This Spring and Summer, 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.
Over the past three columns, we have introduced and explored the basis of modeling a landscape and determining watering requirements through detailed observations and effective calculations, with the goal in building a moisture-based irrigation schedule which saves resources and finances. It is now time to combine the processes and formulas and create a customized watering regiment, that is able to accommodate the various nuances of the existence of microclimates in a single landscape. To complete this project thoroughly and accurately, we will use the laboratory of a hypothetical property in the residential suburbs of somewhere, USA, in illustrating the correct process of identifying water needs and game planning the proper method of delivering artificial precipitation. And now we enter the confines of the lab.
Pigville, USA, is known for residential expanses of suburbs bordering greenbelts and heavy forests. The town enjoys a moderate climate, with hot and dry summers and cool and wet winters. With the proximity to the ocean, the threat of freezing weather is moderated by the maritime influence. A hilly terrain dominates the region as a towering mountain range runs North to South, roughly 60 miles to the East. This means that the threat of potential run-off for the average landscape is very real.
Our property is a quarter acre of western facing property with a slight grade flowing east to west. The landscape consists of well-groomed beds of native shrubs surrounded by turf. The property is bordered by a fence to the North, a residential roadway to the East, a Laurel hedge to the South and a group of tall pines to the West. We will focus on the turf between the roadway and the house to the East, with an area of roughly 1300 square feet. Currently, the homeowner runs employs 5 gear driven pop-up sprinklers at 20 minutes per day during the summer to irrigate the almost triangular shaped piece of lawn. Based on the average cost per cubic foot set by the local utility, the bill for irrigating the microclimate costs nearly $35.00 for the month of July, and the amount is compromised by the fact that run-off occurs after four minutes of runtime. There are no restrictions from the municipality on watering days. Expenses and resources can be saved by applying our new moisture-based schedule expertise to the problem area.
The initial step we need to take is to acquire a soil sample in determining Available Water (AW) and measure the Root Zone (RZ) of the turf. This will allow for the foundation of building an accurate irrigation schedule, that is entirely separate from meteorological factors and isolates the performance of the soil effectively delivering water through infiltration spurred on by gravity, and percolation leading to capillary action. There is complete write-up and tutorial in regards to the specifics of AW and RZ here. It is determined through observation that the soil is a sandy-loam mix and has a corresponding AW value of .12in/in. The RZ is measured at 5 inches.
Next, we need to decide how high of maintenance the turf is in dictating the amount of moisture that will be allowed to diminish between irrigation events. Management Allowable Depletion (MAD) is a tool that gives the end-user the latitude in allocating a specific level of stress on a plant, with the benefits of less water used and in some cases inducing growth and bloom. As the front turf is subject to a heavy level of foot and pet traffic (pigs and dogs), we will substitute heartiness for appearance in choosing a higher MAD value of 60%. You can read a fill description of employing MAD here. By combining the AW, RZ, and MAD, we are able to determine Allowable Depletion (AD) in specifying a variable and illustrating an accurate survey of how soil interacts with water and roots. Thus, .12 AW x 5 RZ x 60% MAD yields an AD of .36in. Now we can employ the climate factors and landscape adjustments to determine a base range between irrigation events.
Water lost to plant transpiration and evaporation is depicted by the powerful equation of Evapotranspiration (ETo). Reference ETo combines, solar radiation, elevation and weather factors to measure net water loss in an area and can be adjusted for time periods as specific as an hour. One of the greatest attributes of the internet, is the access to research databases and ETo readings are available for most cities and towns. In our manufactured climate, the average daily July ETo loss is .21in or over 6.5in for the month. To account for generalizations in data, specific landscape conditions can be introduced to enhance the resolution of ETo for microclimates. The Landscape Coefficient (KL), provides the homeowner with an interface with ETo and takes into account plant species (Ks), microclimate (Kmc), and plant density (Kd), which heightens accuracy and determines Plant Water Requirement (PWR). As the lawn has a low tolerance for water loss and is not in any competition with other plants for water, the key is adjusting the volatile and unrelenting realities of the Kmc. With the proximity to pavement and full sunlight, we will assign a 1.4 magnitude to the Kmc or 140%. The Ks receives a .9, while the Kd will be denoted with a .6. Thus, the PWR value for the turf on an average July day is .16 in, which varies significantly from the .21in value taken from historical trends.
Finally, with water loss values established from plant, soil and meteorologic specifics, we have our basis to start crunching numbers for the inception of a soil moisture schedule construct. Utilizing AD and PWR, the Maximum Irrigation Interval (IRmax) between irrigation events can be determined. The simple calculation of .36in AD divided by the .16 PWR gives an IRmax of 2.3 days between irrigation events, rounded down to 2 days. July’s historical climate dictates that 15 watering events will occur for the month, and the specific reference to frequency provides the foundation of implementing smart irrigation practices.
There is still a lot of work to do in creating the most efficient irrigation schedule in adhering to the specific needs of the turf microclimate and the effectively measuring the performance of the irrigation system through evaluations of uniformity and efficiency will lead to most accurate and efficient watering plan. Basically, this encapsulates scoring the individual sprinkler on hitting the target of the plant, and gives both a high resolution snapshot and broad overview of the interaction between the system and the landscape. The only real downfall to moisture based scheduling practices is the existence of a poorly planned and substandard irrigation infrastructure. Innovation is no match for incompetency.
Available Water (AW)
Root Zone (RZ)
Plant Available Water (PAW)- AW x RZ = PAW
Management Allowable Depletion (MAD)
Allowable Depletion (AD)- PAW x MAD= AD
Evapotranspiration (ET) ETr x .83 = ETo
Landscape Coefficient (KL)- Ks x Kmc x Kd = KL
Plant Water Requirement (PWR) Eto x KL = PWR
Maximum Irrigation Interval (IRmax)- ETo X PWR= IRmax
Visit the Irrigation Association here.