Best Management Guidelines for Sustainable Irrigated Agriculture

7 Reviewing the Seasons Irrigation

At the end of the irrigation season, a review should be carried out to determine the success or otherwise of the irrigation strategy used, and to get a measure of the overall performance of the irrigation system.

Assessing the effect of irrigation on production, profit, the environment and the community, needs to be carried out. Further to this, knowing what the water, energy and labour cost has been for the season will be helpful in planning the next season’s irrigation.

The recommended indicators can be used to do this. The current indicators need to be compared to previous indicators to determine the direction of change. If indicators are moving in a positive direction, the changes implemented at the previous review have been effective. If the indicators are moving in a negative direction, the previous changes have had adverse effects on the system, and the reasons for the adverse effects must be identified before making the next change.

The following notes refer to some of the options available to farmers to assess the indicators and give suggestions for improvements.

7.1 RESOURCE CONSENT CONDITIONS

What is the relationship between the peak flow and the resource consent conditions? Are the weekly and seasonal volumes consistent with the conditions?
Relevant indicator

Resource consents obtained and complied with.

You will need to check your daily records to determine volumes abstracted and compare them to the resource consent conditions.

If the actual abstractions are higher than allowed, the reasons should be identified and measures taken to comply with the consent. In particular, you should note whether it was a one-off event or a regular occurrence. Note that the reasons for the higher flows or volumes could be technical or managerial. It may have been due to a technical failure such as pipe leakage, or may have been a result of inefficient irrigation, or simply because the area and/or type of crop irrigated requires more water than the resource consent allows for.

If the peak abstraction was lower than allowed, then the resource consent has been complied with.

7.2 DAILY AND SEASONAL VOLUMES OF WATER USED

Is water use as expected? How does the seasonal volume used change over time? Are there any unexpected changes in flow rates when operating the system at different locations? Is the irrigation system applying the required depth? How efficient was your irrigation? What percentage of your total water supply was actually used by plants? Is the peak flow equal to the design capacity of the system? Are the flow rates or daily volumes consistent with the design capacity of the system.

These are some of the many questions you could ask relating to water use.
Relevant indicators

daily volumes of irrigation water flowing onto the farm for each crop;

daily percentage of water flowing onto farm that is stored in the root zone;

daily visual assessment of the amount of surface runoff;

maximum water abstraction rate each season.

If water use too low, then either it has been a very wet year, or the system is not performing to capacity, or crops are being underwatered. This could be caused by:

worn pumps;

blockages in the distribution system (e.g. clogged filters);

problems with the water supply;

problems with the soil moisture measuring equipment;

wrong size nozzles on irrigation equipment;

poor management.

Pumps, wells, filters and the water supply in particular should be checked. If necessary, replace or recalibrate soil moisture measuring equipment.

If the flow rates are variable, likely causes are:

operating outside system specifications;

unacceptable variation due to pipeline sizes (poor design);

pump duty problems;

unaccounted for changes in elevation;

Check that the system is being operated within the limits specified in the design. Also check pressure losses in the system. A hydraulic analysis may be required to do this. If poor pump performance is suspected, a pump test or hydraulic analysis is recommended. You may need to change the operation of the system, system components or flow rates if a design problem is identified.

If water use is too high, then you have probably been irrigating inefficiently. Either water was lost before it reached the soil, or water was lost as drainage.

To find out how much water is lost before it reaches the soil surface, your daily records should be checked. The amount lost should range from 0-20 percent of the total amount extracted, with the actual amount dependent on the type of irrigation system.

If losses are greater than this, likely causes are:

leaks in the distribution system;

surface ponding;

wind-drift;

watering areas outside of paddock area; or

the system is applying higher irrigation depths than expected.

You should:

fix any leaks;

change the system application rate or depth to reduce surface ponding;

adjust irrigation strategy for windy conditions;

adjust irrigator positions to prevent overthrow; or

change the sprinkler or irrigation settings to reduce the depths applied.

Check daily records to determine maximum daily volume abstracted. If the peak flow has reduced over the last season, you should identify the causes and rectify them. Possibilities are:

pump wear;

irrigation plant wear;

water supply degeneration;

well levels dropping;

screen blockages;

diversion problems; or

channel deterioration.

7.3 DRAINAGE DEPTHS AND VOLUMES

How much water was lost to drainage through the root zone? Does the soil moisture reading taken below the root zone agree with the calculated drainage depth?

Relevant indicator

Daily percentage of water flowing onto farm that is stored in the root zone.

Losses to drainage through the root zone should be examined to find out if there has been substantial drainage flow.

Based on the daily information, examine how much water applied to the soil (irrigation plus rainfall) was lost as drainage. For a good sprinkler system the amount lost should be less than 20 percent of the total amount applied during the season. For a border dyke system, up to 80 percent of the applied water could be lost on a seasonal

Try to find out the cause of most of the drainage. Was it caused by rainfall or over-irrigation? Check the actual irrigation depths that each irrigator has applied

The likely causes of problems in this area are:

an irrigation depth was applied that exceeded the depth of water the soil could absorb;

excessive rainfall;

uneven application depths (low uniformity);

surface redistribution;

design effects;

component manufacture effects;

irrigator is applying more than the design (or expected) depth.

Usually, better irrigation management can significantly reduce problems in this area. Check to make sure that your irrigation strategy takes into account the relevant factors described in the Planning Section.

