Indicators of Sustainable Irrigated Agriculture

10 Water Indicators

The sustainability goal for water is to minimise any adverse effects on the water source and on any water bodies that receive irrigation water through wind-drift, surface runoff, or drainage to groundwater. Adverse effects on water bodies relate to effects on ecosystems and other uses (e.g. recreation, drinking water supply) that they support. Water sources may in some cases benefit from irrigation - for example, management of dams can prevent a watercourse from flowing below a minimum value - maintaining an even flow. Increasing drainage flows can also have positive effects. The supply of water to downstream ecosystems and users of the receiving waters will be increased, and the maintenance of adequate drainage to groundwater is important in preventing salt build-up on prone land, provided that groundwater levels are not close to the surface.

In most cases, individual farmers cannot determine the direct effects of their irrigation on the associated water body. This is particularly true for groundwater systems. The actual effects on a water body will be the cumulative impact of all other irrigated farms, power schemes, domestic water supplies, contaminated sites, discharges etc. These cumulative effects can only be assessed and controlled by the regulatory authority responsible for protection of the water resource. Where a farmer is the only irrigator or one of a small group taking water from a small surface stream or river, the abstraction may result in a large percentage reduction in stream flow, and in extreme cases, use all of the available flow. There are very visible effects on the flow, but the environmental effects resulting from the reduced flow may be less obvious.

Regulatory authorities use resource consents and associated conditions to control how much water-use and disposal is permitted by each user. Making sure that the correct resource consents are held and continue to be renewed is important in ensuring that irrigation can continue.

Farmers could assume they are not causing an adverse effect on water bodies if they are complying with their resource consents. However, this assumes that the regulatory authorities have good information about the relationship between the restrictions they impose and the effects of activities on the water bodies. Because this type of information is often not available, particularly with respect to drainage water, authorities usually take a precautionary approach. Farmers who measure their actual effects can provide precise information to councils and potentially improve the quality of decisions relating to their resource consents.

10.1 INDICATORS FOR EFFECTS ON WATER SOURCE

10.1.1 Issues

The amount and timing of water abstractions and the location of the abstraction point determine the impact of irrigated agriculture on the water source. Regional Councils generally issue water permits (resource consents) which specify a maximum abstraction rate and the total volumes of water that can be abstracted on a daily or weekly basis. These amounts are allocated so that the minimum flow required for the water source to maintain its life-supporting capacity is met, and the water source does not suffer adverse effects.

The quantity of water lost from the farm will also impact on the water source; the more water that either drains to groundwater or surface watercourses and is not stored in the soil, the more water that must be abstracted from the source to meet crop requirements. In some cases the water source will also be the receiving waters (e.g. a return channel on a border-dyke irrigation scheme). These aspects of water use will be measured by indicators relating to the receiving waters which are covered in Section 10.2.

10.1.2 Potential Indicators

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

weekly irrigation volumes;

monthly irrigation volumes;

seasonal water use;

maximum water abstraction rate each season;

resource consents - held and renewed.

10.1.3 Rationale for Selecting Indicators

The quantity of water coming onto a farm is relatively straightforward and unambiguous to measure. Piped irrigation systems can use flow meters to assess volumes pumped and maximum flow rates. For irrigation systems using gravity-fed channels, the same information can be measured using ultrasonic flow meters, calibrated weirs or flumes. These flow measurements can be made at any frequency: hourly, daily, weekly or monthly.

Daily irrigation volume is the most suitable indicator for describing the timing and volume of irrigation water flowing onto a farm. While daily measurements of irrigation volume may seem a lot of information to be recording, irrigation decisions such as how much water to apply and which area to water are made on a daily basis. And, as will be discussed in the "Best Management Guidelines" project, measuring daily irrigation volume is an important component in effective management of irrigation systems. If the daily volume of irrigation water used is measured it can be summed to give any other irrigation water quantity information which is required by a farmer or council (e.g. weekly, monthly or seasonal volumes).

Several of the other indicators (for example, productivity, economic) require that the amount of water used for each crop be known. Therefore, daily information on which crop was irrigated will need to be kept. If two crops were watered in one day, then the number of hours each crop was irrigated should be recorded and the water apportioned to each crop accordingly. Flow meters on irrigation equipment can provide this information directly, and are also useful in managing efficient water use.

The maximum abstraction rate should also be measured because it is a requirement of most resource consents. It is also easy to measure and directly relates to the impact of irrigation on the water source.

10.1.4 Recommended Indicators

Resource consents held and renewed
Daily volume of irrigation water flowing onto the farm for each crop(m³)
Maximum water abstraction rate each season (m³/hr)

10.1.5 Related Information

The following information will be necessary in order to interpret the indicators, and make management decisions based on the results of the monitoring:

location of water supply;

farm size;

resource consent conditions;

which crop is watered each day;

area of each crop on farm.

