If you spend enough time around water professionals, you'll eventually hear someone toss around the term "evapotranspiration" (often shortened to "ET") and nod along as if it's obvious. It's one of those words that sounds technical but describes something everyone has witnessed: a wet lawn drying out on a sunny afternoon, or a forest that seems to breathe moisture into the air. For drinking water utilities specifically, ET is far more than a backyard curiosity — it shapes how much raw water is available to treat in the first place, and increasingly, it's being explored as a tool for dealing with some of the sector's toughest contaminants.

The Basic Definition

Evapotranspiration is the combined process by which water moves from the Earth's surface into the atmosphere. It's a portmanteau of two separate physical processes:

• Evaporation — water turning from liquid to vapor directly from soil, pavement, and open water bodies.
• Transpiration — water absorbed by plant roots and released as vapor through small pores in the leaves called stomata.

Because these two processes happen simultaneously and are difficult to measure separately in the field, hydrologists and water managers typically treat them as one combined value: ET.

Why It Matters to Drinking Water Professionals

ET is a major player in the water cycle, and in many watersheds it accounts for a larger share of water loss than runoff or infiltration combined. For utilities responsible for maintaining a reliable drinking water supply, that makes it a critical planning variable, not just a hydrology footnote.

It directly affects source water availability. Utilities and water resource managers use ET rates to estimate how much water a reservoir or watershed will lose to the atmosphere rather than deliver downstream to a treatment plant. In arid and semi-arid regions especially, ET can rival or exceed precipitation, directly shaping how much raw water is actually available to treat and distribute to customers.

It is an emerging tool against PFAS and other persistent contaminants. Traditional drinking water treatment for PFAS (per and polyfluoroalkyl substances) focuses on removing them from water via granular activated carbon, ion exchange, or reverse osmosis. ET-based approaches take a different angle: engineered wetlands and other vegetation-driven systems use transpiration to help concentrate, immobilize, or move contaminants out of the water cycle entirely, rather than simply relocating them from one waste stream to another. For utilities under growing pressure to address PFAS at the source, that distinction matters.

A One Water, whole-cycle perspective. Because ET moves water — and, potentially, what's dissolved in it — out of surface water and groundwater systems altogether, it's a useful lens for utilities thinking beyond the treatment plant fence line. A One Water approach treats drinking water, wastewater, and stormwater as one interconnected system rather than three separate problems, and ET touches all three, making it a natural entry point for utilities exploring source water protection.

Drought monitoring and long-term supply planning. As climate patterns shift, ET rates are changing too: generally trending upward with rising temperatures. That has real implications for how much water a utility can count on in the coming decades, and it's an increasingly common data point in long-term water supply and climate resilience planning.

How It's Measured

ET is notoriously tricky to measure directly, so professionals typically rely on a combination of methods:

• Pan evaporation — measuring water loss from a standardized open pan as a proxy.
• Lysimeters — sealed containers of soil and vegetation that measure water balance precisely, often used as a research-grade benchmark.
• Remote sensing — satellite data increasingly used to estimate ET across large landscapes.
• Modeled estimates — equations like the Penman-Monteith formula, which combine weather data (temperature, humidity, wind speed, solar radiation) to calculate ET without direct field measurement. This is the most widely used approach in the U.S.

A Term Worth Knowing

Evapotranspiration might sound like a mouthful reserved for hydrology textbooks, but it's a quiet force behind decisions drinking water utilities make every day — from how much raw water a system can count on, to how source watersheds are protected, to how emerging technologies are tackling persistent contaminants like PFAS. As more utilities look for treatment approaches that address contamination at its source rather than after the fact, understanding ET is a useful starting point for following where that conversation is headed next.

This piece was created with the help of generative AI tools and edited by our content team for clarity and accuracy.