these factors.
The key to an efficient pumping system is to select the best motor/pump combination at the design stage. If the better combination is more expensive, an economic evaluation should be made to determine if the possible savings in energy outweigh the additional capital cost.
Operating a pump at a flow at or beyond its maximum efficiency point will reduce the cost per cubic metre of water pumped.
As pumps wear, they become less efficient. Monitoring performance with pump tests will tell you whether or not loss of performance is significant enough to warrant repairs.
5.14.5 Electrical Load Management
Irrigators do not have much influence over electricity prices or the reliability of supply. There is a trend for some electricity supply companies to tailor tariffs to suit irrigation, or to offer load management options, but these are not yet common.
Where cheaper night rate electricity options are available, there is a temptation to design irrigation systems to only operate within those hours, even during peak irrigation periods. For a ten-hour night rate period, irrigation systems have to be designed with over twice the capacity as they do if the system is operated over the full day. This means bigger pipes, pumps and system components, and a much higher cost system.
Given that irrigation systems only need to operate at their peak capacity for part of the season, it is usually much cheaper overall to design systems to utilise the full day, operate at night whenever possible, and extend irrigation into day time rates when demand requires it.
Contact for Enquiries
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