10.2 INDICATORS FOR EFFECTS ON RECEIVING WATERS

10.2.1 Issues

For the receiving waters, the impact of irrigated agriculture relates to both water quantity and water quality.

Water is lost from the farm by surface runoff or wind-drift to rivers, streams and water races, or infiltration through soil to groundwater (often also leading to surface waters). The difference between the volume of water lost from a farm and the volume coming onto the farm is the amount of irrigation water that is actually effective in increasing crop production. This provides a measure of how efficiently the irrigation system is designed and managed in terms of water use, which can be used on a daily basis to assess the efficiency of a single irrigation application, or used on a seasonal basis to provide an annual (irrigation season) water budget for the irrigation water. Both daily and seasonal information is useful to determine ways to save water and either reduce future water demand or increase crop production.

Irrigation water may also be lost through wind-drift over the paddocks, and the irrigation of areas not directly affecting crop growth (e.g. over the fence). This is more significant in some areas than others, but should be considered when assessing irrigation system efficiency.

From a water quality viewpoint, the amount of nutrients, pesticide residues and faecal coliforms running off or draining to receiving waters are important and will be a function of the runoff/drainage flows and the concentration of nutrients and contaminants in the drainage water. Regulatory authorities can combine these nutrient loads with information on the size, assimilative capacity and other uses of the water body to assess the effects on the receiving waters. The resource consent process will address the cumulative effects on the receiving water.

10.2.2 Potential Indicators

Quantity of water going to receiving waters

Groundwater depth.

Groundwater flow.

Soil moisture measurements.

Amount of water used by crops.

Irrigation system efficiencies,

Deep percolation - drainage below the root zone,

Losses to evaporation.

Overland flow losses.

Visual measures of surface runoff.

Quantity of water stored in the root zone.

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

Quality of water going to receiving waters

Groundwater salinity.

Groundwater nutrients.

Groundwater agrichemicals.

Groundwater water related diseases.

Sediment quantity.

Surface water nutrients.

Water quality in streams receiving runoff.

Amount of growth in surface waterways.

Direct measure of quality of drainage flows below farm.

Direct measure of quality of surface runoff from farm.

Quantity of water draining to groundwater or surface water.

Quality of irrigation water applied (e.g. level of nitrogen in wastewater used for irrigation).

10.2.3 Rationale for Selecting Indicators

Quantity of water going to receiving waters: Some of the potential indicators relate to measurements of the groundwater system such as groundwater depth and flow. While some may relatively easy to measure, there are usually too many other influences on a groundwater system for a farmer to make a direct connection between farming actions and groundwater characteristics. For example, the water level in the Canterbury aquifers responds to rainfall in the foothills of the Southern Alps, rainfall on the Canterbury plains, and river levels in the major rivers on the plains. Farmers will not be able to identify whether their actions or other factors influenced the groundwater levels or flows.

Unlike the water coming onto the farm, the drainage water does not generally leave the farm at a single point; water drains from all over the farm and at different rates depending on soil type, crop, current soil moisture conditions, rainfall, irrigation depths and timing. If a drainage system such as mole or tile drains is installed on the farm, there is a possibility that the quantity and quality of drainage water can be measured for the farm as a whole. However, drainage systems cannot always provide adequate information regarding the origin of the water, i.e. whether it is irrigation water, rainfall, or high groundwater.

Lysimeters are currently the only commercially available devices for directly measuring drainage flows at a given point on farms. There are two types of lysimeters - drainage and suction lysimeters. Both of these require permanent installation and are fixed in place to take measurement of the flow and quality of drainage water at a given point on the farm. However, both are expensive to install, and because they are buried in the soil column, disrupt some farming practices such as ploughing.

From a cost and practicality viewpoint, where suitable drainage systems do not exist, it is not appropriate to recommend that farmers directly measure drainage flows.

Rather than directly measure drainage flows, it is suggested that a measure be made of the percentage of water flowing on to the farm that is stored in the root zone. This will give an indication of how much water is actually effective in increased production and conversely how much is lost due to surface runoff or drainage.

The proposed method of establishing the effectiveness of irrigation in providing water for plant uptake is to measure the percentage of water flowing into the farm that is stored in the root zone. This can be determined by taking soil moisture measurements in and below the root zone, and using the information about soil water holding capacity to calculate the volume of water. The "Best Management Guidelines" recommend daily soil moisture monitoring as an important component in good irrigation practice. Matching the amount of water applied to the soil moisture deficit will minimise or even eliminate any drainage to groundwater.

Ideally, soil moisture measurements should be made for each crop type and each soil type on a farm. In practice, crop factors can be used to relate water use among different crops. The soil property that has the most influence on the percentage of applied water used by the crop is the soil moisture holding capacity. If a farm contains soils with very different moisture capacities then drainage will need to be measured on each soil type.

