Evapotranspiration (ET)

Overview

Most of the precipitation that falls on the Colorado River Basin returns to the atmosphere via evapotranspiration (ET), a term which encompasses evaporation of moisture from soils and open water, transpiration from plants and crops, and sublimation from the snowpack. Since ET is such a large component of the water budget from the field scale to the basin scale, ET can strongly influence the moisture status (dryness/wetness) of the land surface and ultimately, the volume of runoff.

Strictly speaking, ET refers to the actual loss of water (depth per unit time) from the land surface. ET is driven by the atmosphere's evaporative demand (E₀)--its "thirst" for surface moisture--but the amount of ET (water loss) is necessarily limited by the available water at the surface. So cumulative ET over an extended period (e.g., one year) cannot exceed precipitation (plus irrigation water, in the case of irrigated cropland).

Evaporative demand (E₀) is not limited by available water at the surface, and is often greater than than precipitation (+ irrigation) and ET. Evaporative demand is functionally equivalent to Potential ET (PET). Reference ET (ET₀) is closely related to E₀ and PET; it is an estimate of the upper bound of ET losses (i.e., consumptive water use) from irrigated cropland. In principle, with a fully watered crop, Reference ET would be the same as (actual) ET, but in practice, Reference ET is often greater.

Reference ET and PET can be estimated using an equation that inputs meteorological variables (ideally: temperature, solar radiation, humidity, winds), or derived from remote sensing data using a land-surface model. Direct in-situ measurements of PET and Reference ET (e.g., pan evaporation) are very sparse across the CRB. Actual ET is more challenging to quantify. ET can be estimated using a land-surface (hydrology) model with meteorological inputs, or by assimilating satellite observations of land-surface temperature into an energy-balance model. More direct measurements of ET can be made using Eddy Covariance methods, which requires multiple instrumentation packages arranged vertically on a single tower.

ET and related variables have large seasonal variability relative to their annual variability, with a maximum in summer and minimum in winter. ET and related variables are subject to long-term trends driven by warming temperatures.

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Relevance

In the Colorado River Basin, estimates of ET and evaporative demand (Reference ET, PET) are used in reservoir operations, irrigation scheduling, and demand and consumptive use modeling. Estimates of watershed-scale ET are also used to validate the water budget simulations in hydrologic models, such as those used by the CBRFC for streamflow forecasting. Estimates of monthly reservoir evaporation and consumptive use by agriculture are also important terms in the Reclamation operations and planning models, and in Reclamation's calculations of natural flows.

Data and tools

Reference ET and PET

Several weather-station networks serving the agricultural sector in one or more basin states record all of the variables (temperature, solar radiation, humidity, winds) needed for robust, real-time estimates of Reference ET. Typically, Reference ET is calculated for each day, to the nearest 0.01".

CoAgMET

CoAgMET has real-time data for 90 stations in agricultural areas across Colorado.

Actual ET

OpenET

OpenET is an open-source platform and data viewing tool that brings together several satellite-based ET estimation methods to provide daily, monthly, and annual ET estimates at the field scale. Free registration required to view data.

Additional resources

State of the Science Report

Chapter 5 of the State of the Science report, Section 5.5 describes evaporation, ET, and evaporative demand, and sources of data, in greater detail.