What it measures
A tree pulls water from the soil through its roots and releases most of it as vapor through its leaves. Evaporation from the soil surface adds to that. Heat, wind, sun and dry air all raise the rate, which is why a tree needs more water on a hot, windy afternoon than on a cool, still one.
ET is usually given in inches of water per day or per month, and the figure for a region comes from weather stations.
Why it matters in Northern Nevada
Our valleys get little rain, a lot of sun and steady wind, so evaporative demand in summer is high compared with the rain that falls. The NOAA climate normals show how little precipitation the valley stations record (see climate normals by station). A tree that goes into summer with dry roots scorches (see leaf scorch) and can dieback. See watering mature trees in the high desert.
Winter matters too, because evergreens keep losing water when the ground is frozen.
What an arborist does about it
Arborists use the idea to read a stressed tree: is it losing water faster than it can draw it? Signs include scorched edges, early fall color, thin crowns and dead tips. If the tree is declining we tell you whether pruning dead wood or removal is the answer. We do not sell irrigation systems; the Cooperative Extension guide covers landscape watering. See drought stress and dieback.
In plain terms, a tree on a windy west-facing slope in July loses far more water than the same tree in a sheltered yard in April. Deep, infrequent watering for the whole root zone is the idea the Extension guide builds on.
Sources
- University of Nevada Cooperative Extension: Gardening Guide for High-Desert Urban Landscapes (read October 17, 2026)
- NOAA NCEI U.S. Climate Normals 1991-2020 (read October 17, 2026)