In cooperation with DuPage County Department of Engineering, 

Stormwater Management Division

Comparison of Potential Evapotranspiration Calculated by 

the LXPET (Lamoreux Potential Evapotranspiration) Program

and by the WDMUtil (Watershed Data Management Utility)

Program

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2005-1020
Comparison of Potential Evapotranspiration 

Calculated by the LXPET (Lamoreux 

Potential Evapotranspiration) Program 

and by the WDMUtil (Watershed Data 

Management Utility) Program

By Elizabeth A. Murphy

In cooperation with DuPage County Department of Engineering, 

Stormwater Management Division

Open-File Report 2005-1020

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.
v

Contents

Abstract ................................................................................................... 1

Introduction ................................................................................................ 1

Comparison of Potential Evapotranspiration Equations ............................................. 2

The Lake-Correction Factor ..................................................................... 5

Hourly Distribution .............................................................................. 5

Hydrological Simulation Program-Fortran (HSPF) ................................................... 6

Summary ................................................................................................... 6

References Cited .......................................................................................... 7

Appendix A: Computing evapotranspiration time series using LXPET ........................ 11 

Appendix B: LXPET Fortran code .................................................................. 16

Figures

1-2. Graphs showing—

 1. LXPET (Lamoreux Potential Evapotranspiration) and WDMUtil (Watershed Data 

 Management Utility) double-mass curve and the 0.7 factor for daily 

 evaporation calculated at Argonne National Laboratory in Illinois for the 

 period January 1, 1990,–September 30, 2003 ............................................. 5

2. Evaporation distribution by LXPET (Lamoreux Potential Evapotranspiration) and 

WDMUtil (Watershed Data Management Utility) at Argonne National Laboratory in 

Illinois on A) January 15, 1995 (winter), and B) July 15, 1995 (summer). ............... 6

Table

1. Comparison of the LXPET (Lamoreux Potential Evapotranspiration) and WDMUtil 

 (Watershed Data Management Utility) equations used to generate the evaporation 

 time series for this study. ..................................................................... 4
vi

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

mile (mi) 1.609 kilometer (km)

Energy

Langleys per day (Lg/d) 279.12 Watts per square meter (W/m2)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8
Comparison of Potential Evapotranspiration Calculated 

by the LXPET (Lamoreux Potential Evapotranspiration) 

Program and by the WDMUtil (Watershed Data 

Management Utility) Program

By Elizabeth A. Murphy

Abstract

Potential evapotranspiration can be estimated using observed meteorological data, and many methods have been 

developed to accomplish this estimation. This report, prepared by the U.S. Geological Survey in cooperation with 

DuPage County Department of Engineering, Stormwater Management Division, describes the LXPET (Lamoreux 

Potential Evapotranspiration) program and compares the daily and hourly values calculated by LXPET with the 

values calculated by WDMUtil (Watershed Data Management Utility). The inputs to both LXPET and WDMUtil 

are total daily solar radiation in Langleys per day, total daily wind movement in miles, and average daily dewpoint 

temperature in degrees Fahrenheit. Average air temperature in degrees Fahrenheit is used in LXPET, whereas either 

maximum and minimum daily air temperature or average daily air temperature in degrees Fahrenheit is used in 

WDMUtil. The latitude (degree, minute, second) where evaporation is calculated is a required WDMUtil input.

The daily evaporation time-series data generated by LXPET are smaller by a factor of 0.7 than the daily timeseries data generated by WDMUtil. The 0.7 factor is the same as the 0.7 correction factor commonly applied to 

pan evaporation data to approximate lake evaporation. The time series from either program can be used in hydrologic modeling depending on the expected model input. The Hydrologic Simulation Program-Fortran (HSPF), the 

hydrologic model used in this study, uses potential evapotranspiration as input. Because lake evaporation is used 

as potential evapotranspiration in hydrologic modeling, LXPET time-series values should be used as HSPF input or 

the WDMUtil time-series values should be multiplied by 0.7 before being input into HSPF. However, because the 

WDMUtil time series is adjusted for season of the year, this time series may give more realistic evaporation values 

when HSPF is run over multiple seasons.

Introduction

The U.S. Geological Survey (USGS), in cooperation with DuPage County Department of Engineering, Stormwater Management Division, has been conducting hydrologic and hydraulic modeling studies in DuPage County, Ill. 

since 1997 using Hydrologic Simulation Program-Fortran (HSPF) as the hydrologic model. Potential evapotranspiration is an important parameter in hydrologic modeling, and it can be estimated using observed meteorological data. 

A study was begun in 2003 to compare the potential evapotranspiration time series calculated by LXPET (Lamoreux 

Potential Evapotranspiration) and WDMUtil (Watershed Data Management Utility), a program packaged with HSPF 

as part of the BASINS (Better Assessment Science Integrating point and Nonpoint Sources) software system (U.S. 

