Evapotranspiration

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Responsible:Niccolò Calandri

Layer Evapotranspiration on real sites

Real Estate UK

Evapotranspiration

Catania

Evapotranspiration

Mina el Cerrejon

Evapotranspiration

Overview

Evapotranspiration (ET) measures the combined rate of water loss from the land surface to the atmosphere through two processes: evaporation from bare soil and water surfaces, and transpiration from plant stomata. The result is expressed in millimetres per day (mm/day) and provides a direct indicator of how much water an ecosystem is moving from the land into the atmosphere.

ET is fundamental to the hydrological cycle. It links the water cycle to the energy balance and vegetation dynamics, influencing water resource management, climate regulation, and crop growth. High ET values indicate active, productive vegetation or high soil moisture conditions (wetlands, irrigated cropland), while low ET values can indicate either dormant/arid conditions or well-managed dry ecosystems.

The indicator is computed for both the monitored site (ROI) and a reference control area (CA), enabling comparison of local conditions against the surrounding landscape.

Important: Two separate "evapotranspiration" concepts exist on the platform. This KPI uses MODIS satellite data. A separate parameter uses Open-Meteo FAO reference ET potential and appears only within the Historical Climate and Future Climate trend charts — it is a completely different data source and product.

Calculation Methodology

ET is derived from the MODIS MOD16A2 (Terra satellite) and MYD16A2 (Aqua satellite) products, both available on Google Earth Engine at native 500 m resolution. These products implement the Penman-Monteith algorithm internally, combining meteorological data, land cover classification, and stomatal resistance estimates.

\text{ET (mm/day)} = \frac{\text{mean(pixel-wise composite} \times 0.1)}{8}

where 0.1 = official MODIS ET scaling factor, 8 = number of days in the MODIS cumulative period

The platform reports the spatial mean of the resulting daily ET composite across the site polygon (ROI) and the control area polygon (CA). Data are sourced from MODIS Terra and Aqua collections at 500 m native resolution (upsampled to 250 m for display).

What You See on the Platform

Assessment > Microclimate > "Evapotranspiration" (Gauge)

Gauge. A semicircular arc gauge displaying both the site value (ROI) and the control area value (CA) positioned along a five-segment quality arc.

Purpose: How much water is the site ecosystem losing to the atmosphere each day, compared to the surrounding control area?

Description: The gauge displays the ROI ET value as a marker on the arc and the CA value as a comparison label below the centre. A delta badge shows the difference (ROI minus CA) — a negative delta means the site has lower ET than the control area, suggesting less vegetation activity or drier conditions. A tooltip icon shows the full ET definition.


3.2mm/dayEvapotranspiration
010

How it's calculated: The displayed value is the spatial mean of the pixel-wise mean MODIS ET composite (converted to daily mm/day) over the site polygon for the selected period. The CA value is computed identically over the control area polygon.

Note: This indicator is inverted — lower values are not necessarily better. Interpretation depends strongly on ecosystem context. In arid zones, low ET may indicate water-stressed or degraded vegetation. In wetlands or forests, high ET is expected and ecologically healthy.

Legend:

Level	ET (mm/day)	Color	Meaning
A	0 -- 0.99	■ `#00A67A`	Low ET — arid zones or dormant vegetation
B	1.0 -- 2.49	■ `#00DF80`	Moderate ET — sparse to moderate vegetation
C	2.5 -- 4.99	■ `#FFD21E`	Active ET — forests and productive ecosystems
D	5.0 -- 7.99	■ `#FF8B16`	High ET — wetlands or irrigated cropland
E	≥ 8.0	■ `#FF367F`	Very high ET — significant water loss

Interpretation example:

If the gauge shows 3.2 mm/day (level C) with a CA of 4.8 mm/day, the site is losing less water to the atmosphere than its control area — this could indicate less vegetated or more drought-stressed conditions relative to the surrounding landscape.

Maps > "Evapotranspiration" (Map Layer)

Map Layer. An interactive raster map layer visualising the spatial distribution of daily mean evapotranspiration across the site and surrounding area, rendered as a continuous colour gradient.

Purpose: Where are the areas with the highest and lowest water loss across the landscape?

