This Spring, the Self-Deprecating Gardner in conjunction with Liberty Park Press will offer a series of irrigating tools and tips that will hopefully help to improve the health of plants and save money on the Summer watering bill. This tutorial is geared towards the homeowner and the DIY enthusiast.
In our last article we introduced and explored the specifics of creating a soil moisture based irrigation schedule and emphasized the importance of how different soil types and plant species can be assigned a management value that dictates the exact range between watering events. Management Allowable Depletion (MAD) gives the end user the flexibility to assign an exact value reflected by a percentage of moisture that will dissipate from completely saturated soil through plant transpiration and evaporation, before the next irrigation occurs. The decision maker can induce various water levels of stress on plants and turf in order to maximize resources and dictate growth and bloom. MAD refines the Allowable Depletion (AD) variable to specify a desirable window, based on soil profile and root depth, between timed watering events.
As the soil moisture and plant relationship has been established, we can now focus on the environmental aspect of the scheduling paradigm and the loaded concept of Evapotranspiration. In combining the total amount of available water in a specific area, Evaportranspiration or ET, is the net amount of water lost to the transpiration of plants and evaporation due to meteorological factors. These readings are separate from rain that reaches the root zone or effective rain. The basis for the modern ET equation first originated in the late 1940’s by British scientist Howard Penman after studying the effects of solar radiation, wind, humidity and temperature in predicting water loss on pans. Fellow countryman John Monteith soon joined the effort and the Penman-Monteith ET equation was born as the foundation of identifying a specific amount of water loss relative to time. The modern version of the equation or the Modified Penman-Monteith ET formula, is a collaboration between various researchers and scientists in the agriculture and physics community in providing a universal mechanism for understanding and calculating how water, plants and local conditions interact. The Federal Agricultural Organization recognizes the equation as the foundation for identifying water needs for crops and landscapes.
The beauty of ET is the specificity that it provides in showing the amount of potential water that needs to be replaced, based on the quantity of energy flux. The baseline form of ET, ETr, applies to observed and studied water loss over Alfalfa crops. In our tutorial, we will be focusing on the ET of observed and studied water loss over turf grasses, or ETc. ETc is roughly 83% the magnitude of ETr. During the height of Summer, ETc numbers typically range from .10 inches on a cloudy and moist day to .45 inches in extreme climates found in the desert Southwest. Even though clouds can persist, short wave and long range solar energy and wind speed weighted more heavily than temperature, thus the erroneous decision making of choosing to water simply by what is construed as a “postcard” day is eliminated. While the numerous calculations which comprise the formula are daunting, many resources exist online in the form of daily tables and season averages and the subsequent data is built within the hardware infrastructure of numerous modern irrigation control systems. The Modified Penman-Monteith equation also provides water loss readings tailored to time duration, thus specific values can be calculated hourly, daily, weekly and monthly in producing an accurate and efficient schedule. Though the system has accuracy limitations in modeling unstable atmospheric conditions, the margin for error is small and is not a detriment for scheduling purposes.
To illustrate the effectiveness of ETc in applying a magnitude to a landscape, the following example will be used. What is the estimated ETc value for an average July day in Seattle? The Irrigation Water Management Services for the Seattle Region website shows that historically for the month, July averages a total of 4.46in of moisture lost to ETc combined with .17in of rain that actually reaches the root zone. To calculate a daily average, simply subtract the effective rain amount of .17in from the ET of 4.46 to get 4.29 and then divide the number by 31 (days in July) to get an ETc of .14in per day on average. This is the raw foundation for building an irrigation schedule based on meteorological factors. While the reading may seem incomplete and slightly ambiguous, thankfully there are tested methods to provide clarity and substantiation in applying the number to the realm of scheduling.
The Landscape Factor or KL allows the decision maker to customize the ET reading according to specifics in terrain, plant species and density of competing root zones. This prevents a lone and incomplete ETc value in compromising scheduling accuracy. The simple equation of KL= Ks (species factor) x Kmc (microclimate factor) x Kd (density factor) is utilized by industry professionals in tailoring a heightened resolution of ETc. Each factor in the the equation is based on a management decision between .2 and 1.4 and can interpreted as a percentage. The practice was first developed for application in the harsh microclimates of California, where turf and ornamentals are prone to high than average water loss magnitudes. The beauty of KL is that the equation can be applied on hyper local level, globally. Using the Seattle example from above, a KL will be determined to specify the .14in ET based on the three KL factors in a simulated watering area.
We will make our contrived landscape an expanse of cool weather turf grass bordering a roadway which receives full sunlight (a very real possibility in the typical Seattle summer). The following logic can be applied to achieve a workable KL- as a species factor, the cool weather turf garners a .8, as a result of the full sunlight conditions adjacent to pavement the microclimate is 1.2, finally as the root competition of turf is moderate, the density factor is 1.0. Thus, 8 (Ks) x 1.2 (Kmc) x 1 (Kd) = .96 KL. The KL is then multiplied by the average daily ETc reading from above to achieve a landscape specific variable- .96 KL x .14in ET = .13in.
The ultimate reflection of our created yard at .13in is then labeled as Plant Available Water or PWR. The PWR value highlights the specificity of the ETc reading interacting with the unique nuances of a landscape. This concept is extremely important in numerically defining the reality that small areas of land can possess extremely diverse watering requirements based on the exposure of sunlight, the susceptibility certain plant species to low moisture and the overall competition of roots for water rights. PWR illustrates effectively the amount of water that will be lost to ETc over a period of time and the overall replacement value needed to sustain a healthy and efficient landscape. The variable can be combined with AD in planning a reasonable and effective range of irrigation events over the course of a watering season.
In our next tutorial, we will explore the powerful collaboration of PWR and AD and introduce a battery of sprinkler performance testing equations to establish specific ranges between watering events and the amount of time and volume of resources required to effectively and efficiently irrigate.