Irrigation Efficiency

There are many ways to assess the efficiency of an irrigation system. Selecting the ones to use depend on what you want to know. A good place to start is reading the case study, “Hawke’s Bay Cropping Farm: Optimising irrigation efficiency in a drought prone region” which is available from the MPI website. More details are given in a 2006 report (Edkins, R, 2006 Irrigation Efficiency Gaps – Review and Stock Take) prepared by Aqualinc Research.

In broad terms, irrigation efficiency might be grouped into economic, environmental and social indicators.

Economic indicators

Economic indicators consider the benefits to the farm business of irrigating, and include things such as

• the amount of water used per unit of farm output (e.g. m³/tonne), or alternatively crop production per volume of water used. This could be measured in cases of vegetables grown per cubic metre extracted, tonnes per millimetre applied or other similar ratios. The aim is obtaining the most production from the least amount of water.
• Return on water use (Profit per volume water used). Similar to above, this could be measured as increased profit per millimetre applied or increased revenue Return on water use (Profit per volume water used). Similar to above, this could be measured as increased profit per millimetre applied or increased revenue.
• Energy used to pump water, taking into account the total volume pumped. It is affected by pump selection, pipe size and other design criteria, as well as system maintenance and site topography.
• Labour per irrigated hectare. How many hours does it take to move, manage and maintain the system?

Environmental Indicators

Environmental indicators consider how irrigation may be affecting the environment outside the farm, and include

• the amount of water taken that was effectively used (a percentage)
• the effects of abstraction on others – instantaneous rates can affect nearby bores, seasonal takes can affect the water table in an area
• effects of nutrient or chemical losses from leaching on the health of water bodies.

Social Indicators

Social indicators include assessments of the effect irrigating has on the community. Positive factors include

• increased employment
• increased economic activity resulting from more production
• more families filling the local school

But there can also be undesirable effects such as

• reduced water levels in streams or lakes
• less opportunity for some recreational activities
• or reduced water quality from more nutrients

Good irrigation managers consider all the effects of their actions and take steps to ensure the best possible outcomes can be achieved.

On-Farm Indicators

Efficiency indicators used in irrigation system performance assessment include

• Application Efficiency – The percentage of applied water that is retained in the root zone, or in the target area, after an irrigation event.
• System Capacity – The flow of water per unit of irrigated area normally expressed as litres per second per hectare (L/s/ha) or mm per day (mm/d) calculated on the basis of the system operating 24 hours per day.
• Return Interval – The typical period between one irrigation event and the next. It is usually calculated for the most demanding period so that the irrigation system can meet water demand most of the time.
• Water Use Efficiency – We define water use efficiency as the volume of water taken for irrigation per unit of farm output, described as cubic metres extracted per tonne of produce (or crates of vegetables, or broccoli heads….).
• Hydraulic Efficiency – measures the headloss (pressure loss in the system) from the headworks to the outlet.
• Energy Efficiency – The amount of energy the system requires to pump a certain volume of water, typically kWh/m³.

Application Efficiency

Application efficiency considers what proportion of water applied to a crop is stored in the soil available for plant growth. It is what most people seem to mean when they say irrigation efficiency. An estimate of Application efficiency can be made for a single event, but seasonal or annual application efficiency values are more relevant.

The two main factors affecting application efficiency are the applied depth and the uniformity of distribution. If you apply more than the soil can hold, it will run off or drain out the bottom. Distribution uniformity is a measure of how evenly the system applies the water to the crop. It is one of the key things measured in system performance monitoring. We have a whole section devoted to it later in this course! Typically, distribution uniformity measurements are made at the top of the crop canopy in field crops, and above the soil surface in any case. This means there are opportunities for the plants and the soil itself to further influence the evenness.

When considering uniformity, it is very important to think what the evenness is applying to. We take individual plants as the unit to be considered, as they are what we are trying to manage.  For a beetroot crop that means the whole soil volume across the field needs to be evenly watered. But for grapes or fruit trees, only the main root volume of each plant has to be considered. Knowing the target is essential to decide what to measure when we are assessing uniformity.

Note also that it is not economically viable to aim perfect uniformity in most cases, although some drip irrigation systems can be getting very close especially when new. But for sprinkler systems on broad acre field crops, getting as close as you can by sensible design, maintenance and decision making will increase profitability.

Application In-Efficiencies

As it often the case, the biggest losses are not the ones that seem most obvious. Indeed, in our experience, the biggest losses are some that you can only determine by careful measurement.

The table below gives examples of the loses that might occur from an irrigation system.

We often hear people discussing the first five points above, but they are not where we normally put our attention. Leaks are usually obvious (not always if pipes are well buried!) and usually quickly fixed, in part because they can make a mess. Studies have shown that the air cannot evaporate very much water before irrigation droplets hit the ground, and even in very hot dry conditions is less than 3%. Canopy interception can be significant, especially when only small irrigation depths are applied. But when the water evaporates, it tends to reduce crop transpiration and soil evaporation so is not as much of an issue as people think.

Wind can blow water off target, and using a gun on a blustery day is likely to be inefficient. Even if the water stays in the crop, it is likely to be inefficiently applied because it causes uneven application (see more soon). Surface run off – typically because the application intensity is too high (thunderstorm rather than shower) or too much is applied at once (saturated soil doesn’t absorb water quickly) – can be a problem. If you see it, change your soil management as well as your irrigation management!

The biggest losses by far are caused by applying more water than the soil can hold. This happens across the field if the target depth was too much, and in parts of the field if the irrigation is not distributed evenly. Application Depth and Distribution Uniformity are two critical factors and can really only be assessed by careful measurement (see Assessing a Traveling Irrigator later in this course).

Attainable Application Efficiencies

Different irrigation types by their very design have different attainable irrigation efficiencies. Key New Zealand types are shown in the chart based on the work of A.J. Clemmens below. In this table, 60% efficiency means that 60% of water applied through the irrigation system is available to support plant growth – essentially by replacing that lost by evapotranspiration.

Drip and micro-irrigation systems can be very efficient if well designed, installed, maintained and managed. Each has to be right if best results are to be achieved. Spray irrigation is typically less efficient and because water is thrown for far the expected performance of big gun irrigators is lower than others, and very poor in windy conditions. Solid set systems and travelling booms are difficult to get perfectly overlapped between runs and can also be affected by wind. Because there is a high labour demand shifting them, they are usually set to apply large irrigation depths and can have excess drainage as a result. Centre pivot and linear move irrigators offer the highest potential application efficiencies. The sprinklers are set to optimum spacings, they can be set near to the ground, and are often set to apply lower depths at each pass.

In the 2000s, together with Page Bloomer Associates we undertook a number of irrigation system evaluations. Some typical results are shown in the chart below.

Measurements of systems found a wide range of performance. This slide has a sample of results from Hawke’s Bay, Marlborough and Canterbury. The green star in micro-spray (bottom line at 35%) is the value determined from a poorly maintained under-tree micro-sprinkler system in which one block had excessively low pressure and flow. We made adjustments and the result is shown by the blue star at 75%.

The green star in multiple sprayline (35%) represents measurements made with impact sprinklers run at minimum pressure. The second green star (50%) is the same system with pressure raised to the maximum recommended sprinkler pressure. The brown star (60%) is the same system with different spray heads (rotators) and the pressure boosted still further. We have found such systems seldom perform well.