Description: The ET map layer appears in the layer selector panel within the Maps view, under the Microclimate group. When the layer has not yet been generated, a thumbnail card with a "Generate" button is shown. Once generated, selecting the layer loads the raster overlay on the map. The legend header shows the date of the composite (e.g. "Evapotranspiration in January 2024"). The colourmap scales dynamically to the data range of the rendered image.

How it's calculated: Each map layer is generated as a monthly MODIS ET composite for the selected site at 250 m resolution (bilinearly upsampled from 500 m native). One map layer covers one calendar month.

Legend: The map uses a jet-style continuous gradient colourmap:

Evapotranspiration (mm/day)
Low
High
Low ET (bare soil, urban)High ET (dense vegetation, wetlands)

Interpretation example:

If the map shows a predominantly cyan-to-green palette over a forested area and dark-blue tones over bare urban surfaces, the vegetated zones are transpiring actively while impervious surfaces have near-zero ET.

Data Sources

Source	Provider	Coverage	Resolution	Period
MODIS Terra ET (MOD16A2)	NASA LP DAAC via Google Earth Engine	Global	500 m native (250 m output)	2021-01-01 -- present
MODIS Aqua ET (MYD16A2)	NASA LP DAAC via Google Earth Engine	Global	500 m native (250 m output)	2021-01-01 -- present

Detailed Methodology

The processing pipeline follows five steps:

Image collection — MODIS Terra MOD16A2 and Aqua MYD16A2 collections are filtered by date and area of interest on Google Earth Engine. Both version 6.1 collections are used.
8-day to daily conversion — Each 8-day cumulative image is divided by 8 to obtain mean daily values.
Scale factor — The official MODIS ET scaling factor (multiply by 0.1) is applied to all pixel values.
Temporal compositing — A pixel-wise mean is calculated over the requested time period across all available images from both Terra and Aqua.
Reprojection and upsampling — The composite is reprojected to EPSG:3857 at 500 m and bilinearly upsampled to 250 m for display.

For the gauge, the spatial mean of the composite is computed over the site polygon (ROI) and the control area polygon (CA). For the map layer, the full raster is rendered as a monthly composite with a dynamically scaled jet colourmap.

Technical Notes

Data availability begins on 2021-01-01; map layers requested for earlier dates will be rejected
The 250 m output resolution is bilinearly upsampled from the 500 m native MODIS grid; fine spatial features smaller than ~500 m may appear smoothed
Both Terra (MOD16A2) and Aqua (MYD16A2) collections are merged to increase temporal coverage before compositing
The ET product measures actual ET (what the ecosystem is losing), not potential ET (maximum possible loss under ideal water availability); in water-limited ecosystems, actual ET can be far below potential ET
Seasonal and year-to-year variability is high: summer months typically show much higher ET than winter months; monthly composites should be compared across the same calendar month across years to avoid seasonal artefacts
The quality scale is inverted for this KPI: the A--E grades run from low ET (A) to high ET (E); whether high or low ET is "better" depends entirely on the ecosystem — the grades should be read as intensity levels, not as absolute ecological health scores
Related KPI: Aridity Index uses precipitation-to-potential-ET ratio as a measure of water availability; high ET combined with low precipitation produces high aridity
For trend analysis of potential ET under future climate scenarios, see the Future Climate section of the platform (separate data source: Open-Meteo FAO reference ET)

Bibliography

Mu, Q., Zhao, M., Running, S.W. (2011). "Improvements to a MODIS global terrestrial evapotranspiration algorithm". Remote Sensing of Environment, 115(8), 1781--1800. DOI: 10.1016/j.rse.2011.02.019
Running, S., Mu, Q., Zhao, M. (2017). MOD16A2 MODIS/Terra Net Evapotranspiration 8-Day L4 Global 500m SIN Grid V006. NASA EOSDIS Land Processes DAAC. DOI: 10.5067/MODIS/MOD16A2.006
Allen, R.G., Pereira, L.S., Raes, D., Smith, M. (1998). Crop evapotranspiration — Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56. Rome: FAO. Available at: fao.org/3/X0490E/X0490E00.htm