BY DAVID ROWE
DEPARTMENT OF PRIMARY INDUSTRIES
& REGIONAL DEVELOPMENT

The announcement of a proposed 10% reduction in licenced groundwater allocations for horticultural businesses in North Wanneroo has caused concern among vegetable growers.

Lower rainfall in response to a changing climate has seen significantly less water recharging the Gnangara system, a resource shared by the Water Corporation for public drinking water. The reduction is designed to balance the system and maintain access for both of these important users. However, to maintain or increase production going forward, improvements in water use efficiency will need to be found.

DRIP lines apply water accurately to the base of this celery crop.

Given that water required to grow a crop is directly related to the crop area being grown and the environmental conditions in which that crop is grown, some of the ways water use savings can be achieved are:
• reducing evaporation
• reducing the irrigated area
• increasing evenness of irrigation (uniformity)
• scheduling irrigation based on crop demand.

Reducing evaporation
Reducing evaporation will deliver savings in water by reducing the amount of water a plant requires to remain healthy. Reducing evaporation can be achieved by influencing environmental factors such as wind speed, humidity, temperature, and solar radiation. This can be done by protecting the crop with windbreaks, shade netting, tunnel houses, or plastic mulch.

In a netted apple trial run by the Department in 2015–16, water use was reduced as a result of increasing the shade factor by placing netting over the crop.

Additional benefits were a greater proportion of first grade or premium quality fruit.

Reducing the irrigated area
Reducing the irrigated area can be achieved by applying water only to areas where the crop can use it. A good example of saving water this way is using drip irrigation. Well-designed drip irrigation systems apply water very evenly and directly to the base of the plant and, unlike overhead sprinklers, water is not applied to wheel tracks and headlands where there are no plants.

Wheel tracks and sprinkler lines typically make up 15–20% of the total area being irrigated by sprinklers, so there is a significant potential for water saving by applying irrigation only to the plant beds. Drip irrigation is also much less prone to losses through wind drift and evaporation compared to overhead irrigation.

Distribution uniformity
Distribution uniformity (DU) describes how evenly an irrigation system applies water over the area being irrigated. It is calculated by dividing the average depth of water applied to the driest quarter of irrigated area by the average depth of water over the total irrigated area. DU is technically a ratio but is commonly expressed as a percentage where a higher percentage indicates greater evenness between the depths of water applied across the field. The minimum industry standard for DU on horticultural crops is 75% for overhead irrigation.

Growing an even crop requires that the whole crop receives a specific minimum amount of water. A system with a low DU will need to run longer to ensure that the driest quarter of the crop receives that minimum amount.

For example, using the Department of Primary Industries and Regional Development (DPIRD)
Irrigation Calculator (which estimates total plant water use of a crop based on crop factors and historical weather data) you can calculate that a lettuce crop planted in November in a sandy soil in Wanneroo will use approximately 4ML/ ha over a 45 day growing period.

Figure 1 shows the plant water requirements plus the additional irrigation water required to compensate for various levels of irrigation inefficiency (DU). An overhead irrigation system delivering water at 85% uniformity would require 18% less water than an irrigation system with 70% distribution uniformity.

Additional benefits of irrigation systems with higher DU are lower electricity cost for pumping and potentially reduced nutrient requirements resulting from reduced losses below the crop.

Scheduling irrigation based on crop demand
Even the most well designed system will fail to irrigate efficiently if not operated effectively. Learning how to schedule irrigation based on crop demand will lead to the most efficient use of any system. Crop water demand is directly related to evaporation, which is influenced more by solar radiation and wind speed, than by temperature and humidity. By using evaporation as a reference point, the daily plant water requirement can be calculated.

Calculating daily water requirement and applying the required amount of water can minimise both overwatering that wastes water and nutrients, and under watering that leads to crop stress and reduced yields or longer growth periods.

Confirming the effectiveness of irrigation should be done with technology such as soil moisture or soil tension sensors that will show if water applied has been used, is remaining in the root zone, or has drained past the root zone and is therefore no longer available. In combination, this multipronged approach can improve water use efficiency.

While it is daunting to face a reduction in water allocations, DPIRD is supporting growers by both demonstrating water saving techniques and assessing the cost versus the benefits of these techniques.

MORE INFORMATION
Keep an eye out for articles in upcoming WA Grower magazines and visit the DPIRD website for tips on irrigation scheduling and soil moisture monitoring.