Environmental Protection Agency, 2001). The purpose of this report is to compare the evapotranspiration time series 

generated by LXPET with that generated by WDMUtil. The differences between the two time series are discussed, 

as well as the time series used for HSPF model simulation. The differences between the two time series are also 

shown graphically using meteorological inputs from the Argonne National Laboratory in Illinois. This meteorology 

station has been operated since 1948 and data from January 1, 1990, to September 30, 2003, are used in this study 

(Argonne National Laboratory, 2005).
2 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 

The USGS was given a program for calculating potential evapotranspiration based on the method of Kohler and 

others (1955) as adapted to digital computers by Lamoreux (1962) by the Northeastern Illinois Planning Commission 

(NIPC). The USGS also was given a program for distributing the calculated daily time series to hourly values (Tom 

Price, Northeastern Illinois Planning Commission, presently with Conservation Design Forum, written commun., 

1997). The program code probably originated as part of the LANDS module of the HSP (Hydrocomp Simulation 

Program) hydrologic model (Hydrocomp International, Inc., 1970) and was coded in PL/1 before it was ported to 

FORTRAN (FORmula TRANslation) in the early 1980s. The program was coded in FORTRAN by Hey and Associates under contract to NIPC (Tom Price, Northeastern Illinois Planning Commission, presently with Conservation 

Design Forum, written commun., 2005). Kohler and others (1955) developed graphs to determine lake evaporation 

from air temperature, dewpoint temperature, solar radiation, and wind movement. Lamoreux (1962) developed 

equations from the graphs, so a computer can be used to calculate the evaporation. The USGS modified and combined the two programs to automatically distribute the daily time series to an hourly time series and to accept average daily air temperature as an input time series rather than a maximum daily air temperature and a minimum daily 

air temperature. The USGS named the program LXPET to reflect the use of the Lamoreux methods and calculation of potential evapotranspiration. No complete documentation is available for LXPET; however, this report will 

describe the equations and methods used by LXPET to calculate potential evapotranspiration and provide instructions for using LXPET in appendix A. The FORTRAN source code for the LXPET program has been included in 

appendix B. A digital version of the FORTRAN source code and the compiled LXPET program can be requested 

from the USGS Illinois Water Science Center.

Comparison of Potential Evapotranspiration Equations

LXPET and WDMUtil can both be used to calculate evaporation. LXPET is based on the method of Kohler and 

others (1955) as adapted to digital computers by Lamoreux (1962). LXPET calculates lake evaporation. The Penman pan evaporation calculations used in WDMUtil are based on the Penman (1948) formula and the method of 

Kohler and others (1955).

The inputs to LXPET are total daily solar radiation in Langleys per day (Lg/d), total daily wind movement in 

miles (mi), average daily dewpoint temperature in degrees Fahrenheit (°F), and average daily air temperature (°F). 

Average air temperature (°F) is required in LXPET, whereas WDMUtil requires either maximum and minimum daily 

air temperatures, or average daily air temperature (°F). WDMUtil also requires the latitude (degree, minute, second) 

where evaporation is calculated. The output of LXPET is an hourly lake evaporation time series (thousandths of an 

inch). The output of WDMUtil is an hourly pan evaporation time series (inches).

The LXPET program contains the following equations for the calculation of lake evaporation (EVAP). 

EVAP = B * A * 1,000 , (1)

where

B = [ 0.015 + ((Ta + 398.36)-2) * 68,554,000,000 * exp (

 -7,482.6 )]-1 

,

 (Ta + 398.36)

A = exp ((Ta – 212) * (0.1024 – 0.01066 * ln(R))) – 0.0001 + 0.0105 * C 0.88 * (0.37 + 0.0041 * Up) ,

where

C = 6,413,300 * [exp (

 -7,482.6 ) – exp ( -7,482.6 )] ,

 (Ta + 398.36) (DEWP + 398.36)
where Ta is the air temperature in °F, R is the total daily solar radiation in Langleys, Up is total daily wind movement 

in miles, and DEWP is the dewpoint temperature in °F.

The WDMUtil Version 2.0 Users’ Manual (2001) states the Penman daily pan evaporation formula as

E = (Q * DEL + Ea * GAM) ––––––––––––—––––– (2) ,

 (DEL + GAM)

where E is pan evaporation in inches, Q is net radiation exchange, DEL is the slope of the saturation vapor-pressure 

curve at the given air temperature, GAM is 0.0105 inch Hg/°F (defined by Bowen’s ratio), and Ea is pan evaporation 

when air temperature equals water temperature.

The empirical equation for Q * DEL is

Q * DEL = exp [(Ta – 212)(0.1024 – 0.01066 * ln(R))] – 0.0001.

(The last term in the equation above is actually listed as 0.000 in the WDMUtil documentation (WDMUtil Version 

2.0 Users’ Manual, 2001), but that was confirmed to be an error (Paul Cocca, U.S. Environmental Protection Agency, 

written commun., 2004.))

