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An Interactive Calculator For Sprinklers

The sprinkler calculator helps you to choose the correct depth to place the FullStops. It can also help you to work out why a FullStop may rarely respond to sprinkler irrigation, or why they may respond all the time.

Below are the instructions on how to use the calculator for sprinkler irrigation. At the bottom of this page is the link to download the calculator.

The top left hand box on the calculator requires you to insert an Active Root Zone depth in mm. We define the active root zone as the maximum depth you would normally want to wet by irrigation. This is the zone containing most of the plant roots. The default setting is 500 mm. Water going below the active root zone is not necessarily wasted, because we expect some root growth below this depth.

The active root zone depth is mostly determined by type of crop grown but is influenced by soil properties such as hard layers that restrict root growth. The irrigation strategy also affects the active root zone depth. Wheat roots may go down 1 m or more, but a centre pivot irrigation system may only rewet the top 150 to 200 mm of soil. This would constitute the active root zone depth for our purposes.

The middle box on the left asks for the Daily Crop Water Use in mm. In temperate climates, daily water use would average around 1 to 4 mm per day in winter and 4 to 7 mm per day in summer. The default value is set at 5 mm per day.

The lower box on the left asks for the Days Since Last Irrigation. The default value is set at 5 days.

The Soil Water Deficit is calculated by multiplying the daily water use by the days since irrigation.

The Soil Water Storage (%) before irrigation is given for two different soils; a light or sandy type of soil and a heavy or clayey soil. When the soil water storage is 100% the active root zone is filled to field capacity. When the soil water storage is 0% the active root zone is at permanent wilting point.

The depth of FullStop installation is based on the depth of the active root zone. For an active root zone depth of 450 mm or greater we use the one third / two thirds rule. The shallow detector is placed one third of the way down the active root zone and the deep detector two thirds of the way down.

The shallowest depth a FullStop can detect a wetting front is 150 mm, or 50 mm from the soil surface to the rim of the funnel. Therefore an installation depth of 150 mm is always used for shallow rooted crops and the deep detector may not be used at all.

The Active Root Zone depth, Daily Crop Water Use and Days Since Last Irrigation boxes can all be changed to simulate your normal practice.

Irrigation is simulated by completing the two boxes on the top right hand side. The first box requires the Sprinkler Application Rate in mm/h. This can be measured by placing catch cups in a pattern around the sprinklers. It is important to note how variable some sprinkler systems can be, particularly mini-sprinklers. The default value is 10 mm/h.

Lastly, you need to Specify the Duration of Irrigation in hours. The default value is two and a half hours, which must be entered as 2.5 hours. The amount of irrigation (duration multiplied by application rate) will be calculated.

Hit the Recalculate button, and the simulation will run.

Two soil profiles (or cross sections) are shown from the 0 mm (surface) to 600 mm depth. The light soil profile is shown on the left and the heavy soil on the right.

The wetting front is very strong while irrigation is taking place. However, water still continues to move downward through the soil profile for hours to days after irrigation. The shaded blue colour represents the depth at which a wetting front is still strong enough to activate a FullStop. We call this the 'trip zone'.

The horizontal green line is the depth to which the front would reach about 24 hours after irrigation. If the green line cannot be seen it may be below the maximum depth on the graph, and the calculator will report “24 h below 600 mm”. If this message is not displayed, and the green line is not on the graph, it means that the bottom of the blue shaded ‘trip zone’ was the deepest the water penetrated.

The yellow line marks the bottom of the active root zone.

The round circles are the FullStops. When a FullStop lies within the 'trip zone' it changes colour from yellow to red.

The pink bar above the surface layer represents the amount of water applied by irrigation.

For the light textured soil, the default simulation shows that the soil water deficit was 25 mm giving a storage of 29% in the root zone. The irrigation was 25 mm, and this activated the shallow but not the deep detector. The trip zone extended to 280 mm.

Note if you want to refill the profile totally, such that the 24 hour wetting front is the same depth as the active root zone, then you must apply the soil water deficit PLUS one day's crop water use.

Two contrasting soils are shown because wetting fronts move differently in different soils. The 'trip zone' is usually a bit smaller in light textured soils.

By changing the active root depth, soil water deficit, and irrigation amount, you can watch the response of the FullStops.

Note that it is possible to create scenarios where water is pushed below the active root-zone without activating the deep FullStop (see the position of the green line). This example represents a limitation to the technique, and is why we recommend that deep detectors are not placed deeper than about 50 cm for sprinkler irrigation.

Before attempting to use this simulation we would recommend that you print out this instruction page.

CLICK HERE   to dowload and start running SprinklerGame.exe (764 kb). This game will not run on a Mac.

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