Irrigation Scheduling

The purpose of irrigation is to ensure plants have the right amount of water available for optimum growth. This is the essence of irrigation management. Recent research demonstrated that commencing irrigation too late impacts upon the potential dry matter (DM) production of many dairy irrigators and, therefore, farm profit. A vital piece of information for deciding when farmers should start irrigating is the amount of water stored in the soil.

The Smarter Irrigation for Profit (SIP1) project highlighted that many irrigators are not maximising the production potential of their sites as management basics are not implemented or executed well. The key fundamental is the right scheduling, or timing, of water applications. The next is making the most of these applications by applying the right amount. By optimising both timing and rate, water productivity can be increased whilst reducing input costs and improving the profitability of irrigation.

Key indicators for irrigation timing and rate

Keeping soil moisture in the optimal zone to maximise plant growth is the aim of irrigation. This zone is called readily available water (RAW) and is different for varying soils and plant types. RAW is determined by knowing the soil texture and the rooting depth of the plant.

SIP1 highlighted that most dairy farmers use their experience and rotations to guide their irrigation decisions, but this is resulting in lost opportunities to optimise plant growth. By monitoring soil moisture through in-situ measurement (soil moisture probes) or using a weather-based water balance calculation, farmers can more accurately schedule irrigation and determine necessary rates to keep soil moisture in the optimal RAW zone.

More can be found on determining RAW on the Smarter Irrigation for Profit page.

Why is RAW so important?

The concept of RAW for the industry has not been adequately explained until SIP1. The RAW zone can be visualised as a bucket that receives water (rainfall and irrigation) and loses water (evapotranspiration [transpiration of the plant and evaporation]).

The RAW is in perfect equilibrium when it is full, but not leaky. At this point, called field capacity, the plant can most easily extract water from the soil and put excess energy into development and growth. When soil moisture becomes depleted below the lower level of the RAW zone, called the refill point, the plant has to work much harder to source water from the soil and its development and growth is impeded.

The lost opportunity

Many dairy farmers have historically used visual 'eye-balling' of the pasture/crop or a fixed irrigation method for scheduling irrigation. In SIP1 it was determined that at the commencement of an irrigation season, or after rainfall, irrigators were starting their irrigation too late, leading to what is now called a 'green drought'.

Historically, energy and water costs associated with irrigation were the determining factors for delaying start-up. Now there is more information about the improved production, and therefore income, that outweighs these input costs when irrigation commences earlier.

Delayed start-up allows the soil moisture to deplete below the RAW after the last rainfall event. When irrigation finally commences, capacity to lift the soil moisture back-up into the optimal zone is limited by the specifications of the irrigation system. The farmer may be forced, if it is possible by their system, to operate the irrigation for longer periods, potentially using peak and shoulder power at higher rates to do so. If the irrigator is started at the right time to simply maintain soil moisture in the RAW zone, unnecessary high use costs can be avoided.

In order to optimise the efficiency of irrigation, there are a number of steps farmers can follow:

1. Monitor soil moisture
2. Adopt a weather-based water balance calculator tool to forecast scheduling requirements
3. Look at water and energy data to monitor usage and costs
4. Measure production (kilograms DM per hectare per day) to evaluate whether the efficiency of the inputs versus outputs is being optimised

Confidence in the data is important, so farmers should ensure they use a reputable irrigation agronomist to discuss their soil moisture monitoring equipment needs. This technology provides a measurement of the effectiveness of rainfall and irrigation on raising or maintaining soil moisture within the RAW zone.

Irrigation scheduling decision support tools

Deploying a fixed depletion level strategy rather than a fixed frequency/ amount is fundamental. There are a number of freely available sources of information that can guide you to ensure you are starting-up and maintain your irrigations above the refill point in the RAW zone.

Weatherwise

Weatherwise is a free seven-day weather forecast notification emailed to farmers' inboxes daily. The data is supplied by SWAN Systems from the Australian Bureau of Meteorology (BOM) using a forecast model with 400,000 points on a grid extending right across Australia.