Storage volumes calculated from soil moisture measurements do not separate out whether the water stored in the soil is from irrigation or rainfall. Irrigation increases soil moisture and therefore causes a greater proportion of seasonal rainfall to drain to groundwater than would have drained from unirrigated farms. The only way to assess how much drainage is caused by rainfall is to use a further set of soil moisture measurements for the same crop without irrigation that is grown in the same year on the same soil type with the same daily rainfall. This is not a practical measure since it is unlikely that a farmer will only irrigate some areas of a given crop. So, rather than separate out the contribution of rainfall to drainage, the recommended indicator - the percentage of water flowing on to the farm that is stored in the root zone - describes the total amount of water stored from both irrigation and rainfall, since this is feasible to measure with soil moisture measurement devices. Daily rainfalls will also need to be recorded and used in the calculations that relate water applied (both through irrigation and rainfall) to changes in soil moisture.

Like drainage to groundwater, losses to surface water and evaporation could occur all over a farm and at varying rates. If there is a single watercourse that collects all surface runoff from a farm, then flow rates in this watercourse can be measured. In most cases, it will not be easy to distinguish where the surface water is going. Because losses to surface water such as wind drift and erosion channels can be easily seen, a visual assessment can be used to give an indication of surface losses. The recommended indicator is visual assessment, which should pick up overland flow paths and ponding associated with high surface runoff. The occurrence of surface water can then be reduced with appropriate management practices.

Quality of the water draining to groundwater or surface water: Measuring the amounts of nutrients, pesticide residues, faecal coliforms etc. leaving the farm (i.e. the nutrients, pesticide residues, etc.) has many of the complications associated with measuring the quantity of drainage water. If a suitable mole or tile drain system is in place, direct measures of drainage water quality can be made.

Many of the potential indicators for water quality measure properties of the whole groundwater system such as the level of nutrients in the groundwater. As mentioned above, it is impossible for farmers to separate the effect of their actions on groundwater quality from the effects of other users and natural processes. The recommended indicators must therefore concentrate on measuring the contribution of a single farm by describing the quality of the drainage water infiltrating below a farm.

In the absence of effective mole and tile drainage systems, lysimeters are the only practicable method to directly measure the quality of drainage water. However, lysimeters are expensive and can disrupt farming practices. There are some developing technologies that could enable soil moisture probes to also measure some aspects of water quality but these are not yet commercially available. Measures of the amount and type of soluble components in the soil could be used but the values would need to be measured daily and combined with daily drainage flows to give information on the seasonal or weekly amounts of nutrients etc. draining to groundwater. Analysis of the soluble components in soil would require laboratory tests, which are likely to be too expensive to carry out on a daily basis.

For clean water irrigation, there is no recommended indicator for water quality. However, by using the indicator of percentage water applied that is stored in the root zone, farmers can manage their irrigation systems to minimise the amount of water drainage to groundwater, and thus will also be reducing their nutrient load on the groundwater. Farmers should however, record the amount of fertiliser and pesticide that they apply each year. This related information will help identify any benefits of irrigation associated with reduced requirement for fertiliser and pesticides (refer Section 9). It should also be noted that the quality of the irrigation water (e.g. brackish water, water with high iron content) may also affect the quality of the drainage water and effects on soil.

For effluent irrigation, the recommended indicator is a direct measure of the nitrogen in the water draining below the root zone measured by lysimeter. Effluent irrigation is predominantly onto permanent pasture land, which does not require frequent tillage operations. Because lysimeters collect drainage volumes they can be read at any frequency. For sustainability concerns, reporting the total seasonal nitrogen leached is sufficient.

Measuring the amount of nitrogen which drains below the root zone is also a far better indicator of the effects of the effluent application on groundwater than the current measure used in resource consents: the amount of nitrogen applied to the surface. There are practical sampling difficulties under a grazing regime, referring to the non-uniformity of nitrogen application by stock. If lysimetry can be used effectively, and farmers can get a good indication of the nitrogen flux below the root zone, there will be financial benefits associated with installing a lysimeter as well as increased flexibility in the timing and depth of effluent application.

10.2.4 Recommended Indicators

Daily percentage of water flowing onto the farm that is stored in the root zone (derived from soil moisture measurements in and below root zone)

Daily visual assessment of the amount of ponding or surface water runoff

Lysimeter-based measurement of nitrogen leaching below the root zone (kg)

(for effluent irrigation only)

10.2.5 Related Information

The following will need to be known to enable the indicators to be used as tools for making management decisions:

daily rainfall;

area of each soil type on the farm;

soil moisture holding capacities of all soil types on the farm;

resource consent permits and conditions.