An Overview of Woody Plants in ASABE Standard S623: Determining Urban Landscape Plant Water Demand

Water use in the urban landscape is scrutinized and prescribed by many organizations and jurisdictions that create programs, codes or standards as rules for sustainable development. Improved landscape water use efficiency is a key element in these programs and codes. Efficiency is usually assumed to consist of restricting water application rate or limiting plant palette. The new S623 standard provides a simple and science-based method to estimate the amount of water landscapes plants require to meet expectations. It can be applied both during landscape design for calculating water budgets and for landscape water management and irrigation scheduling strategies. Here we focus on the complex context for the tree component of the standard. Trees in urban landscapes are often free standing. spatially separate, or above surrounding plant material. Water use by freestanding trees and woody shrubs has been quantified for a number of species and can be reasonably predicted based on trunk dimensions or canopy footprints when normalized to reference evapotranspiration (ETo). The ratio of actual water use to ETo is defined as the plant factor (PF). Freestanding tree canopies are well-ventilated with air water deficits (humidity) typically being imposed at the individual leaf surface. High air water deficits often trigger some degree of stomatal closure that decouples transpiration from ETo and moderates rate of water use. In humid climates with small air water deficits, stomatal closure is typically less intense and prolonged, so water use rate as a fraction of ETo is greater. Consequently, the S623 standard defines PF’s separately for trees in low humidity (PF=0.5) and high humidity (PF=0.7) climates, with a separate category for desert plants (PF=0.3) whose water use is even further decoupled from ETo. These PF’s result in acceptable aesthetic appearance and function when applied to irrigated landscapes. A key assumption is that complex, site-specific landscape factors—mixed species, variable plants densities, and diverse microclimates—offset each other such that water use in a given landscape zone is homeostatic and can be characterized by the PF for the most water demanding plant type within that zone. The Standard provides the first reliable, research-based set of PF’s for landscape plants that can be applied nationally. In addition, it is simple to understand and apply, readily accommodates new plant introductions, and is scientifically and conceptually defensible.  Adoption of the standard will require extensive marketing to landscape architects, professional landscape water managers and non-technical public policy-makers.