Typically a landscape water budget is calculated using Area, Plant Type, Plant Density, Micro-Climate, Irrigation Performance and Historical ET° (reference evapotranspiration).
ET°, simply described, is “inches of water needed to replace water transpired by plants and evaporated from the soil surface surrounding plants”. Next, ET° is modified by the plant type (Kc) or crop coefficient. Water use varies for varying plant types. After adjusting ET° for plant type, adjustments are also made for plant density, micro-climate and irrigation performance. After these adjustments we use this “factor” or adjustment number to calculate water budgets. Yearly ET° is averaged over many years for your location. WatrWise Consulting does not factor rainfall into our water budgets (for Arizona). Rainfall (what little there is) is not consistent and mostly runs off and not available to plantings, especially monsoon rains that deliver water too fast to soak in. Winter rains tend to deliver water slower over an extended period of time and are able to soak into the soil. WatrWise Consulting recommends using these soaking winter rains to help leach salt accumulation in the soil by increasing the depth of watering of the irrigation system. See the post “Salinity Effect on Plant Growth”.
Beware, landscape water budgets may be based on design, management, conservation or sales goals. Possibly leading to deficit watered landscapes.
WatrWise Consulting recommends that you carefully check to be sure that your landscape water budget is a comprehensive landscape water budget that accurately estimates your property’s landscape water requirements by using all of the necessary adjustments. This will ensure that your property is watered correctly.
Landscape water budgets are often manipulated in order to show a return on investment (ROI) by lowering water requirements for the property, in order to reduce water delivered to the landscape (water billing), leading to deficit watered landscapes; ultimately endangering the health of the landscape. This is often done to entice a property owner to buy unneeded irrigation improvements for the property.
“A large tree which has not received supplemental irrigation for many years should be able to withstand the effects of an unusually dry year better than a tree whose regular pattern of irrigation is disrupted. In the latter case, the ability of the tree to take up water is balanced with historically high soil moisture availability. When that availability is reduced, the tree must restore a functional balance between growth pattern and site conditions. As with any change to mature trees, this may take several years. If severe water stress conditions develop as a result of a reduction in site soil moisture, trees may die or lose vigor. A drought situation can be disastrous for a tree which has received irrigation and fertilizers for many years prior to the water stress.” Quoted from – WATER AS A LIMITING FACTOR IN THE DEVELOPMENT OF URBAN TREES – James R. Clark and Roger Kjelgren.
Comprehensive landscape water budgets have the following adjustments –
- ET° (reference evapotranspiration)
- Micro Climate
- Plant factor (type of plant)
- Irrigation Performance (efficiency) sometimes called DU for turf or EU for drip (DU is not measured for landscape drip systems). Sprinkler/spray irrigation systems are typically maintained between 50% and 60% efficiency. Landscape drip systems are seldom tested for efficiency.
- Water Quality or Leaching Fraction (salinity)
- Lost water (including over spray, deep percolation, runoff and evaporation) in addition to efficiency.
Example Budget –
WatrWise Consulting has added three more adjustment factors to our Landscape Water Budgets that are becoming increasingly important and often overlooked; Leaching Fraction/Water Quality, Lost Water and Management. WatrWise Consulting has added these adjustments to better represent landscape water requirements as water quality continues to decline, management either declines or improves and to adjust for lost water. Management relates to the “human factor” of how well irrigation scheduling is performed over time (seasonal/weather adjustments).
Irrigation audits are typically done on high performing irrigation systems – omitting poorly performing irrigation systems – raising the overall performance average of sprinkler systems. Lowering landscape water requirements/landscape water budgets; possibly leading to deficit watered landscapes.
A White Paper by Tim Wilson CSWP, CID, CIC, CLIA, CGIA, CTT+
Paul Brown – Exension Specialist, Biometerorolgy
The University of Arizona , College of Agriculture and Life Sciences – Tucson, Arizona
Kelly Turner and Dorothy C. Ibes
School of Geographical Sciences and Urban Planning Arizona State University
James R. Clark and Roger Kjelgren.