Evapotranspiration (ET)

Overview

Most of the precipitation that falls on the Colorado River Basin returns to the atmosphere via evapotranspiration (ET), which includes evaporation of moisture from soils and open water, transpiration from plants and crops, and usually, sublimation from the snowpack. Since ET and related variables are such a large component of the water budget, from the field scale to the basin scale, they help determine the moisture status (dryness/wetness) of the land surface and the volume of runoff.

ET and related variables have large seasonal variability, reaching a maximum in summer and minimum in winter, relative to their annual variability, which is smaller than that of precipitation. ET and related variables are subject to long-term trends driven by increasing temperature.

The terminology of ET-related variables can be confusing. To start with, we need to distinguish two dimensions of ET:

• AET (Actual ET) is the real loss of water (depth/unit time) from the land surface, limited by available water; AET cannot exceed precipitation (+ irrigation, for cropland). Often AET is just referred to simply "ET."
• PET (Potential ET) is the loss of water that would occur if water were unlimited; PET can and often does exceed precipitation + irrigation

PET, then, is a measure of the "evaporative demand"--the atmosphere's thirst for surface moisture. Other variables, such as Reference ET and E0 ("E-naught"), also measure the evaporative demand and are functionally equivalent to PET, though calculation methods may differ.

ET-related variables are typically estimated using a model that uses meteorological observations (temperature, solar radiation, humidity, winds), or they are derived from remote sensing data using a land-surface model. Direct in-situ observations of these variables (e.g., pan evaporation) are very sparse and while they do not offer an adequate spatial representation at the watershed or basin scale, they can be useful for validating other estimates.

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Figure 1. CAPTION. SOURCE.)

Relevance

In the Colorado River Basin, estimates of open-water evaporation, ET, and PET/evaporative demand 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 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.

Additional resources

State of the Science Report

Chapter # of the State of the Science report, describes...