Which important factors must be considered when calculating the design landscape water requirements?

The calculation of design landscape water requirements, particularly as prescribed by the LEED for Homes v4 rating system, is based on the EPA WaterSense Water Budget Tool. This tool's core formula relies on three primary factors: reference evapotranspiration (ETo), which is determined by the local microclimate; the landscape coefficient (KL), which is a function of the water needs of the chosen vegetation selection (species factor); and the overall irrigation efficiency (IE), which is determined by the irrigation type (e.g., drip, spray). These variables directly quantify the amount of water a landscape needs and how efficiently it will be delivered, forming the basis of the water budget calculation.

A. These are components of an irrigation system or management strategy, not the primary variables used to calculate the landscape's inherent water demand.
C. These factors are primarily associated with stormwater management and erosion control (Sustainable Sites credits), not the calculation of plant irrigation needs.
D. These are related to soil health and stormwater runoff management, not the direct inputs for a landscape water budget calculation as defined by LEED.

1. U.S. Green Building Council. (2016). LEED v4 for Homes and Midrise. Washington, DC: U.S. Green Building Council.

Page 122, WE Credit 1: Outdoor Water Use Reduction, Step-by-Step Guidance: "Calculate the landscape water requirement (LWR) using the EPA WaterSense Water Budget Tool. The tool calculates an annual water budget for the property based on the landscaped area, local climate data, and the water needs of the plant species." This directly links the calculation to climate (microclimate) and plant species (vegetation selection).

2. U.S. Environmental Protection Agency. (2014). WaterSense Water Budget Tool Data Sheet.

Section: "Water Budget Approach," Page 1: The document outlines the formula used: LWR = (ETo – Peffective) (LA KL) / IE. The key inputs are explicitly defined as ETo (Reference Evapotranspiration, a function of microclimate), KL (Landscape Coefficient, a function of vegetation selection), and IE (Irrigation Efficiency, a function of irrigation type).

3. Costello, L. R., & Jones, K. S. (2014). WUCOLS IV: Water Use Classification of Landscape Species. University of California, Division of Agriculture and Natural Resources.

Chapter 2, "Estimating Water Needs of Landscape Plantings," Page 7: This academic guide, foundational to water-efficient landscaping in the U.S., explains the formula ETL = ETo x KL, where ETL is the landscape evapotranspiration. It states, "The landscape coefficient (KL) is a composite value that incorporates the water needs of the plant species (species factor), the density of the planting..., and the environmental conditions of the site (microclimate factor)." This confirms the academic basis for using these specific factors.