7.4 SURFACE PONDING AND RUNOFF

Is any surface ponding evident?
Relevant indicators

daily visual assessment of the amount of surface runoff;

indicator of soil health (soil surface breakdown of aggregates).

If the daily records of visual assessments of surface ponding show that ponding or surface runoff has occurred frequently, likely causes are;

a sealing layer has developed on the soil surface;

insufficient crop cover;

leaks in distribution system;

topography of land is unsuitable for the irrigation method used;

irrigation application rate or depth is too high.

If the problem is occurring on a newly planted crop, the surface runoff or damage to the soil surface may be reduced significantly or eliminated once the crop cover increases.

The impact of a sprinkler jet on the soil surface may be reduced by increasing pressure to obtain smaller droplet sizes, or changing the angle of contact for the jet.

If the problem is occurring on an established crop such as pasture, the design application rate may simply be too high for the soil type. The first thing to try is to reduce the application depth so that less water is applied more often. The higher infiltration rate that normally exists at the start of the irrigation may be enough to solve the problem. If the problem persists, it will be necessary to reduce the application rate of the system. A change of sprinkler type to increase the wetted coverage of the system may be an option. A reduction in pressure to reduce the application rate may be another. If these changes do not solve the problem, a different type of irrigation system may be required.

7.5 SOIL HEALTH

Has the general health of the soil changed?
Relevant indicators

total organic carbon;

nitrogen;

soil pH;

water holding capacity.

Refer to the report Soil Quality Indicators for Sustainable Agriculture in New Zealand‡ for advice on these issues.

7.6 NITROGEN UNDER EFFLUENT IRRIGATION

Does the amount of nitrogen exceed resource consent conditions?
Relevant indicator

Lysimeter-based measurement of nitrogen leaching below the root zone (effluent irrigation only).

The amount of nitrogen leached will be closely related to daily volumes of water infiltrating below the root zone.

If nitrogen amount is too high, likely causes are that daily drainage volumes are excessive or nitrogen loading is too high.

Refer to Section 7.3 for ways to change drainage flows.

7.7 PROFIT

Is farm profit as expected? Has expenditure on irrigation been higher or lower than expected? Has profit per unit of water used increased or decreased?
Relevant indicators

annual net operating profit after tax ($);

profit per unit of water used ($/m³).

Annual net operating profit is influenced by a large number of factors. However, improved irrigation management should result in increasing profits due to irrigation in the long term. If it doesn’t, then irrigation is not sustainable.

To obtain the net income attributable to irrigation, the capital and operating costs of the irrigation system should be subtracted from the income arising from increased yield. If the income has decreased from the previous year, you should try to find out whether the decrease is due to increased expenditure on irrigation, lower yields, or lower product prices. If it is due to lower yields, then perhaps your irrigation management has not been as good as in previous years. If it is due to increased expenditure, you should note whether it is a one-off cost, or a recurring cost, and if recurring, changes may be necessary to reduce this cost in future seasons.

7.8 PRODUCTION QUANTITY AND QUALITY

Are crop yields as high as expected and of appropriate quality? Has more or less water been used to produce the crop?
Relevant indicators

production/unit of water used;

quantity produced/hectare for each crop;

quality of produce (% at each grading level).

If crop yields are higher than expected, then irrigation may have been more efficient than planned. This should be reflected in the water use indicators. Although many factors influence yield, the irrigation strategy has not been detrimental to production if yields have increased.

If crop yields are not as high as expected, or the quality of the crop has not been as high as expected, possible causes are:

Moisture available to the crop was too low either throughout the season or at some critical time.

Losses to drainage were too high and nutrient leaching occurred.

The condition of the soil has deteriorated.

Increasing the system capacity of the irrigation system may be required. It could require changing your irrigation strategy, such as irrigating earlier in the season and avoiding stress periods. If losses to drainage are high, changing the irrigation strategy to apply less water more often may be needed.

Production per unit of water used can be interpreted in two ways.

The first assumes water use is from irrigation only, and will vary significantly from year to year according to rainfall. In wet years, irrigation water use will be low, and the indicator will be high. In dry years, the reverse will be true. Using the indicator in this way will give you an idea of trends in the long-term, but not in the short-term.

The second may assumes water use includes rainfall as well as irrigation, and gives you an idea of how well you have used rainfall and how efficiently you have irrigated. It is more useful to indicate progress in the short term.

7.9 ENERGY USE

Was energy use as predicted? Has the energy use per volume of water pumped changed significantly?
Relevant indicators

annual energy used to operate irrigation system;

energy used per volume of water pumped.

The amount of energy is directly related to pumped volumes and can be changed by adopting any measures outlined in Section 5.14.

7.10 LABOUR USE

Is the amount of time spent moving equipment reasonable, variable, decreasing or increasing? Was more time spent on irrigation than last season? How many hours of irrigation down time were there due to breakdowns?

The causes of downtime should be checked. Possible reasons are:

Component failure due to unusual or excess conditions.

Improper operation.

Design faults.

Failure due to normal ageing of system.

Failure due to lack of maintenance.

If system breakdown has caused a significant increase in labour, replacement of equipment should be considered.

7.11 REGULATORY REQUIREMENTS

Have there been any abatement notices or resource consent renewals in the last season?
Relevant indicator

Record of any abatement notices.

If there have been any abatement notices, the reasons for these should be identified, and steps taken to eliminate them in the future.