Across most farms, more water escapes into the atmosphere than runs off the land or seeps into groundwater, yet until recently, farmers and water managers had little visibility into how much was being lost or exactly where it was going. Tracking this hidden flow of water, known as evapotranspiration, has long been one of the most difficult challenges in agricultural water management.
Evapotranspiration, or ET, is the combined loss of water through evaporation from the soil surface and transpiration through plant leaves. Across a typical growing season, it accounts for the majority of water consumed on agricultural land. Despite its centrality in every irrigation decision, water rights allocation, and assessment of drought stress, ET has historically resisted direct, comprehensive measurement at the scales where it makes a difference.
The Measurement Coverage Challenge
Through most of the twentieth century, the available tools for measuring ET were fundamentally local. Earlier approaches relied on evaporation pans and weighing lysimeters, devices that could quantify water loss under controlled conditions but told practitioners almost nothing about the varied reality of an actual field, watershed, or region. Sap flow sensors and leaf chambers went further still, capturing transpiration at the individual plant and providing rich data for experimental work but offered no path to the landscape scale coverage that water management genuinely requires.
Ground-based eddy-covariance flux towers marked the most significant advance before satellites entered the picture. Installed at representative sites across the world, these instruments measure the continuous exchange of water vapor between vegetation and the atmosphere, producing reliable time series data at the ecosystem level. Their limitation is geographic. While useful at their specific sites, flux towers are islands of data in a world that needs continuous, comprehensive spatial coverage. They capture a great deal about specific locations but tell us nothing about the fields on the farms in between.
What Thermal Infrared Changed
Once sensors capable of detecting thermal energy from orbit were deployed, beginning with early Landsat missions in the 1970s and progressing through instruments including MODIS, VIIRS, and more recently NASA’s ECOSTRESS, researchers have produced ET estimates across entire landscapes simultaneously. The underlying physics is clear. Water-stressed vegetation cannot cool itself through transpiration and runs measurably hotter than well-irrigated crops. Land surface temperature, measured from space through thermal infrared sensors, provides a signal of the energy balance at the surface, and from that energy balance ET can be derived with reasonable accuracy.
What has constrained a satellite approach is the tension between spatial and temporal resolution. Space based platforms like Landsat produce detailed imagery but revisit any given location only once every sixteen days. Agriculture operates on faster timescales. Crop water stress can emerge and worsen within days, and irrigation decisions benefit from information updated daily rather than weekly. Most satellite platforms offer broad spatial coverage without the temporal frequency that farmers and water managers actually need. Geostationary satellites, which do revisit continuously, lack the spatial resolution required for field level insights.
No single existing observational approach has managed to deliver both spatial completeness and revisit frequency at once. Most satellites provide spatial breadth without temporal depth. For decades, practitioners have had to work around this gap rather than through it.
The Case for More Detailed, More Frequent Coverage
Commercial smallsat constellations represent the most credible path toward resolving that tension. By deploying multiple smaller satellites in coordinated orbits, operators can reduce revisit intervals dramatically, approaching the daily cadence that agriculture applications require. The era is still young, but the trajectory is clear, and it is arriving alongside a decade of progress in thermal infrared modelling.
Companies like Hydrosat are now putting decades of thermal infrared research to work in the field. Hydrosat’s IrriWatch platform uses satellite-derived ET data to provide daily irrigation guidance at field resolution, used by growers worldwide. The platform observes land surface temperature from orbit to infer water use, crop stress, and management needs before problems become visible.
Why This Matters Beyond the Farm
Groundwater aquifers are under stress across nearly every major agricultural region, and mountain snowpack, a critical seasonal buffer for downstream agricultural water supplies, is in accelerating retreat. Water rights disputes are becoming more frequent and more contentious. The ability to independently verify what agriculture is actually consuming, rather than relying on modeled estimates or self-reported figures, now carries genuine governance significance. Satellite-based ET measurement offers an auditable, spatially continuous record of water use at field level, a form of accountability that has not previously existed at scale.
For farmers, the absence of accurate ET data tends to push irrigation in one direction–when in doubt, add water. Research consistently shows that ET-guided irrigation reduces water consumption while maintaining or improving yields. At field scale, that is an efficiency gain. At basin scale, aggregated across thousands of farms drawing from the same aquifer or river system, it becomes a meaningful contribution to water security.
Measuring ET everywhere, all the time, at a resolution farmers can act on remains an ongoing challenge rather than a solved one, but it is closer than it has ever been. The convergence of advancing satellite technology, decades of accumulated ET modeling knowledge, machine learning approaches that can fill gaps in cloudy or under sampled regions, and growing commercial investment suggests the gap between aspiration and capability is narrowing.
This blog is based on a peer-reviewed paper published in Global Change Biology, led by Hydrosat Science Lead, Dr. Joshua B. Fisher, which traces the full arc of that measurement challenge. “Evapotranspiration Everywhere, All the Time: Towards a Unified View From Earth Observation” (Fisher, Anderson, Miralles et al., 2026) is the work of a broad collaborative group of researchers from institutions including Chapman University, USDA-ARS, Ghent University, the University of Wisconsin-Madison, and Seoul National University, alongside Hydrosat contributors Joshua Fisher, Science Lead, and Wim Bastiaanssen, Product Ambassador and founder of IrriWatch.
