Evapotranspiration Monitoring System

Cooling, Climate, and the Water That Returns

Evapotranspiration (ET) is the combined movement of water from soil and plants back into the atmosphere. It is one of the largest fluxes in the water cycle and one of the most powerful cooling mechanisms in any landscape. Every litre of water evapotranspired absorbs about 2.5 megajoules of latent heat, the same mechanism that cools the human body through sweat. This is why parks, urban trees, green roofs, and water features cool the air around them, and why outdoor thermal comfort improves so dramatically under canopy cover.

ET can be measured two ways. Indirectly, by algorithm calculated from weather data such as temperature, humidity, and solar radiation, producing a reference value. Or directly, with sensors that capture the actual flux from a specific surface. Both approaches are available at Wetec.

Evapotranspiration monitoring systems can be configured around either or both measurement approaches, depending on the study scale and accuracy required:

• Indirect / calculated ET — weather station data (temperature, humidity, solar radiation, wind, and rainfall) feeds standard algorithms such as Penman-Monteith to produce reference ET (ET₀). Suitable for general water management, regional climate studies, and applications where a reference value is sufficient.

• Direct / measured ET — direct measurement of water vapour flux from a specific surface using eddy covariance — a sonic anemometer paired with a fast-response gas analyser. The most accurate method, capturing actual ET from a real site such as a forest canopy, urban park, farm field, or reservoir surface.

• Supporting measurements — net radiation, soil heat flux, and soil moisture profile sensors enable surface energy balance calculations and water balance validation — bridging between the two approaches.

• Data and analysis — continuous logging, remote access, and outputs compatible with hydrological models, urban climate research datasets, and sustainability reporting.