The equation for Ea is

Ea = (0.37 + 0.0041 * Up) * (es – ea)

0.88,

where (es – ea) is the vapor-pressure deficit between the surface and dewpoint temperatures.

As is shown in table 1 and discussed below, the key difference between the LXPET and WDMUtil 

programs is the slope of the saturation vapor-pressure curve, DEL. 

DEL is calculated in LXPET with the equation

DEL = (Ta + 398.36)-2 * 68,554,000,000 * exp (

 -7,482.6 ) . (3)

 (Ta + 398.36)

DEL is calculated in WDMUtil with the equation

DEL = (

 7,482.6 ) exp (15.674 – 7,482.6 ) . (4)

 (Ta + 398.36)2 (Ta + 398.36)

A side-by-side comparison of the LXPET and WDMUtil evaporation equations is provided in table 1. This 

table illustrates that the programs are based on the same set of equations. The only difference between pan evaporation and lake evaporation is in the equation for DEL. 

Comparison of Potential Evapotranspiration Equations 3
4 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil LXPET WDMUtil EVAP = 1,000( A ) = (Q * DEL + GAM * Ea) — –––––––––––––––––––– , B’ (GAM + DEL) where A = Q * DEL + GAM * Ea and B’ = B -1 = GAM + DEL

E = (Q * DEL + Ea * GAM) ––––––––––—––––––– (DEL + GAM)

Numerator

A = exp[(

Ta – 212)(0.1024 – 0.01066 * ln(

R))] – 0.0001 + 0.0105 * (0.37 + 0.0041 * Up) *

C 0.88 ,where 

C = es – ea

Q * DEL = exp [(

Ta – 212)(0.1024 – 0.01066 * ln(

R))] – 0.0001

Ea = (0.37 + 0.0041 * Up) * (es – ea)0.88GAM = 0.0105

Table 1. Comparison of the LXPET (Lamoreux Potential Evapotranspiration) and WDMUtil (Watershed Data Management Utility) equations used to generate the evaporation time series for this study.[EVAP, lake evaporation in thousandths of an inch,; 

Ta, air temperature in degrees Fahrenheit (°F); 

R, total daily solar radiation in Langleys; 

Up, total daily wind movement in miles; DEWP, dewpoint temperature in °F; 

E, pan evaporation in inches; 

Q, net radiation exchange; DEL, slope of the saturation vapor pressure curve at the given air temperature; GAM, 0.0105 inch Hg/°F (defined by Bowen’s ratio); 

Ea, pan 

evaporation when air temperature equals water temperature; (es-ea), vapor-pressure deficit between the surface and dewpoint temperatures]

Denominator

B’ = [0.015 + ((

Ta + 398.36)-2) * 68,554,000,000 * exp( -7,482.6 )] ————–— (Ta + 398.36) DEL + GAM = 

( 7,482.6 ) exp (15.674 – 7,482.6 ) + 0.0105 —————–– —————– (Ta + 398.36)2 (Ta + 398.36)

DEL = (

Ta + 398.36)-2 * 68,554,000,000 * exp ( -7,482.6 ) —————–

(Ta + 398.36)

= (

Ta + 398.36)-2 * 4.7988 *1010

* exp ( -7,482.6 ) —–––——– –————— 0.7 (Ta + 398.36)

DEL = ( 7,482.6 ) exp (15.674 – 7,482.6 ) —————–– —————– (Ta + 398.36)2 (Ta + 398.36)= (

Ta + 398.36)-2 * 7,482.6 * exp(15.674) * exp ( -7,482.6 ) –———–—–

(Ta + 398.36)

= (

Ta + 398.36)-2 * 4.799 * 1010 * exp( -7,482.6 ) ——–——— (Ta + 398.36)
The Lake-Correction Factor

When the daily-evaporation time series from these two programs were compared, the LXPET values were 

smaller than the values calculated with the WDMUtil program. When the LXPET values were divided by the 

WDMUtil values, the result always was approximately 0.7. A double-mass curve with a constant line of 0.7 is 

depicted in figure 1 to show the consistency of the 0.7 factor over the period January 1990-September 2003. 

The DEL equation in the LXPET calculations can be rewritten to reveal a 0.7 term not present in the WDMUtil DEL equation (table 1). This 0.7 factor is described as a lake correction by Kohler and others (1955) and is used 

to convert from Class A pan evaporation to lake evaporation. According to Viessman and others (1989, p. 102), 

“most investigators have assumed that potential evapotranspiration is equal to lake evaporation as determined from 

National Weather Service Class A pan records.” However, Viessman and others (1989) also explain that the potential 

evapotranspiration is a function of soil type and vegetation, so the simple use of lake evaporation is not theoretically 

correct.

Hourly Distribution 

The LXPET program calculates daily lake evaporation values and distributes the daily values to hourly values. 

