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 around ET-related variables can be confusing and there are several "synonyms"--terms which are identical in practice. 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; it cannot exceed precipitation (+ irrigation, for cropland)
• PET (Potential ET) is the loss of water that would occur if the water supply were unlimited; it can exceed precipitation + irrigation

PET is also known as the "evaporative demand"--the atmosphere's thirst for surface moisture. Other variables capture evaporative demand and are equivalent to PET including Reference ET and E0 ("E-naught").

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

To support a variety of water resource management decisions, estimates of open-water evaporation, evapotranspiration (ET), and evaporative demand are required at varying timescales: daily (reservoir operations), weekly (irrigation scheduling), and seasonally (demand and consumptive use estimation) (Hobbins and Huntington 2017). Estimates of watershed-scale evapotranspiration are also used to validate the simulated water budget in hydrologic models, including that 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 their flow naturalization calculations.

Additional resources

State of the Science Report

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