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.
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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
Abstract
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.
Figure 1
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.
Louis Hooffstetter says:
April 11, 2013 at 6:44 pm
Careful, it may be more analogous to discovering water freezes at the same temperature (but boils at different temperatures) at five different regions and applying that to come up with estimates everywhere else on the planet.
Would you call that BS?
LOL
Absurd total sample size, no replicates (n=1), no random site selection; so what could possibly go wrong ?
“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.”
Yeah.
So they took measurements at five (5!) weather stations and are basically claiming that the results can be extrapolated to any weather station in the world, Plus they yap one about this as if it is some new idea. They could have incorporated this study form NOAA using over 400 pans across the US:
http://www.nws.noaa.gov/oh/hdsc/PMP_related_studies/TR33.pdf
Where data was collected for 15 years.
Or this study using 493 pans
http://journals.ametsoc.org/doi/pdf/10.1175/1525-7541(2000)001%3C0543%3APETIDA%3E2.0.CO%3B2
Or this one from China that spanned over 40 years:
http://onlinelibrary.wiley.com/doi/10.1029/2011GL047929/abstract
Or this one which amongst other things identifies a convergence problem with pan studies not unlike Jones and Mann’s tree rings. They go the wrong direction from where they are expecting them to go, casting doubt on the measurement method itfelf.
In other words this study is not comprehensive and pretends to be new when it fact is has been done for decades and shows that the data is problematic. I had some links to European studies at one point as well but can’t lay my finger tips on the this moment. My recollection was they they too posted results disparate enough that you could not extrapolate them to other sites.
Ric,
When I read CoCoRaHS, I immediately assumed it was a class project at the high school in CoCoRa. I thought what a quaint name for a town. I wonder what state it’s in.
I just finished a couple of courses in hydrogeology which cover exactly what they are talking about. If they are investigating a new method to measure the rate of evapotranspiration which improves upon the present methods then I wish them success. By their press release, however, it sounds like their hubris is rising to the top. If they are claiming that by using 5 different stations, all located in the lower 48 states of the USA, they can extrapolate their results globally, then they are mistaken. The word evapotranspiration is used because part of the process involves plants – their intake of water and the transpiration of water from them. The role of plants in evapotranspiration is the main complicating factor in developing equations to calculate that rate. The plants in those areas which they used to get their data from are not representative of all the plants in the world, therefore any method that they develop, although useful for the United States (except Hawaii and Alaska), can not be used to calculate the rates outside of those states any better than the old formulas.
I remember watching a BBC enviromentary where they were using evaporation pans to convince the viewers that it was all getting much hotter. They were in exposed places and being compared to historical records from sheltered places.
I didn’t see anything about the huge role wind plays, and not really here either. Anyone who has a swimming pool will know that when it is windy it is like starting a high capacity pump to empty it. It can be boiling hot for days and the water level barely moves, add a stiff breeze and it goes down really quickly.
That’s why windy days are better for drying the washing too.
Speaking on a different subject Mark Styne wrote on his blog, “once you put reality up for grabs, all kinds of pathologies suddenly become viable.”
Sounds like Styne was right again.
If you are willing to ignore enough reality and truth you can have any answer you want.
cn
These people, in addition to the averred CU employees who are actually part of the gaggle at GISS?
How many thousands of these pretend scientists are there?
The extrapolation is BS. What does the class A wet pan give that a standard wet/dry bulb not give? I know it is difficult to ensure constant wetness for the wet bulb in hot conditions but these things are read hourly so this is a simple fix by the observer.
Reality Check: WJR Alexander et al. found a significant correlation between the ~22 year Hale Solar Cycle and floods in the South African region – but there was NO correlation with evaporation and the solar cycle.
WJR Alexander Causal linkages between solar activity and climatic responses, Water Resources & Flood Studies, U. Pretoria, 1 March 2006
WJR Alexander et al. Linkages between solar activity, climate predictability and water resource development 2007
W.J.R. Alexander & and F. Bailey, Solar Activity and Climate Change – A Summary, Energy & Environment Vol. 18, No. 6, 2007, pp 801-804
Other WJR Alexander’s publications
Similarly:
Andreas Prokopha, et al., Influence of the 11 year solar cycle on annual streamflow maxima in Southern Canada, Journal of Hydrology Volumes 442–443, 6 June 2012, Pages 55–62
Apologies to Mr. Steyn
cn
Interesting they are looking at 4713 BC with real time measurements? Must have perfected the Wayback machine. Twits don’t even know what Julian days are.
I would like to express my appreciation to Anthony and all the commenters for providing me with some hope that we will eventually come out of this craziness somewhat intact – not not without wasting an obscene amount of money along the way. You reassure me that there are actually sane people who are reading the tripe and asking WTF? Everytime I see a article in the paper or hear a new item I cringe. Thanks people for the sanity check.
“…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)….” Modeling the global climate, however, is a piece of cake. Just twiddle the CO2 knob – it goes to 11.
“…the press release (probably written by a person wholly unfamiliar with the science)…”
That’s 3/4 of the problems right there. You can’t blame the public for mistaken belief if their basic information is wrong. But the scientist should know better, and correct it.
Actually there probably are corrections somewhere for the more outlandish mistakes, but they are attached to the originals so you can’t find them. Tha t way they can say they were issued, but the original is all the public remembers.
All they need to do is go to the Engineering Toolbox and find the equation for water evaporation.
http://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html
The major variables are the difference in humidity ratio from the humidity ratio at saturation and the wind velocity.
johchlarson
Are you sure this paper isn’t an elaborate April Fool’s joke? Up close, it just sounds plausible to make some sort of sense, but from a longer view, it smells like a prank.