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
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.
An experiment based upon that pan, with markings on just part of its perimeter, sat on a wooden pallet with variable air-gap beneath, and bearers in random orientation, that may or may not be in wind- and/or solar-shadow can be replicable?
Five stations? Sounds like a school project. And these guys turn it into something global?
Seriously – Where are the adults? This kids need to be grounded and have their pocket money taken off them. We need more of a backbone from the MSM and to see these guys and gals hauled up and shown to the world in their true light and their poor form.
This is yet another appalling claim they are making! If they are not ashamed, they should be shamed in the public eye. Only the MSM can do that. Nothing is going to change until this behavior is stepped on and stomped out.
MSM – take a HINT, already! Sheesh!
I would certainly be interested in understanding the correlation of this release and their funding needs\opportunity. Then again I could pick up 5 Lotto tickets an proclaim similar success without proof. >$arc<
I would be curious how they control for other variables such as altitude (easier evaporation with less air pressure), barometric pressure, cloud cover, relative humidity, time of day, etc. Hmm, maybe I should apply for a grant 😉
Their claim seems to be that evaporation rates are constant over the diurnal cycle:
Maybe that is just a poorly written sentence. Could they have meant to say that “the vertical variance of the relative humidity profile is less than previous understanding predicted would occur for increased or decreased evaporation rates …”? As written, their claim is that the lack of vertical variance in the relative humidity profile indicates a constant evaporation rate, which sounds absurd on its face. Evaporation is negative at night when dew precipitates out and positive when the sun comes up and evaporates the dew.
If this is how poorly they write then how well can they reason?
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.
Can someone explain what minimizing the variance means.
Think Globally – Evaporate Locally!
.Let’s don’t forget that this is the institution that hired convicted terrorist and murderer Katherine Power as a”professor.” This latest bit of flatulence sounds consistent with Columbia’s attitude otherwise towards truth, justice and liberty.
What a comedown, what a degradation for a one-proud, once-admirable institution now sunk so deeply into the moral and political morass. How the mighty are fallen.
“The censors invent what the cattle will swallow/Till newsprint and backsides sound equally hollow” (words from Songs for Voice and Orchestra, Op. 18 No. IV, 1972, copyright Chad J. Wozniak).
Looks like a great bird bath and watering hole.
Typical standardized site (irrigated pasture) with a U.S.W.B. Weather Bureau Class ‘A’ pan, tank
Not relevant to the study, but the picture shows a class A weather site with evaporation pan in the middle of an irrigated field. How anyone thinks they can get climatically useful information from this site is beyond me.
Minimising variance? Beggars belief … around here rate of evaporation can be so intense you can stand and watch it going on.
Also – why bother? My local station has daily evaporation readings going back decades, all accessible.
http://www.bom.gov.au/climate/dwo/IDCJDW4128.latest.shtml
Wow – temp min was 19° yesterday – no wonder I was dreaming about driving in snow ….
I’m sort of curious. I see the U.S.W.B. Weather Bureau Class ‘A’ pan, but is the skid below it an officially licensed skid? In any case, were licensed union workers employed to move the equipment? Or, were young impressionable college students exploited for this work? Worse, were any qui pro quo’s exchanged? Was OSHA contacted to sweep the area to determine if dangerous vermin were present, and also, to determine that adequate hydration was available for the workers? The caption reads that this is a standardized irrigated pasture site. Is it a pasture site for grazing animals which might be used in human consumption. If so, has the USDA properly evaluated it. Is the water quality in the pan checked on an hourly basis to insure that no contaminants enter the water and thus the bodies of the food animals? Speaking of water, is this now an officially designated wetland? There is water there as we can see. Therefore, has approval been granted by the EPA? And let’s not forget the impact this could have on endangered species.
But, I think, the most important consideration is the impact this has on, yes, global warming. I know you think that: What, it’s just a pan of water? But it’s a ‘pan of death.’ Water has 9,300% the impact as CO2. And that water, in that pan, is intended for no other reason than to evaporate, out of that pan, and into the atmosphere we so dearly love and turning it into a hateful atmosphere. Ok, I’m a little overemphasizing this and I’m not climate scientist so I have no excuse. Carbon offsets?
Adds new meaning to the term “vaporware”.
Ask a farmer who irrigates. Evaporation rates are figured into irrigation needs. This present research is a poor second to what farmers have been doing for decades.