The distribution process is incorporated into the program and is not seen by the user as a separate step. In contrast 

to the WDMUtil method described below, the LXPET distribution process does not change seasonally. The LXPET 

program always distributes daily evaporation over the hours from 6:00 to 18:00.

The WDMUtil hourly pan evaporation time-series calculation takes two steps. The daily evaporation time series 

is calculated, then must be disaggregated to an hourly time series as a second step. WDMUtil requires the latitude 

of the site as an input for the disaggregation process. The seasons at the site are determined based on the latitude. 

WDMUtil disaggregates the evaporation differently depending on season. The winter evaporation is spread over 

fewer hours (for example, 7:00-15:00) than the spring or fall (for example, 5:00-17:00) with evaporation occurring 

over the most hours in the summer (for example, 4:00-18:00). This disaggregation method more realistically divides 

the daily evaporation over the hours of the day than the LXPET method, which does not change seasonally. 

The difference in the disaggregation on a winter day and a summer day at the meteorological observation station 

at Argonne National Laboratory in northeastern Illinois is illustrated in figure 2. In winter, LXPET peak evaporation 

is lower than WDMUtil, whereas in summer, LXPET peak evaporation is higher than WDMUtil. LXPET distributes 

Figure 1. LXPET (Lamoreux Potential Evapotranspiration) and WDMUtil (Watershed Data Management Utility) double-mass curve 

and the 0.7 factor for daily evaporation calculated at Argonne National Laboratory in Illinois for the period January 1, 1990,–September 30, 2003.

Comparison of Potential Evapotranspiration Equations 5
6 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 

the evaporation over more hours in winter than WDMUtil. In summer, LXPET distributes the evaporation over 1 

hour less than WDMUtil. 

Hydrological Simulation Program-Fortran (HSPF)

The HSPF program uses potential evapotranspiration as a model input. The following discussion of the appropriate HSPF input time series is taken from the HSPF manual (Bicknell and others, 1997) regarding the input of 

potential evapotranspiration to the model.

“ET potential or demand is supplied as an input times series, typically using U.S. Weather Bureau Class A pan 

records plus an adjustment factor. The data are further adjusted for cover in the parent subroutine PWATER.”

This data description indicates that pan evaporation time series should be adjusted before it is input to HSPF. 

A conclusion of the present study is that either the LXPET time series should be used or the WDMUtil time series 

should be multiplied by 0.7 before it is input into HSPF.

Summary

The U.S. Geological Survey, in cooperation with DuPage Department of Engineering, Stormwater Management Division, has compared the evaporation time series calculated by LXPET and WDMUtil. Either program can 

be used to calculate an evaporation time series for hydrologic modeling. The programs use similar equations and 

require similar input data. The LXPET program generates a lake evaporation time series, whereas WDMUtil generates a pan evaporation time series. The difference between the programs is the equation used for DEL, the equation 

of the saturation vapor-pressure curve. The LXPET daily values are adjusted by a lake correction factor of approximately 0.7 compared to the WDMUtil daily values of pan evaporation. 

The LXPET daily evaporation values are disaggregated to hourly values in a uniform manner that does not 

change based on season of the year or latitude. WDMUtil uses latitude to alter the disaggregation from daily to 

hourly values based on seasonal and regional differences. 

HSPF is a hydrologic model used to simulate the amount of runoff during storms in watersheds. The HSPF 

program requires potential evapotranspiration as an input time series. Because lake evaporation is used by HSPF 

modelers as potential evapotranspiration, either LXPET time-series values should be used as HSPF input or the 

WDMUtil pan evaporation time-series values should be multiplied by 0.7 before being input into HSPF. However, 

because the WDMUtil time series is adjusted for season of the year and latitude, multiplying it by 0.7 may give more 

realistic potential evapotranspiration values than the LXPET output time series when the HSPF model is run over 

multiple seasons.

Figure 2. Evaporation distribution by LXPET (Lamoreux Potential Evapotranspiration) and WDMUtil (Watershed Data Management 

Utility) at Argonne National Laboratory in Illinois on A) January 15, 1995 (winter), and B) July 15, 1995 (summer).
The FORTRAN source code for the LXPET program has been included in this report. A digital version of the 

FORTRAN source code and the compiled LXPET program can be requested from the USGS Illinois Water Science 

Center.

References Cited

Argonne National Laboratory, 2005, Argonne Data: accessed February 14, 2005, at URL http://www.atmos.anl.gov/ANLMET/

Bicknell, B.R., Imhoff, J.C., Kittle, J.L., Donigan, A.S., and Johanson, R.C., 1997, Hydrologic Simulation Program-Fortran, 

User’s Manual for Version 11: U.S. Environmental Protection Agency, EPA/600/R-97/-90, 755 p.

Hydrocomp International, Inc., 1970, The LANDS Module of the Hydrocomp Program: Palo Alto, California.

