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    Using MAD to Control Irrigation Schedules

    WeatherTRAK Controllers

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    HydroPoint wants to provide you with every opportunity to irrigate in ways that best match your unique needs.

    One of those ways is to use MAD to help guide your irrigation schedules.

    Unfortunately, there are two MAD standards within the irrigation industry:

    • Maximum Allowable Deficiency, as defined by the Irrigation Association, is used to estimate the amount of water that can be used without adversely affecting the plant and is defined as the ratio of readily available water to available water.

    • Management Allowed Depletion is defined by the IA as the desired soil moisture deficit at the time of irrigation.

    This second definition is the one that HydroPoint uses since maximum isn't as specific. You can water a landscape beyond its maximum field capacity, or wait to irrigate until after the soil is beyond its maximum moisture deficiency. Therefore, it's not a real limit.

    With Management Allowed Depletion (which will be the only term referenced as MAD for the remainder of this article), the goal is to get the soil as close to depletion as possible before irrigating.

    To break this down more easily, picture a clear glass of water with a plant in it. We'll use this as a representation of the water available to a plant on your site. If the glass is full of water, then the landscape is at field capacity. All of the available water is stable in the soil. It is not overflowing the glass to create puddles or mud, but rather evenly distributed.

    As evapotranspiration begins to occur, which is the amount of water that evaporates from the soil and plant surfaces and transpires from vegetation, the amount of available water in the glass decreases. It is called available water, as it is water that is available to the plant root.

    With MAD, the idea is to let the level naturally decrease from field capacity and irrigate before the available water reaches the plant's permanent wilting point.

    Then there is the question of when you irrigate. With MAD, the idea is that you want to get as close to MAD before you irrigate, so as close as you can but not exceed, then you can irrigate that day.

    Depletion and field capacity measures the exact opposite percentages. When the water glass is at field capacity, it is at 100% available water and 0% depletion. These two variables always add up to 100, whether it is 75% available water and 25% depletion or 62% depletion and 38% available water.

    HydroPoint typically sets "Target MAD" at 50%, although you can change that number to anything you prefer. This value tells our scheduling system how to determine whether irrigation is necessary, in addition many other variables such as tomorrow's weather forecast.

    Put simply, once you set a Target Mad value, your irrigation schedule will look ahead and, based on the evapotranspiration rate at which your field capacity is being depleted, determine that if the landscape will exceed Target MAD tomorrow, irrigate today.

    The reason HydroPoint doesn't use Maximum Allowed Depletion as its standard is that there are situations where it is necessary to exceed the maximum. If you manage a site with sports turf and, due to team practices and games, you are unable to irrigate on Friday or Saturday, the system will irrigate on Sunday. But because you were unable to irrigate for two days in a row, the landscape will exceed the Maximum Allowed Depletion. And the soil may not be able to absorb all of that water necessary to bring the levels back up, and instead of the field being ready for Monday's practice, the team has a wet, squishy field for practice.

    This is where engaging the MAD Limit Switch feature for your controllers on WeatherTRAK Central is a better solution. In this case, whether the system determines to irrigate every day, or skip a few days, it will never let the soil exceed Target MAD.

    Using the MAD Limit Switch enables your system to be more predictive by telling it to look ahead and irrigate in advance, rather than waiting for the level to actually exceed Target MAD. For example, if your landscape is at field capacity today, which is 0% depletion, your depletion rate is 20% per day, and your Target MAD is set at 50%, then the first day after you irrigated, depletion will be at 20%, so the system will not irrigate. The second day, your landscape would be at 40% depletion, which is still above Target MAD, but the system decides that if the landscape is going to exceed Target MAD tomorrow, it will irrigate today.

    When it comes to the sports field scenario, if the system knows it will be unable to irrigate for two days because of a Friday practice ahead of a Saturday game, it may irrigate to field capacity on Thursday because it knows it can't irrigate again until Sunday. Even if the depletion rate for those two days will put it above Target MAD, it can only irrigate to field capacity, and then fill up the landscape as much as possible again on Sunday.

    The only time the MAD Limit Switch would not be useful is if you're in an area with extreme water rationing. In those cases, you will want to put as much water down when you are able to, and not tune it to depletion, since the site may need to be brought up to field capacity more than being linked to depletion levels.

    Because the MAD Limit Switch adds more of a predictive nature to how your WeatherTRAK system decides to irrigate your site, HydroPoint recommends turning this feature on as a default setting (unless water rationing is an issue in your location). The feature was originally designed for customers with sports turf, but the benefits seem to reach beyond that segment with benefits across a wide range of landscapes.