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Irrigation Audit Application Methodology


The purpose of this application is to simplify irrigation auditing and make it possible for any field worker or homeowner, anywhere in the world, to put a basic, water-conserving schedule into a sprinkler controller and, by so doing, dramatically increase the quantity and quality of water-conserving schedules. The underlying methodology for calculating this basic schedule is based on the principles in the Irrigation Association's handbook, Landscape Irrigation Auditor, 3 rd Edition.

Simplifying Departures

The following are simplifying departures from Landscape Irrigation Auditor which are necessary to either minimize the amount of time or specialized knowledge a field worker or homeowner needs to have, or to make a full-season watering schedule work for a traditional non-wireless controller without requiring frequent changes to the watering schedule:

  1. As a simplifying step, we use basic intake rate as the basis for determining the base runoff case, and then recommend that further adjustments be made based on observed runoff.
  2. In order to accommodate a combination of spray and rotor zones on a single program, we use the highest required number of cycles (i.e., the highest precipitation rate spray zone) as the basis for the recommended number of cycles for all zones, since most traditional controllers require that all stations on a program have the same number of start times.
  3. To ensure that the runtime for each zone can stay constant for every month of the year, we use the "budget" or "seasonal adjustment" feature found on most traditional clocks to accomplish this. We recognize that this may differ from the expected use of this feature, but it makes implementation of the monthly watering schedule very fast, since runtimes do not need to be adjusted for each zone every month.

The end result of the combination of the Landscape Irrigation Auditor methodology and the simplifying approach above is a schedule that produces a difference that is typically within ±5% of gross water applied during any given month, as compared with the results obtained only from the Landscape Irrigation Auditor, 3 rd Edition methodology. The Sample Audit page allows you to enter data for a single zone and compare Irrigation Auditing Applications' results with those from the Irrigation Association's Auditing and Scheduling Calculator.

Reference Evapotranspiration Dataset

Evapotranspiration is the sum loss of water from the soil both by evaporation from the soil surface and by transpiration from the leaves of the plants growing on it. Our reference evapotranspiration data is based on the IWMI ArcGRID Dataset, which uses the Standardized Penman-Monteith Equation to derive ETo (uniform-density cool season grass, 4-6 inches, controlled conditions) values by month. It has been computed for all landmasses of any significant size around the world. While real-time atmospheric inputs into the equation may produce more accurate results than historical inputs, the vast majority of controllers are not weather-data-connected so scheduling must be kept simple and infrequent in order for field workers and homeowners to implement the schedule. Monthly data is more than satisfactory for the purpose of deriving a reasonably accurate schedule.

These historical datasets span a long period of time and the principle of long-term averages (regression to the mean) dictates that adherence to the recommended schedule over a long period of time will lead to essentially equivalent water usage to ETo based on real-time atmospheric data. The primary weakness of historical ETo is that landscape appearance could be affected during periods of unseasonal (above or below average) conditions, requiring user intervention at such times. However, we recommend returning to the base monthly schedule at the beginning of each month in order to maximize water savings over the long term.

Additional Considerations

Complexity has been a primary barrier to implementing large-scale landscape irrigation audits. Various factors contributing to this complexity and the solutions contained in Irrigation Audit Applications are as follows:

  1. The traditional Landscape Irrigation Auditor methodology worksheet is geared toward large-scale field application. Each worksheet only covers a single zone for a single month. The Irrigation Audit Application provides twelve months of scheduling for all zones on a program. The only user interventions required are monthly changes to the interval and budget.
  2. ETo data have been hard to find. The Irrigation Audit Application database is the first one we have found that allows a user to enter a zip code or city name from anywhere in the world and retrieve ETo results for all twelve months of the year.
  3. Landscape factor, soil type and root zone depth are subjective. These three factors require judgment, and different auditors often come up with different inputs for each, even for the same site. Of these three inputs, landscape factor has the greatest impact, since it affects both water usage and landscape appearance, while soil type and root zone depth primarily affect landscape appearance. The Irrigation Audit Application allows the field worker or homeowner to start with a specific landscape factor for a hydrozone and then, based on performance of the zones within that hydrozone over time, easily make adjustments to the landscape factor to achieve acceptable appearance. The default factor for the application is 0.7. For soil type we have identified five broad textural classes, similar to the Auditing and Scheduling Calculator, and recommend that field workers and homeowners not select sandy (coarse) or clay (fine) unless they are confident that the soil in a hydrozone is consistently at one of those extremes, since the field capacity and basic intake rates for those textural classes are markedly different than for the moderate soil textures. Root zone depth is perhaps the most subjective of all, and can have significant impact on runtimes. The conventional wisdom to water deeply and infrequently to encourage deep roots is actually quite conditional. Infrequent watering in the spring will typically encourage deep-rooting, but it is also important to maintain sufficient plant available water in the root zone during the hot, summer months, which requires increasing the irrigation interval. We recommend that field workers and homeowners create their water schedule early in the spring, using the target root zone depth for the particular hydrozone. The default for the application is 6 inches.
  4. Effective Rainfall is difficult to measure without sophisticated devices. In Irrigation Water Needs, FAO highlights the challenges of accounting for effective rainfall. Our recommendation is that only experienced users should adjust the watering schedule to account for effective rainfall. In some cases, it may be aesthetically desirable or even legally required to shut off sprinklers during periods of rainfall; in those cases, we advocate continuing the irrigation cycle at the first available opportunity after the rainfall.
  5. The Irrigation Association prefers scheduling multiplier $\mathsf{ \left( SM = \frac{1}{0.4 + 0.6 \times DU_lq} \right) }$ over distribution uniformity as the basic for determining gross water to apply. The reason for this preference is that scheduling multiplier reduces overwatering in cases of very poor distribution uniformity, and it takes into account the capillary movement of water through soil. The Irrigation Association's Auditing and Scheduling Calculator allows the user to specify a scheduling multiplier between a lower (no multiplier) and upper (full multiplier) bounds. For simplicity, Irrigation Audit Application uses the upper bound and recommends that the user adjust the runtime on a zone-specific basis, if symptoms of overwatering are observed.


For large-scale audits, we believe that Certified Landscape Irrigation Auditors will continue to be essential for the creation of water management plans that optimize landscape appearance and minimize water usage. As a complement to the work of the professionals, this application provides a highly-scalable approach to water management that empowers professionals, field workers, and homeowners alike to create an effective water management plan for every controller.

We maintain the highest respect for the Certified Landscape Irrigation Auditors accreditation. Accordingly, we invite each professional in the industry to read Landscape Irrigation Auditor, take the Landscape Irrigation Auditor class, pass the exam, and stay certified. We wish to extend our appreciation to the Irrigation Association, the IMWI, and the scores of volunteers who have contributed tremendous knowledge and experience to the field of landscape water management.