Kohler, M.A., Nordensen, T.J., and Fox, W.E., 1955, Evaporation from Pans and Lakes: U.S. Weather Bureau Research Paper, 

no. 38, 21 p.

Lamoreux, W.W., 1962, Modern Evaporation Formulae Adapted to Computer Use: Monthly Weather Review, v. 90, p. 26-28.

Penman, H.L., 1948, Natural evaporation from open water, bare soil, and grass: Proceedings of the Royal Society of London, 

Series A, Mathematical and Physical Sciences, v. 193, p. 120-146.

U.S. Environmental Protection Agency, 2001, Better Assessment Science Integrating point and Nonpoint Sources, BASINS, 

Version 3.0: EPA-823-B-01-001, 337 p.

Viessman, W., Lewis, L., and Knapp, J., 1989, Introduction to Hydrology (3d ed.): New York, Harper & Row, 780 p. 

WDMUtil Version 2.0 Users’ Manual, 2001: accessed November 8, 2004, at URL http://www.epa.gov/waterscience/basins/

bsnsdocs.html.

References Cited 7
8 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 
APPENDIXES

Appendixes 9
10 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 
Appendix A: Computing evapotranspiration time series using LXPET (Lamoreux 

Potential Evaporation)

LXPET is a program that computes an hourly potential evapotranspiration (lake evaporation) time series (EVAP) 

using daily average dewpoint temperature (DEWP), daily average temperature (TEMP), daily total wind movement 

(WIND), and daily total solar radiation (SRAD) data files as input. The use of LXPET with input data files created in the GenScn (Generation and Analysis of Model Simulation Scenarios) program (Kittle and others, 1998) is 

described below, but the input data files can be from any source as long as they are properly formatted. Instructions 

for generating input data files using GenScn, correctly formatting these data files, and using LXPET as a DOS application are given in the steps below. Tips on troubleshooting common problems also are included.

Obtaining the data from the WDM (Watershed Data Management) with the Generation and Analysis 

of Model Simulation Scenarios (GenScn) Program 

Note: These instructions are based on GenScn version 2.3 (Kittle and others, 1998). Certain details may 

vary in other versions.

(1) Open GenScn and the WDM file with the DEWP, SRAD, TEMP, and WIND data. 

(2) Select OBSERVED, the constituent (for example, DEWP), and the location (for example, Argonne). Add this 

 combination to your Time Series frame (see figure A1 below).

(3) Enter the beginning and ending dates needed for the computed EVAP.

(4) From the T step, Units menu, select the appropriate units for the data. “Sum” should be selected for SRAD and 

 WIND, “Average” for DEWP and TEMP.

(5) Once the T step unit is chosen, another drop-down box will appear. Make sure “1 Day” appears. If not, select 

 “Day” from the drop-down box.

Appendix A: Computing evapotranspiration time series using LXPET (Lamoreux Potential Evaporation) 11

Figure A1. The Generation and Analysis of Model Simulation Scenarios (GenScn) program with the Argonne National Laboratory in 

Illinois dewpoint time series selected.
12 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 

(6) Select the List option (second from the left in the Analysis menu at the bottom of the screen). 

(7) The date format should be 4-digit year, space, 2-digit month, space, 2-digit day. If the data are not in this for 

 mat, select edit and date format to adjust the format of the date.

(8) Select File and Save to Text File. Save the text file as a space delimited file. Make sure that the data files are 

 saved in the same directory as the LXPET program. Choose a file name with no more than eight characters and 

 an extension with no more than three characters. This file name format is necessary because the data file will be 

 used on a DOS platform.

Repeat steps 4-7 for the other constituents.

Formatting the data for LXPET

(1) Open the data file in a text editor.

(2) Delete the header lines of the file (lines at the top that contain information about the data set). These header 

 lines will cause an error message from LXPET. 

(3) Make sure there are 21 spaces between the last character of the date and the first character of the value for 

 that date. The first character of the value may be a minus sign in the case of temperature and dewpoint files. 

 Make sure there are no tabs, only spaces. The LXPET program does not accept tab characters. An example 

 of a properly formatted file (with the spaces visible) is included as figure A2 below.

(4) Save the changes.

Repeat these steps for each of the other data files.

Running LXPET

(1) Make sure that LXPET and the four input data files are in the same directory.

(2) Run LXPET by either double-clicking on LXPET.exe from the Windows Explorer or typing LXPET at the 

 prompt from a DOS screen or window from its directory. In the DOS window, change to the directory where 

 LXPET and the data files are located (see figure A3 below). No parameters need to be added to LXPET at the 

 command line. It is preferable to start from a DOS window, so the error messages are shown in the DOS win 

 dow if LXPET does not work correctly. If the LXPET.exe is opened by double-clicking from Windows, the 

 DOS window will close automatically if an error occurs.

(3) When prompted, enter the name of the solar radiation data file including the extension. Hit enter when fin 

 ished.