Just out of curiosity, if the basic approach is valid, as Anthony seems to suggest, how many stations, and with what kind of siting and monitoring, would be necessary to pass muster? Stations in Death Valley, or the Sahara or Atacama deserts, would yield little in the way of useful evapotranspiration data, unless they were counterbalanced by stations in rain forests and monsoon-prone areas like South Asia .And at that point what are you really proving?
CoCoRaHS is collecting evapotranspiration reports, it’s a new project started last year and hasn’t spread widely yet. If readers want to join (and buy their own equipment), see http://www.cocorahs.org/Content.aspx?page=et
I imagine a lot of use report precip data to CoCoRaHS. A little about them:
CoCoRaHS is an acronym for the Community Collaborative Rain, Hail and Snow Network. CoCoRaHS is a unique, non-profit, community-based network of volunteers of all ages and backgrounds working together to measure and map precipitation (rain, hail and snow). By using low-cost measurement tools, stressing training and education, and utilizing an interactive Web-site, our aim is to provide the highest quality data for natural resource, education and research applications. We are now in all fifty states.
From the press release:
. . .daily maps of evaporation around the world that will enable scientists to evaluate changes in water table, . . .”
The statement and the wording of this statement is odd. Were I to construct such a statement using American rather than English, I would write “the water table” but I see nothing else to suggest such lack of the word. That’s just to suggest that the press release may be the work of a person that has not a clue as to the meaning of “water table.”
However, if they can “evaluate changes in water table” from air temperature and humidity there will be thousands of folks wanting such information. They may actually mean to speak of “soil moisture” rather than the water table, but that’s just a guess.
CoCoRaHS is collecting evapotranspiration reports, it’s a new project started last year and hasn’t spread widely yet. If readers want to join (and buy their own equipment), see http://www.cocorahs.org/Content.aspx?page=et
I imagine a lot of report precip data to CoCoRaHS. A little about them:
CoCoRaHS is an acronym for the Community Collaborative Rain, Hail and Snow Network. CoCoRaHS is a unique, non-profit, community-based network of volunteers of all ages and backgrounds working together to measure and map precipitation (rain, hail and snow). By using low-cost measurement tools, stressing training and education, and utilizing an interactive Web-site, our aim is to provide the highest quality data for natural resource, education and research applications. We are now in all fifty states.
It’s not entirely clear to me what they are finding, I think your photo is misleading, it sounds like they’re doing more than measuring evaporation pans. This quote sent me off into various silly fantasies that didn’t help:
The pan in the photo looks like it might be galvanized steel, not concrete. Of course, concrete doesn’t evaporate well at typical temperate temperatures.
Maybe I’ll try reading it again tomorrow.
“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. ”
Yeah… And your spatially upscaled model that provides all of this global information is based on water evaporating from five pans in North America? I gotta call BS.
Thanks Anthony,
I still get two newspapers thrown near my driveway everyday.
One of them has a circulation of 422,335 in Chicago, the other is a local paper reaching about 144,000.
Considering that global warming/climate change is gonna kill us all, the lack of stories (lately) informing the unwashed masses of their impending doom concerns me.
I miss the shrill warnings.
Those crossplots (charts C, F, I, L, O) of Estimated vs Measured appear to have dreadful R^2. You estimate 20 and the measured could be anywhere from negative (dew) to 70.
L and O also show that the method significantly underestimates the measured values in the high range, and possibly overestimates the low values.
Which begs the question, how are the “measured” amounts derived?
Ya know, how was the evaporation rate evaluated, considering the grounds it is on was developed for cattle in the sample site example shown.
Just sayin, man made, ya think>>?
Please help me out,,,,,,,,
Is not an evaporation rate contingent upon humidity levels in the end?
Symptom syndrome?
Tom J says:
April 11, 2013 at 5:24 pm
That just makes me cringe. Really it does!
I am in process of buying some land and the governmental impact echo is astonishing.
LOL, seems YouTube has an issue with linking a video here. Thanks Google (remember who they are and what the support) !
No matter, here it is 🙂
[+emphasis]
Imagine that! Who woulda thunk it?
Did not see how this field is irrigated. From below: Raising or lowering the ground water levels or from above, through some sort of sprinkler system. From the freshly mowed grass inside the test station and the lush grass outside the station it is most likely some sort of sprinkler system. If that is the case, do they cover the pan when the sprinkler is operating? Just wondering.
Let’s not be two hard on these folks. The climate scientists have graduated from one tree [Yamel] to five kiddy wading pools.