The impact of urban tree shade on residential irrigation demand in a semi-arid Western U.S. City

Abstract

This research examines how shade offsets residential landscape irrigation demand in a semi-arid city in the western United States. Using over 40,000 customer records from Denver Water, the relationship between residential irrigation and quantities of turf, tree canopy, and shade was studied, controlling for other factors related to the vegetation, parcel, and residence. LiDAR and imagery were used to model turf and tree canopy area and daily hours of shade falling on turf. Simultaneous Autoregression determined that each additional hour of daily building and tree shade falling on turf was associated with 18,672 and 11,683 fewer liters of irrigation per growing season, respectively, for the average parcel (turf area = 476 m2), equating to reductions of 39.2 and 24.5 liters per m2 per growing season for a single hour of shade. These numbers, incorporating spatial spillovers, suggest that for landscapes with both trees and turf—the dominant pattern in many Western US cities— mature trees could potentially be arranged to produce enough shade to reduce water demand of underlying turf by an amount equal to, or even greater than, the trees' irrigation demand. This benefit was not found for shorter and younger trees, which have higher irrigation rates per unit area.