(4) Next, the multiplication factor for solar radiation is requested. Type in 1 for this multiplication factor. Use 1 

 as the multiplication factor for the other data types as well. 

(5) Enter the names of the other data files as prompted by the program.

(6) The last step prompts for the name of the evaporation file. This file is where LXPET saves the computed 

 EVAP time series. Choose a name no longer than eight characters and an extension of no more than three char 

 acters. It is recommended that .hsp is used as the extension. The .hsp format is the default output format and 

 will make it easier to input the results to a WDM file using HSPF.

Open the EVAP file in a text editor. If a blank file, all zeros, or some other unexpected output is listed, then 

refer to the troubleshooting tips below.
Appendix A: Computing evapotranspiration time series using LXPET (Lamoreux Potential Evaporation) 13

Figure A3. The LXPET (Lamoreux Potential Evapotranspiration) program, in a DOS window, with all the prompts displayed.

Figure A2. An example of a properly formatted input file of the dewpoint data for LXPET (Lamoreux Potential Evapotranspiration).
14 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 

Troubleshooting Tips

Problem: The output file shows 24 sets of 24 values for each day.

Cause: This problem is caused by using hourly data instead of daily data. LXPET treats each hour as a day. 

To fix this problem, go back into GenScn and retrieve daily data. Rerun LXPET with daily data.

Problem: The output file shows 0.00 for all the values.

Cause: This type of error could be caused by entering the wrong file name for each data type (for example, 

the file name of the wind data was entered when the program prompted for solar radiation data). Rerun LXPET and 

make sure the right filename is entered for each data type. If this solution doesn’t work, check that the data files are 

named correctly. Another potential cause of this problem is that one or more of the data files were not formatted correctly. As mentioned earlier, there should be 21 spaces between the last character of the date and the first character 

of the value. 

Problem: Evaporation values that look appreciably low.

Cause: One of the data files was not formatted correctly. The LXPET program does not use improperly 

formatted data files in its calculations; therefore, the evaporation would be lower than the correct value.

Problem: Output for evaporation has 0.00 for the beginning and end of the day and ****** characters for the 

rest of the day.

Cause: This problem is caused by the evaporation value being calculated as too high; for example, if 11 

were typed instead of 1 for the multiplication factor for DEWP, TEMP or SRAD. Be aware that if this problem 

occurs from an improperly formatted wind data file, the evaporation values would be larger than the correct values 

but not high enough to make it obvious that a problem is present.

Problem: After the program is run, an error message results as follows:

Invalid decimal character * was detected (unit=**) 

(the relative position causing an error in a record = *)

 DAILY ***** OBSERVED *** at ARGONNE

 ?

 Error occurs at or near line *** of (underscore)getday(underscore)

 Called from or near line *** of (underscore)MAIN(underscore)

Cause: This message appears if the first three lines from the data file(s) are not removed. Delete the first 

three lines of the data file(s) and run LXPET again. 

Problem: While entering the names of the data files, the following error results:

 The value of the STATUS specifier in an OPEN statement does not match the file status (unit=**). Error 

occurs at or near line ** of (underscore)MAIN(underscore)

Cause: This error occurs when the file name entered is not valid. Check to be sure that all required data 

files are in the same directory as LXPET. This error also is caused by misspelled file names entered at the LXPET 

prompt. Also make sure the file name was not misspelled when it was saved in GenScn.
References Cited

Kittle, J.L., Jr., Lumb, A.M., Hummel, P.R., Duda, P.B., and. Gray, M.H., 1998, A Tool for the Generation and Analysis of 

Model Simulation Scenarios for Watersheds (GenScn): U.S. Geological Survey Water-Resources Investigations Report 98-

4134, 152 p.

Appendix A: Computing evapotranspiration time series using LXPET (Lamoreux Potential Evaporation) 15
16 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 

Appendix B: LXPET Fortran code 

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C/* UTILITY PROCEDURE LXPET */

