Hydrosat’s Science Lead, Josh Fisher, led a key NASA Applied Sciences Water Resources project in 2023. While speaking with State water managers, the team introduced the managers to some of the data and tools we’ve been developing for satellite-based evapotranspiration (ET), as part of the project. Most of the focus was on agriculture, drought, and wildfire.
During this meeting, it was highlighted how the cutting-edge tools provided through Hydrosat’s IrriWatch platform can provide crop stress detection and irrigation recommendations. NASA’s Evaporative Stress Index (ESI) provides better resolution than existing drought tools, allowing the State Climatologist and team to make decisions on drought categorization with greater clarity and connection to the people and ecosystems beneath otherwise coarse US Drought Monitor maps. The ESI also provides forest stress maps, which, in conjunction with ET, improves the predictive power of wildfire spread and intensity.
Of the 50 slides presented, the team included n=1 on NASA’s reservoir evaporation capabilities. This one slide was disproportionately well-received. Apparently, the State uses a hand-drawn map from the 1970s for open water evaporation, for which they use a lot. Any hesitations or insecurities about the uncertainties of NASA’s ECOSTRESS Mission data immediately evaporated; with the team knowing that whatever they produced would be a significant improvement. There is strong demand for data on evaporative loss from reservoirs and lakes for water management. Water managers need that data more than ever as water resources become increasingly variable and on a collision course with rising demand for water both by expanding populations and atmospheric aridity.
State water managers can sometimes move water from one reservoir to another to reduce loss from evaporation. Half the water loss from a reservoir may be from evaporation alone, on par with human abstraction. In sum those should be roughly equal to total water inputs. But, with leakage and other water uses such as river health maintenance, if the outputs exceed inputs, then the reservoir shrinks. Hence, knowledge of evaporative loss is crucial for making these management decisions.
In the absence of sophisticated reservoir evaporation measurements, one could make a guess based on pan evaporation. Or do a quick model calculation with a nearby weather station, this is what people have done for decades. Then they do their best to calibrate because both these approaches are known to be remarkably inadequate.
The processes controlling terrestrial water body evaporation are dynamic in space and time. They’re similar to those that control land and plant ET — radiation, humidity, wind, and air and surface temperature — but with different sensitivities. Plants and land introduce additional surface, stomatal, and aerodynamic resistances, as well as variable access to water. Open water, on the other hand, has much deeper radiation-absorbing and energy-moving characteristics that may result in evaporation in an entirely different place than where that energy was absorbed because heat is circulated throughout the water body. These characteristics are strongly determined by the physical structure of the water holding landform and underlying bathymetry (e.g., you feel warm water when you start walking into the ocean from the beach that rapidly turns cold when you get into deeper water). Wind events could impact open water more significantly than forests that are buffered physically and structurally.
Ultimately, radiation continues to be the dominant driver of both land and open water evaporation at large space and time scales; but, the process shifts to atmospherically-controlled at short time scales. For these reasons, simple estimates of open water evaporation can easily be in error.
However, measurement of open water evaporation is very challenging. While forest eddy flux towers are difficult to set up in their own right, they tend to exist on relatively stable ground, unlike water eddy flux instrumentation, bobbing around, and inevitably the prime spot to rendezvous for birds and other animals. Rigorous eddy covariance assumptions about stability and fetch are often ill-constrained, potentially contaminated by land advection, and these point measurements are supposed to represent the entire water body. Monitoring networks such as the US Bureau of Reclamation’s Open Water Evaporation Network (OWEN), the Great Lakes Evaporation Network (GLEN), the Global Lake Ecological Observatory Network (GLEON), and the Western Reservoir Evaporation Network (WREN), among others have done admirable and invaluable job tackling these challenges to provide these critical in situ data.
Remote sensing can overcome the spatial representation problem, especially given high spatial resolution measurements of surface temperature. Such resolution can identify not only spatial variability in evaporation, but also dynamically changing surface area related to water height, total volumetric water change, and changes in bathymetric impacts on evaporation, particularly relevant to sinuous and finely shaped water bodies. But, terrestrial water bodies have been masked out in remotely sensed ET datasets primarily due to the fact that it’s just a different retrieval approach from land/plant ET, and most remote sensing ET scientists have been interested in primarily land/plant science and applications.
To respond to the demand from water managers and scientists interested in reservoir, lake, and inland sea evaporation, a retrieval approach was implemented for terrestrial open water evaporation into his operational data production software. Then adding it to the ECOSTRESS and OpenET operational data production software systems. It was collected in situ open water evaporation measurements across the world in among the largest open water evaporation validations to date (Fisher et al., 2023. Remotely sensed terrestrial open water evaporation. nature.com/articles/s41598–023–34921–2). The team then found that the retrieval does a pretty good job — not as good as people do for land/plant ET, but there may be more error in the open water in situ measurements than in land flux measurements. Comparatively, alternative machine learning models were run but found that they did not necessarily do much better than the process-based retrieval approach. Much of the uncertainty at the instantaneous level was due to acute high wind events; this sensitivity minimizes when moving to daily temporal integration. Still, the analysis gave a baseline of error that practitioners can decide is tolerable or not for their applications, and it gave some room to improve.
The Landsat record continues to be supported with regular launches to replace aging satellites. ECOSTRESS will stay on the ISS pretty much close to the end of the ISS life. SBG, TRISHNA, and LSTM will eventually provide consistent, high quality, and well-calibrated data. Finally, Hydrosat will provide the highest spatiotemporal surface temperature measurements at 50 m, daily, multiple times per day. Other commercial companies such as ConstelIR will continue to add TIR measurements. Additionally, synergies with radar measurements from SWOT enable new monitoring capabilities of changes in reservoir and lake water height levels. Coupling these measurements with reanalyses and in situ networks, along with NASA’s operational systems such as ECOSTRESS and OpenET, expand the instantaneous remote sensing measurements throughout the day. Taken together, NASA’s ability to operationally monitor open water evaporation from millions of water bodies is key to ensuring water management and analysis of changes in climate and hydrological cycling into the future. And hopefully this is a game changer, extremely useful, and overall epic.
This article, originally written by Hydrosat’s Science Lead, Josh Fisher, can be found at the link here.
