Simplified landscape irrigation demand estimation: Slide rules

Irrigated urban landscapes must increasingly maintain economic and ecosystem value with less water in response to drought amplified and shifted by climate change. Efficient landscape water management requires estimating water amount demanded by plants that can be replaced by irrigation to meet minimum performance expectations. The extant approach to estimating landscape water demand is conceptually muddled and often regionally inappropriate. Simplified Landscape Irrigation Demand Estimation (SLIDE) Rules distills scientifically credible assumptions about urban landscape biological and physical complexity into guidelines for estimating water demand that are conceptually accessible and operationally useful. SLIDE Rules are: 1) oasis urban reference evapotranspiration (ETo) effectively represents water use of urban turf seasonally and for day-to-day irrigation scheduling, but is less accurate for estimating water use of non-turf surfaces, especially in dry climates; 2) a discrete number of Plant Factors (PF) adjust ETo to estimate water demand of general landscape plant type categories-turf, non-turf, and desert-that are adjusted for temperature and drought responses; 3) a hydrozone controlled by one irrigation valve is the smallest landscape unit manageable for water, thus overall zone irrigation is governed by the highest water demand plant within that hydrozone; 4) for hydrozones <80% plant cover, water demand can be estimated as volume of water based of individual plants using planar leaf area expressed as projected canopy area. SLIDE is a framework for designing and regulating water-efficient urban landscapes based on selection of appropriate PF's in combination with lower density planting and hardscape. SLIDE is aimed at stakeholders in urban landscapes who primarily design and regulate, but also manage, urban landscapes to use less water.