From Columbia University , something that made my B.S. meter ping. My first thought was that evaporation pans aren’t new, going back to the beginning of the U.S. Weather Bureau COOP network, so what is this all about?
Typical standardized site (irrigated pasture) with a U.S.W.B. Weather Bureau Class ‘A’ pan, tank and DWR agroclimatic station. Source: http://www.water.ca.gov/landwateruse/annualdata/agroclimatic/
This looks to be a case of “test locally, extrapolate globally”. Read on.
New technique measures evaporation globally
First method to use weather station measurements to obtain daily evaporation rates
New York, NY—April 11, 2013—Researchers at Columbia Engineering and Boston University have developed the first method to map evaporation globally using weather stations, which will help scientists evaluate water resource management, assess recent trends of evaporation throughout the globe, and validate surface hydrologic models in various conditions. The study was published in the April 1 online Early Edition of Proceedings of the National Academy of Sciences (PNAS).
“This is the first time we’ve been able to map evaporation in a consistent way, using concrete measurements that are available around the world,” says Pierre Gentine, assistant professor of earth and environmental engineering at Columbia. “This is a big step forward in our understanding of how the water cycle impacts life on Earth.”
The Earth’s surface hydrologic cycle comprises precipitation, runoff, and evaporation fluctuations. Scientists can measure precipitation across the globe using rain gauges or microwave remote sensing devices. In places where streamflow measurements are available, they can also measure the runoff. But measuring evaporation has always been difficult.
“Global measurements of evaporation have been a longstanding and frustrating challenge for the hydrologic community,” says Gentine. “And now, for the first time, we show that simple weather station measurements of air temperature and humidity can be used across the globe to obtain the daily evaporation.”
Evaporation is a key component of the hydrological cycle: it tells us how much water leaves the soil and therefore how much should be left there for a broad range of applications such as agriculture, water resource management, and weather forecasting.
Gentine, who studies the relationship between hydrology and atmospheric science and its impact on climate change, collaborated on this research with Guido D. Salvucci, professor and chair of the Department of Earth and Environmental Sciences at Boston University and the paper’s lead author. Using data from weather stations, widely available across the globe, they focused on evaporation and discovered an emergent relationship between evaporation and relative humidity that gave them the evaporation rates.
Gentine and Salvucci plan to provide daily maps of evaporation around the world that will enable scientists to evaluate changes in water table, calculate water requirements for agriculture, and measure more accurate evaporation fluctuations into the atmosphere.
“Sharing our data with researchers around the world will help us learn more about the Earth’s hydrologic cycle and assess recent trends such as whether it is accelerating,” adds Gentine. “Acceleration could greatly impact our climate, locally, nationally, and globally.”
The research has been funded by the National Science Foundation.
As is typical with poorly organized people who do science by press release these days, they don’t give the name of the study, DOI, link to abstract, or any way to locate the study, further, they have conflicting dates April 1/11 both of which are PNAS publication dates, so I have to go look it up. (Update: Holly Evarts, the person who wrote the PR, says the issue may lie with Eurekalert as the engineering.columbia.edu/new-technique-measures-evaporation-globally at Columbia shows an intact link to the paper. These things should be checked before they go out. )
The only way I found it was with the last name of one of the authors. It gets worse though.
Emergent relation between surface vapor conductance and relative humidity profiles yields evaporation rates from weather data
The ability to predict terrestrial evapotranspiration (E) is limited by the complexity of rate-limiting pathways as water moves through the soil, vegetation (roots, xylem, stomata), canopy air space, and the atmospheric boundary layer. The impossibility of specifying the numerous parameters required to model this process in full spatial detail has necessitated spatially upscaled models that depend on effective parameters such as the surface vapor conductance (Csurf). Csurf accounts for the biophysical and hydrological effects on diffusion through the soil and vegetation substrate. This approach, however, requires either site-specific calibration of Csurf to measured E, or further parameterization based on metrics such as leaf area, senescence state, stomatal conductance, soil texture, soil moisture, and water table depth. Here, we show that this key, rate-limiting, parameter can be estimated from an emergent relationship between the diurnal cycle of the relative humidity profile and E. The relation is that the vertical variance of the relative humidity profile is less than would occur for increased or decreased evaporation rates, suggesting that land–atmosphere feedback processes minimize this variance. It is found to hold over a wide range of climate conditions (arid–humid) and limiting factors (soil moisture, leaf area, energy). With this relation, estimates of E and Csurf can be obtained globally from widely available meteorological measurements, many of which have been archived since the early 1900s. In conjunction with precipitation and stream flow, long-term E estimates provide insights and empirical constraints on projected accelerations of the hydrologic cycle.
To add to insult, even though this is publicly funded work by NSF, it is paywalled at PNAS.
Curious as to how they pulled off this global analysis, I searched found a free copy though at Columbia, here is the link. (PDF).
Compare this headline: New technique measures evaporation globally
To the actual technique:
The hypothesis is tested at five hydrologically, climatically, and biophysically diverse AmeriFlux (14) sites: Vaira Ranch, a grass-land in California; the Duke Forest, a hardwood forest in North Carolina; the Audubon Research Ranch, a desert grassland in Arizona; Fort Peck, a semihumid grassland in the northern great plains of Montana; and Mead Rainfed, an agricultural plot in Nebraska.
Yes that’s right, five stations in the USA, not global, not even out of the country. No wonder the PR lacked basic details about the paper.
The premise itself is probably fine, since they are only defining a technique that could be expanded upon, but the press release (probably written by a person wholly unfamiliar with the science), takes it to a global level as if it is already a reality, when in fact, it isn’t even close yet.
Our main finding is that the surface conductance estimated by minimizing the variance of the RH profile predicts the measured E and sensible heat flux accurately. This finding is demonstrated in Fig. 1, where, for each site, three plots are presented: (i) the mean seasonal cycle of predicted and measured latent heat flux (i.e., the energy equivalent of evapotranspiration),filtered with an 11-d moving window average (Fig. 1A,D,G,J, and M); (ii) a single year or season of results highlighting the covariability of the measured and estimated daily averaged fluxes (Fig. 1B,E,H,K, and N); and (iii) a scatter plot of the daily-estimated and measured fluxes, along with a root mean square and mean bias estimate (Fig. 1 C ,F,I,L, and O). The fit between the measured (green) and estimated (red) fluxes, at both seasonal and synoptic scales, across five significantly different field sites, corroborates the hypothesis that the RH profile evolves to a minimum variability with respect to evaporation.
All well and good, and that surface conductance technique they test may in fact be accurate, but I think you’ll find differences outside of the USA in the way evapotranspiration data is gathered, as well as the quality of it. As our friends constantly remind us about my investigation into siting problems in the USHCN, it may not hold up outside of the USA. Only a global scale study can tell you for certain.
I would do some additional investigation in other countries before I declared this ready to be globally scaled, because as it stands, with five stations, I don’t buy it as being ready for prime time yet.