Using Weatherwise

Reference evapotranspiration (ETo) is based upon an actively growing pasture grass of about 12 centimetres high. It gives farmers an indication of the amount of water being used or lost from the soil profile.

The rainfall data provides an indication of whether the soil profile may be refilled over the coming period. The shortfall is where irrigation scheduling (rate and timing) comes into play, better known as a water balance calculation:

Rainfall minus ETo (millimetres/day) = supplement irrigation required (mm/day)

Where the rainfall amount is greater than the RAW for the soil type and plant species, then only the RAW amount is used as the rest of the rainfall amount is lost as runoff or infiltration below the root zone.

This simple method is a good starting point for irrigators who are currently not using a scheduling tool to plan their irrigation requirements.

Accessing Weatherwise

It takes two minutes to sign up for Weatherwise at www.weatherwise.swansystems.com.au

IrriPasture

IrriPasture is an online weather-based water balance scheduling and recording tool developed by the University of Southern Queensland’s Centre for Engineering in Agriculture. This tool can be used on a PC or via a smartphone app.

IrriPasture uses weather data (ETo/rainfall) from the closest BOM weather station as well as taking into account the rooting depth of the plant (over development and at maturity) and the texture of the soil.

Using IrriPasture

Multiple water balance calculations can be set up for different areas of a farm. RAW should be determined for soil types prior to setting up IrriPasture so the refill point can be set.

IrriPasture provides an overall irrigation amount (mm) and timing (days) forecast for each site and graphs soil moisture trends to help visualise the current predicted water content of the soils.

Rainfall minus ETo (mm/day) plus soil texture and pasture plant water demand (crop coefficient (Kc) of 1)= supplement irrigation required (mm required over certain number of days)

Accessing IrriPasture

IrriPasture can be accessed via its website, https://irripasture.com

IrriSAT

IrriSAT has been extensively used and validated in the grain and cotton industries. The cloud-based app automates satellite processing and information delivery of Landsat and Sentinel satellite data and provides water management information across a range of scales. It uses this information to determine the crop coefficient (Kc) of an actively growing plant.

Whilst plant water-use conversation for ETo of 1.0 is used for pastures, other crops will have a different water requirement. For example, lucerne at early flowering stage or a tall actively growing crop of maize or sorghum is likely to use more water and the appropriate Kc will be approximately 1.2. This means that these crops require 120% more water than good pasture under the same conditions to achieve top growth rates and yields.

Using IrriSAT

Multiple water balance calculations can be set up for different areas or crops. The Kc is used to calculate water requirements for the crop within the past three to seven days as the last satellite has passed over the site. The tool provides an overall irrigation amount (mm) and timing (days) forecast for each site.

Rainfall minus ETo (mm/day) plus soil texture and crop water demand (Kc as determined by IrriSAT) = supplementary irrigation required (mm required over certain number of days)

Accessing IrriSAT

The IrriSAT app can be found via the website and accessed on most web enabled devices including smartphones, tablets, laptops and PCs. Once on the site, click on the information icon for videos and guides to establish sites.

More information is available in A quick guide to the use of the cloud-based IrriSAT app.

Other resources

Dairy SA tools

DairySA has an irrigation requirement spreadsheet that is used locally and a weekly Irrigation Requirement Report for the Mt Gambier and Mt Compass regions.

Contact DairySA's Kylie Boston for further information by emailing kylie@dairysa.com.au.

Weekly Irrigation Requirement$ Summary

The Murray Dairy region is serviced by an Agriculture Victoria Weekly Irrigation Requirement$ Summary. This includes an interactive scheduling tool to determine the most productive timing for the next surface irrigation tailored to the unique circumstances of each farm.

It uses the ETo information sourced from local weather stations and operates with Microsoft Excel on mobile devices and desktop computers..

Contact Agriculture Victoria Senior Irrigation Officer Robert O’Connor to access the weekly email service by emailing Robert.OConnor@agriculture.vic.gov.au.