Reservoir Evaporation

[Philip Selby]

More of a hydrology question than SWAT per-se....

Does anyone know how to relate reservoir evaporation to ETo (Reference evapotranspiration). I notice in the SWAT theoretical documentation it is calculated as 0.6*PET but there is no reference for the method.

I have ETo but I am not needing to calculate PET at present so I am wondering if I can use ETo and a different conversion factor.

[Jim Almendinger]

Phil et al. --

I have tried to find a good answer for you (since I would like to know for myself) -- but I never arrived at a definitive answer.  I was looking for a paper that stated "Lake E may be estimated as calculated PET", perhaps with a coefficient, but I could not find such paper. 

The closest I came was in a statement in Ward and Trimble (2004), p. 92:  "As discussed, lake evaporation is a good surrogate for PET, and the closest surrogate for a lake, and thus for PET, is the buried pan."  That is, pan measurements converted to lake evaporation (with coefficients differing from 0.6-0.8 across the USA) are better than lake-E and PET calculations.  They don't directly say that "PET estimates may be used to estimate lake E", but I guess we may infer that this may be the case (perhaps friends at Ohio State Univ. could walk down the hall and ask Andy Ward if he'd agree or not...). 

Ward, A. D., and S. W. Trimble. Environmental Hydrology. CRC, 2004.

A very good review of methods is given by McMahon et al. (2013):

McMahon, T. A., M. C. Peel, L. Lowe, R. Srikanthan, and T. R. McVicar. “Estimating Actual, Potential, Reference Crop and Pan Evaporation Using Standard Meteorological Data: A Pragmatic Synthesis.” Hydrology and Earth System Sciences 17, no. 4 (2013): 1331–63.

They cite work by Morton:

Morton, Fred I. “Operational Estimates of Lake Evaporation.” Journal of Hydrology 66, no. 1 (1983): 77–100.

where differences between potential evaporation over land and water bodies is discussed, but I couldn't find a definitive statement regarding the use of PET to estimate lake E.  Their definition of PET allowed for greater humidity and lower temperatures over water bodies, which may not be appropriate for the way SWAT deals with things.  It could be construed to infer that open water E was less than PET, if PET were calculated based on higher T and much lower humidity values over land than water -- but SWAT makes no such differentiation, and land-based T and humidity values are applied across the subbasin without regard to the surface wetness of HRUs.  (Actual ET, not PET, takes into account soil moisture, and I suppose crop coefficients, too.) 

In any case I found nothing in the literature (yet) that stated that lake E = 0.6*PET.  I'm not sure why that's the default in SWAT.  I've generally had a positive water balance in my models and I've changed the evaporation coefficients from 0.6 to 1 to maximize my evaporation from reservoirs, ponds, and wetlands.  My first presumption was that the 0.6 coefficient looked suspiciously like a pan coefficient, and that if so, it wasn't appropriate for converting PET to lake E. 

All for now,

-- Jim

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From: "'Philip Selby' via SWAT-user" <swat...@googlegroups.com>
To: swat...@googlegroups.com
Sent: Tuesday, July 29, 2014 9:48:05 AM
Subject: [SWAT-user:4914] Reservoir Evaporation

More of a hydrology question than SWAT per-se....

Does anyone know how to relate reservoir evaporation to ETo (Reference evapotranspiration). I notice in the SWAT theoretical documentation it is calculated as 0.6*PET but there is no reference for the method.

I have ETo but I am not needing to calculate PET at present so I am wondering if I can use ETo and a different conversion factor.

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Dr. James E. Almendinger
St. Croix Watershed Research Station
Science Museum of Minnesota
16910 152nd St N
Marine on St. Croix, MN  55047
tel: 651-433-5953 ext 19

[Philip Selby]

Jim,

The pan evaporation coefficient (usually 0.7 or 0.75) relates pan evaporation to evaporation but this accounts for the fact that the pan has metal sides which get hot with the sun and increase evaporation from the pan.

This is interesting

"Rosenberg (1974) notes that in humid regions (i.e., where advection of sensible heat is unimportant) pan evaporation gives realistic estimates of PET.  In arid localities and where advection is considerable, pan evaporation may give unrealistic values; in fact, he notes that the difference between pan evaporation and PET may be "very pronounced."  In an effort to present a consistent methodology applicable uniformly throughout the country, PET rate is estimated in the module using well-accepted formulations (e.g., the Penman method), as opposed to being assumed equal to pan evaporation rate." (http://mepas.pnnl.gov/mepas/formulations/source_term/5_0/5_13/5_13.html)

If I find anything else enlightening I will post it.

Regards,

Phil

[Jim Almendinger]

Phil et al. --

On the MEPAS web page given below, the main quote from Rosenberg (1974) is --

Rosenberg (1974) notes that the concept of PET has been widely accepted,and he defines it as follows:

"Potential evapotranspiration (PET) is the evaporation from an extended surface of short green . . . (vegetation) . . . which fully shades the ground, exerts little or negligible resistance to the flow of water, and is always well supplied with water.  Potential evapotranspiration cannot exceed free water evaporation under the same weather conditions." (pp. 172)

This would imply that lake E > or = to PET.  But the SWAT default is that E < PET, by a factor of 0.6, i.e., E = 0.6*PET.  Seems to me that the factor should always be at least 1, and never < 1. 

What is the Rosenberg (1974) reference?  I can't seem to find it in Google Scholar, and I don't see a citation to it in Rosenberg's later papers. 

-- Jim

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From: "'Philip Selby' via SWAT-user" <swat...@googlegroups.com>
To: swat...@googlegroups.com

Sent: Wednesday, July 30, 2014 6:49:49 AM
Subject: [SWAT-user:4917] Re: Reservoir Evaporation

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[Philip Selby]

The reference is 

Microclimate: The Biological Environment

 By Norman J. Rosenberg 1974

http://books.google.co.uk/books?id=c6MI6VvTqTsC&lpg=PR17&ots=GouPjhaMa2&dq=Rosenberg%2C%20N.%20J.%201974.%20Microclimate&lr&pg=PA272#v=onepage&q&f=false