Evapotranspiration
The combined process of water evaporation from soil and plant surfaces, and transpiration through plant stomata. Estimated from satellite thermal and vegetation data, it is a critical variable for water resource management, irrigation scheduling, and drought monitoring.
Overview
Evapotranspiration (ET) is the combined process by which water moves from the land surface to the atmosphere through evaporation from soil, water bodies, and wet surfaces, and transpiration through plant stomata. ET is the second-largest component of the water cycle over land (after precipitation) and in many regions exceeds runoff, making it critical for water resource management, irrigation planning, and drought monitoring. Global terrestrial ET is estimated at approximately 65,000 km³/year — over 60% of precipitation falling on land returns to the atmosphere through ET.
How It Works
Satellite-based ET estimation uses the surface energy balance: available energy (net radiation minus soil heat flux) is partitioned into sensible heat (warming the air) and latent heat (evaporating water). ET equals the latent heat flux divided by the latent heat of vaporization. Algorithms like SEBAL (Surface Energy Balance Algorithm for Land), METRIC, and SSEBop use satellite-derived land surface temperature and vegetation indices to estimate this partitioning.
Cooler surfaces (relative to surrounding dry areas) are transpiring more water, while hot surfaces indicate water stress. The OpenET platform operationally produces field-scale ET maps across the western US using an ensemble of six models applied to Landsat and Sentinel-2 thermal and optical data.
Key Facts
- ET accounts for ~60% of precipitation over land — the dominant water loss pathway in most regions.
- Units: typically mm/day or mm/year (1 mm = 1 liter per square meter).
- SEBAL, METRIC, and SSEBop are the most widely used satellite-based ET models.
- OpenET provides field-scale ET for the western US using Landsat and Sentinel-2 data.
- Thermal infrared bands (land surface temperature) are the key satellite input for energy balance ET models.
- Global terrestrial ET is approximately 65,000 km³/year.
Applications
Irrigation Scheduling
ET maps show which fields are water-stressed and which are adequately irrigated, enabling precision water application that reduces waste and improves yields.
Drought Monitoring
ET anomalies — departures from normal — reveal emerging drought conditions before visible vegetation damage occurs.
Water Rights and Allocation
Satellite ET provides objective, spatially explicit water consumption data for adjudicating water rights disputes and managing basin-level water budgets.
Ecosystem Water Use
Quantifying how much water forests, wetlands, and natural ecosystems consume supports environmental flow requirements and conservation planning.
Limitations & Considerations
Satellite ET models require cloud-free thermal imagery, which limits temporal frequency — Landsat thermal bands have 16-day revisit at 100 m resolution. ET is modeled, not directly measured — different algorithms can disagree by 20–30% for the same field. Validation relies on expensive eddy covariance flux towers or lysimeters with limited spatial representation. Instantaneous satellite observations must be extrapolated to daily ET using assumptions about diurnal patterns. Sub-field ET variability may not be captured at Landsat resolution.
History & Background
Penman's (1948) equation first described ET from meteorological variables. The FAO Penman-Monteith equation (Allen et al., 1998) became the standard reference method. Bastiaanssen developed SEBAL in 1998 as the first operational satellite-based ET model. Allen adapted it into METRIC for western US irrigation districts. The USGS SSEBop model simplified the approach for continental-scale applications. OpenET (launched 2021) democratized access to satellite ET by ensembling six models into a freely accessible platform.
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