C/* PURPOSE: */

C/* THE LXPET PROCEDURE COMPUTES DAILY EVAPORATION IN 1000THS */

C/* OF INCHES FROM DAILY WIND MOVEMENT (MPD), RADIATION */

C/* (LANGLEYS/DAY), MAX-MIN AIR TEMPERATURE (DEGREES FAHREN- */

C/* HEIT), AND DEWPOINT (DEGREES FAHRENHEIT) USING THE METHOD */

C/* OF KOHLER, NORDENSEN, AND FOX (1955) AS ADAPTED TO */

C/* COMPUTERS BY LAMOREUX (1962). */

C/*

C/* MODIFIED TO READ AVERAGE AIR TEMP, IT IS READ INTO MAX AIR */

C/* ARRAY, THE MIN AIR ARRAY ISN’T USED IN THIS VERSION */

C/* CALLS DISTRIB.FOR TO DISTRIB THE DAILY DATA INTO HOURLY */

C/* DATA */

 INTEGER YR,MTH,DAY

 INTEGER I,J

 INTEGER RFILE,MXFILE,MNFILE,DFILE,WFILE,EFILE

 INTEGER RESULT

 INTEGER B3,B6

 REAL A,A11,A12,A13,A2,A3,A31,A4,A5

 REAL B,B1,B2,B4,B5

 REAL C,C1,C2,C3

 REAL E,TA

 REAL RAD,TMAX,DEWP,WIND,EVAP

 REAL RFAC,MXFAC,DFAC,WFAC
 CHARACTER*64 RFNAME,MXFNAME,DFNAME,WFNAME,EFNAME

C**********************************************************************

C* SETUP CONSTANTS *

C**********************************************************************

 A = 0.0

 A11 = 212.0

 A12 = 0.1024

 A13 = 0.01066

 A2 = 0.0001

 A3 = 0.0105

 A31 = 0.88

 A4 = 0.37

 A5 = 0.0041

 B = 0.0

 B1 = 0.015

 B2 = 398.36

 B3 = -2

 B4 = 68554000000.0

 B5 = -7482.6

 B6 = -1

 C = 0.0

 C1 = 6413300.0

 C2 = 7482.6

 C3 = 398.36

 E = 2.71828

 TA = 0.0

 

C**********************************************************************

C* GET INFORMATION FROM USER *

C**********************************************************************

 WRITE (*,’(1X,A)’) ‘ENTER SOLAR RADIATION FILENAME:’

 READ (*,’(A)’) RFNAME

 RFILE=10

 OPEN (RFILE,FILE=RFNAME,STATUS=’OLD’)

 WRITE (*,’(1X,A)’) 

 $ ‘ENTER MULTIPLICATION FACTOR FOR SOLAR RADIATION:’

 READ (*,’(F10.0)’) RFAC

 IF (RFAC .EQ. 0) RFAC=1.0

 WRITE (*,’(1X,A)’) ‘ENTER AVERAGE TEMPERATURE FILENAME : ‘

 READ (*,’(A)’) MXFNAME

 MXFILE=11

 OPEN (MXFILE,FILE=MXFNAME,STATUS=’OLD’)

 WRITE (*,’(1X,A)’) 

 $ ‘ENTER MULTIPLICATION FACTOR FOR AVERAGE TEMPERATURE:’

 READ (*,’(F10.0)’) MXFAC

 IF (MXFAC .EQ. 0) MXFAC=1.0

 

 WRITE (*,’(1X,A)’) ‘ENTER DEWPOINT FILENAME : ‘

 READ (*,’(A)’) DFNAME

 DFILE=13

 OPEN (DFILE,FILE=DFNAME,STATUS=’OLD’)

 WRITE (*,’(1X,A)’) 

Appendix B: LXPET Fortran code 17
 $ ‘ENTER MULTIPLICATION FACTOR FOR DEWPOINT:’

 READ (*,’(F10.0)’) DFAC

 IF (DFAC .EQ. 0) DFAC=1.0

 

 WRITE (*,’(1X,A)’) ‘ENTER WIND FILENAME : ‘

 READ (*,’(A)’) WFNAME

 WFILE=14

 OPEN (WFILE,FILE=WFNAME,STATUS=’OLD’)

 WRITE (*,’(1X,A)’) 

 $ ‘ENTER MULTIPLICATION FACTOR FOR WIND:’

 READ (*,’(F10.0)’) WFAC

 IF (WFAC .EQ. 0) WFAC=1.0

 WRITE (*,’(1X,A)’) ‘ENTER EVAPORATION FILENAME : ‘

 READ (*,’(A)’) EFNAME

 EFILE=15

 OPEN (EFILE,FILE=EFNAME,STATUS=’UNKNOWN’)

C**********************************************************************

C* READ IN A DAY OF DATA FROM DATA FILES *

C* SOLAR RADIATION FILE DETERMINES WHICH MONTH/DAY WE’RE ON *

C**********************************************************************

100 CONTINUE

C +++ RADIATION DATA +++

 YR = 0

 MTH = 0

 DAY = 0

 CALL GETDAY (RFILE,YR,MTH,DAY,RAD,RFAC,RESULT)

 IF (RESULT.EQ.3) GOTO 900

 IF (RESULT.NE.0) THEN

 WRITE (*,’(1X,A)’) ‘RADIATION FILE : ‘

 GOTO 800

 ENDIF

C +++ AVERAGE TEMPERATURE DATA +++

 CALL GETDAY (MXFILE,YR,MTH,DAY,TA,MXFAC,RESULT)

 IF (RESULT.NE.0) THEN

 WRITE (*,’(1X,A)’) ‘AVERAGE TEMPERATURE FILE : ‘

 GOTO 800

 ENDIF

C +++ DEWPOINT DATA +++

 CALL GETDAY (DFILE,YR,MTH,DAY,DEWP,DFAC,RESULT)

