Estimating Long-Term Average Monthly
Evapotranspiration from Pan Evaporation Data at
Seven Locations in Puerto Rico
Eric Harmsen1, Antonio Gonzalez2 and Amos Winter3
Abstract: Pan evaporation is a method widely used to schedule irrigation because it is easy
and inexpensive to use. The UPR Agricultural Experiment Station promotes this method in their
“Conjunto Tecnologico” guidance publications for various crops. Reference evapotranspiration,
needed to estimate crop water use, is calculated by multiplying the pan coefficient (Kp) times the
pan evaporation rate. During the 1980s, the Experiment Station developed monthly Kp values
for the seven Agricultural Experiment Substation in Puerto Rico using the equation: Kp =
ETo/Epan, where ETo is the long-term average monthly reference evapotranspiration and Epan is
long-term average monthly pan evaporation. The Substations are located in Adjuntas, Corozal,
Fortuna, Gurabo, Lajas, Río Piedras, and Isabela, PR.
In recent studies, the method used to calculate reference evaporation in the above
equation (NRCS Blaney Criddle method) has been found to produce large errors (over
estimation on average by 17% in humid regions and underestimation on average by 16% in arid
regions). Currently, the United Nations Food and Agriculture Organization (FAO) recommend
the Penman-Monteith method for calculating reference evapotranspiration over all other
methods. The method has a high degree of accuracy because it incorporates the effects of
minimum and maximum air temperature, dew point temperature, wind speed and solar
radiation.
The goal of this project was to update pan coefficients values for the seven Substations
using the Penman-Monteith reference evapotranspiration, and to incorporate twenty years of
additional pan evaporation data. Long-term monthly reference evapotranspiration was
estimated using a recently developed computer program called PR-ET, which estimates the
Penman-Monteith reference evapotranspiration at any location in Puerto Rico. The program
uses estimation procedures to obtain long-term average monthly climate parameters required
by the Penman-Monteith method. Updated pan evaporation data were obtained from NOAA
Climatological Data Summary Sheets.
Statistical evaluation of trends in pan evaporation over the last forty years is also
presented. Preliminary analysis indicate that average pan evaporation during the last twenty
years is higher than during the previous twenty years at all of the Substations except at the
Lajas Substation. The increasing observed pan evaporation rates are inconsistent with global
trends, which indicate decreasing pan evaporation rates. The global downward trend in
evaporation rates has been explained by the observed reduction in solar irradiance and the
associated changes in diurnal temperature range and vapor pressure deficit.
1
Associate Professor, Department of Agricultural and Biosystems Engineering, University of Puerto Rico,
Mayagüez, PR 0061. email: eric_harmsen@cca.uprm.edu
2
Undergraduate Research Assistant, Department of Agricultural and Biosystems Engineering, University
of Puerto Rico, Mayagüez, PR 0061.
3
Professor, Department of Marine Science, University of Puerto Rico, Mayagüez, PR 0061.
2
Background
The pan evaporation method is widely used for irrigation scheduling. The method is popular
because it is inexpensive and easy to apply. Currently in Puerto Rico, the University of Puerto
Rico (UPR) Agricultural Experiment Station is promoting its use for vegetable production ( REF).
The method involve multiplying the pan evaporation rate by a pan coefficient to obtain the
reference evapotranspiration.
ETo = Kp Epan
where ETo is reference evapotranspiration, Kp is the evaporation pan coefficient and Epan is
the evaporation rate. The potential crop evapotranspiration is then equal to the reference
evapotranspiration times a crop coefficient:
ET = Kc ETo
where ET is the potential crop evapotranspiration and Kc is the crop coefficient. During the
1980s Goyal and ___ (1989) developed monthly average pan coefficient based on long term
evaporation data records. These records covered the period from the early 1960s to 1980. The
reference evapotranspiration was estimated using the Soil Conservation Service Blaney-Criddle
method (REF). This method has been shown to produce significant errors in both humid and
arid regions as compared to weighing lysimeter data. Harmsen et al. (2001) reported large
differences between the SCS Blaney-Criddle method and the Penman-Monteith method in a
study that compared seasonal consumptive use for pumpkin and onion at two locations in
Puerto Rico. The maximum observed differences were on the order of 100 mm per season.
Because of the potential errors associated with the SCS Blaney-Criddle method, and because
approximately twenty years of pan evaporation data are now available, reevaluation of the Kp
values in Puerto Rico is merited. The goal of this study was to update the Kp values of the
seven Agricultural Experiment Substations by using more recent pan evaporation data and
using a more accurate method for estimating the reference evapotranspiration. As part of the
study, long-term pan evaporation rates were evaluated to access the effects of climate change.
The United Nations Food and Agriculture Organization (FAO) has recommended the PenmanMonteith method as the single method for estimating reference evapotranspiration throughout
the world. FAO recommends using the Penman-Monteith method over all other methods even
when local data are missing. Studies have shown that using estimation procedures for missing
data with the Penman-Monteith equation will generally provide more accurate estimates of ET o
than will other available methods requiring less input data (Allen et al., 1998).
Methodology
Pan Evaporation Data
Historical pan evaporation data were evaluated. As part of the study by Goyal and Gonzalez
(1989), Kp values were based on data from the early 1960s through 1980. In this study we
incorporate data from 1981 through 2002. One dataset (Lajas) includes data from 1949 through
2002. The goal of the analysis was to determine if the
most recent twenty year period was significantly different than the previous twenty year period, if
it was, to derive the new pan coefficient on the new data.
