Estimating Water Needs of Urban Landscapes

Many water agencies and jurisdictions in urban areas of the western U.S. are using climate-based reference evapotranspiration (ETo) data to determine landscape water budgets and water prices.  Nationally, the WaterSense program sponsored by the U.S. Environmental Protection Agency promotes ETo-based landscape water budgets as a means for certifying new homes as water efficient.  Consequently, landscape architects, landscape management professionals, and the tools they use, such as software and smart irrigation control devices, are incorporating this approach to derive irrigation schedules.  Formulae and algorithms used to produce these ETo-based water budgets and irrigation schedules require multiplying local ETo estimates by an adjustment factor, known as a crop coefficient (Kc) or a plant factor (PF), that accounts for each particular plant’s water need as a function of local climate.  These numbers are usually expressed as a decimal less than 1.0 or a percentage less than 100%.  However, reliable research-based data on landscape plant water needs in terms of Kc or PF values is extremely limited.  Few information sources offer any form of quantitative estimates of landscape plants’ water requirements.  Most of those that do, including the widely-referenced publication, “Water Use Classification of Landscape Species” (WUCOLS), are not based on scientific data.  Using the PF or Kc values available from these sources gives users a false sense of precision in the water budgets and schedules they calculate or that their smart controller develops.  Fortunately, there have been some scientific field studies completed over the past several years that provide ETo-based water need estimates or ranges for selected groups of landscape plants and afford effective means to evaluate the reliability of WUCOLS and similar information sources.  The newer field research data on landscape plant water needs, how it relates to WUCOLS values, and how it can be used to calculate accurate and meaningful irrigation budgets and schedules will be presented.