 IF (RESULT.NE.0) THEN

 WRITE (*,’(1X,A)’) ‘DEWPOINT FILE : ‘

 GOTO 800

 ENDIF

C +++ WIND DATA +++

 CALL GETDAY (WFILE,YR,MTH,DAY,WIND,WFAC,RESULT)

 IF (RESULT.NE.0) THEN

 WRITE (*,’(1X,A)’) ‘WIND FILE : ‘

 GOTO 800

 ENDIF

C**********************************************************************

18 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 
Appendix B: LXPET Fortran code 19

C* CALCULATE EVAP *

C**********************************************************************

 C = C1*(E**(-C2/(TA+C3))-E**(-C2/(DEWP+C3)))

 IF (C .LT. 0.00001) C = 0.00001

 B = (B1+((TA+B2)**B3)*B4*E**(B5/(TA+B2)))**B6

 A = E**((TA-A11)*(A12-A13*LOG(RAD)))

 $ -A2 + A3*(C**A31)*(A4+A5*WIND)

 EVAP=B*A*1000.0

 IF (EVAP .LT. 0.0) EVAP=0.0

 CALL DISTRIB(EFILE,YR,MTH,DAY,EVAP)

 GOTO 100

800 CONTINUE

 WRITE (*,2010) RESULT

2010 FORMAT (‘ ERROR : ‘,I2)

900 CONTINUE

 CLOSE (RFILE)

 CLOSE (MXFILE)

 CLOSE (DFILE)

 CLOSE (WFILE)

 CLOSE (EFILE)

 END

C**********************************************************************

C* SUBROUTINE GETDAY / GET A DAY OF DATA FROM FILE *

C**********************************************************************

 SUBROUTINE GETDAY(FILE,TY,TM,TD,DATA,MFAC,RESULT)

C +++ ARGUMENT VARIABLES +++

 INTEGER FILE,TY,TM,TD,RESULT

 REAL DATA,MFAC

 

C +++ LOCAL VARIABLES +++

 INTEGER YR,MTH,DAY,I

1000 FORMAT (I4,1X,I2,1X,I2,20X,F10.2) 

 RESULT = 0

100 CONTINUE

 READ (FILE,1000,END=800) YR,MTH,DAY,DATA

C +++ CHECK TO SEE IF WE GOT THE MONTH/DAY WE WANT +++

 IF ((0.EQ.TY) .AND. (0.EQ.TM) .AND. (0.EQ.TD)) THEN

 TY = YR

 TM = MTH

 TD = DAY

 GOTO 900

 ENDIF

 IF (YR.EQ.TY) THEN

 IF (MTH.EQ.TM) THEN

 IF (DAY.EQ.TD) GOTO 900
 20 Comparison of Potential Evapotranspiration from the LXPET Program and WDMUtil 

 IF (MTH.LT.TM) GOTO 100

 ENDIF

 RESULT=2

 GOTO 900

 ENDIF

 IF (YR .LT. TY) GOTO 100

 RESULT = 2

 GOTO 900

800 CONTINUE

C +++ UNEXPECTED END OF FILE +++

 RESULT = 3

900 CONTINUE

 IF (MFAC .NE. 1.0) THEN

 DATA=MFAC*DATA

 ENDIF

 RETURN

 END

C *****************************************************************

C * SUBROUTINE DISTRIB *

C * CONVERT DAILY EVAP DATA TO HOURLY EVAP DATA BASED *

C * ON DISTRIBUTION FROM LANDS PROGAM *

C *****************************************************************

 SUBROUTINE DISTRIB(EFILE,YR,MTH,DAY,DAYDAT)

 REAL HRDAT(24),DAYDAT,EVAPDIST(24)

 INTEGER YR,MTH,DAY,I,J,EFILE

 DATA EVAPDIST / 0.000,0.000,0.000,0.000,0.000,0.000,0.019,0.041,

 $ 0.067,0.088,0.102,0.110,0.110,0.110,0.105,0.095,

 $ 0.081,0.055,0.017,0.000,0.000,0.000,0.000,0.000/

 DO 200,J=1,24

 HRDAT(J)=DAYDAT*EVAPDIST(J)

200 CONTINUE

 WRITE(EFILE,2000) YR,MTH,DAY,1,(HRDAT(J),J=1,12)

 WRITE(EFILE,2000) YR,MTH,DAY,2,(HRDAT(J),J=13,24)

2000 FORMAT(8X,I4,1X,2(I2,1X),I1,12F5.2)

 RETURN

 END
Murphy—Comparison of Potential Evapotranspiration Calculated by the LXPET Program and by the WMDUtil Program—

Open-File Report 2005-1020

Director

U.S. Geological Survey

Illinois Water Science Center

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Urbana, IL 61801

http://il.water.usgs.gov