3
Reference Evapotranspiration
The evapotranspiration term in equation 1 was estimated from the following equation:
ETc = Kc ETo
(3)
where Kc is the crop coefficient (dimensionless) and ETo (mm/day) is the reference
evapotranspiration obtained using the Penman-Monteith method. Initial values of the crop
coefficient were obtained from the literature for sweet pepper for the initial, mature and end crop
stages (FAO Paper No. 56). Adjustments of Kc were made during the calibration of equation 1
as described later in this section. ETo was estimated on a daily basis using a spreadsheet
program. The calculation methodology is described by Allen, et al. (1998). ETo is a function of
the average daily measured solar radiation, minimum and maximum air temperature, relative
humidity, and wind speed. These data were obtained from the weather station located near the
study area. Wind speeds obtained from the 10 m high tower were adjusted to the 2 m wind
speed, required by the Penman-Monteith method, by means of an exponential relationship.
Evaporation Pan Coefficient
Pan evaporation coefficients were estimated from the following
Results and Discussion
Evaporation (inches)
Evaluation of Historical Pan Evaporation Data
10
8
6
4
2
0
1 2 3 4 5 6 7 8 9 10 11 12
Adjuntas
Corozal
Fortuna
Gurabo
Isabela
Lajas
Rio Piedras
Month
Long term average pan evaporation for the seven sites. Note that Fortuna is theh greatest.
4
Pan Evap (in)
Lajas Pan Evaporation Data
10
January
July
Linear (January)
Linear (July)
5
0
1940 1960 1980 2000 2020
Year
Lajas shows significant downward trend in pan evaporation for January and July.
Decreasing Pan Evaporation
Evaporation
(inches)
9.00
8.00
7.00
Lajas
Rio Piedras
6.00
5.00
4.00
3.00
1950
1960
1970
1980
1990
2000
Year
Fortuna and Rio Piedras showed a significant downward trend in the average monthly pan
evaporation
5
Pan Evaporation
Evaporation
(inches)
7.00
6.00
Adjuntas
Gurabo
5.00
4.00
3.00
1960
1970
1980
1990
2000
Year
Adjuntas and Gurabo showed a significant upward trend in the average monthly pan
evaporation
Pan Evaporation
Evaporation
(inches)
9.00
8.00
7.00
Isabela
Corozal
Fortuna
6.00
5.00
4.00
3.00
1960
1970
1980
1990
2000
Year
Corozal, Isabela and Fortuna did not show any significant trends in average monthly pan
evaporation
6
Reference Evapotranspiration
Evaporation (inches)
6
Adjuntas
Corozal
Fortuna
Gurabo
Isabela
Lajas
Rio Piedras
5
4
3
2
1 2 3 4 5 6 7 8 9 10 11 12
Month
ETo (mm/day)
Long term average reference evapotranspiration.
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
2/13
Isabela
Fortuna
4/4
5/24
7/13
Date
Four months during 2002, ETo for Isabela exceeded ETo for Fortuna by 10%
7
Pan Evaporation Coefficients
Pan Evaporation Coefficients Based on 1981 through 2002 pan evaporation data.
Adjuntas
Corozal
Fortuna
Gurabo
Isabela
Lajas
Rio Piedras
Average
Jan
0.82
0.90
0.63
0.67
0.82
1.07
0.91
0.83
Feb
0.92
0.95
0.69
0.76
0.88
1.18
0.93
0.90
Mar
0.86
0.86
0.66
0.70
0.80
1.06
0.84
0.82
Apr
0.89
0.88
0.68
0.71
0.78
1.06
0.82
0.83
May
0.87
0.89
0.73
0.75
0.83
1.01
0.88
0.85
Jun
0.81
0.84
0.72
0.72
0.85
1.10
0.87
0.85
Jul
0.83
0.83
0.69
0.73
0.82
1.12
0.84
0.84
Aug
0.84
0.84
0.67
0.72
0.83
1.04
0.89
0.83
Sep
0.83
0.90
0.73
0.78
0.87
0.95
0.92
0.85
Oct
0.85
0.90
0.71
0.75
0.87
1.05
0.88
0.86
Nov
0.87
0.96
0.69
0.76
0.89
1.09
0.93
0.88
Dec
0.82
0.89
0.64
0.70
0.84
1.10
0.93
0.84
Annual
0.85
0.89
0.69
0.73
0.84
1.07
0.89
0.85
Values of kp greater than one for Lajas indicates that the long term average reference
evapotranspiration was greater than the pan evaporation.
Summary and Conclusions
Investigation should be undertaken to explain the significant decrease in the evaporation at
Lajas. Also, a study in which actual evapotranspiration is measured should be conducted to
confirm the values of the reference evapotranspiration used in this study.
References
Goyal, M. and E. A. Gonzalez Fuentes, 1989. Coeficiente de Evaporimetro para Puerto Rico. J.
Agric. Univ. P.R. 73 (1):00
Zapata, R., R. R. López, and D. Vázquez, 2001. Collection of Meteorological Data for
Improvement of Building Codes for Puerto Rico. Proceedings of the Sixth Caribbean
Islands Water Resources Congress. Editor: Walter F. Silva Araya. University of Puerto
Rico, Mayagüez, PR 00680.
8