ESTIMATION OF PAN EVAPORATION COEFFICIENTS - Management, Performance, and Applications of Micro Irrigation Systems - page 105

K p = ET o/ E pan (3)

where: K p is the pan coefficient; ET o is reference or potential ET; and E pan is
the pan evaporation rate. Mean daily values of pan evaporation were derived
from a University of Puerto Rico Agricultural Experiment Station document
Climatological Data from the Experimental Substations of Puerto Ric o [12].
Goyal and González [11] estimated the potential ET by using the Soil Conser-
vation Service (SCS) Blaney-Criddle method [13]. In a recent study by the
American Society of Civil Engineers (ASCE), the SCS Blaney-Criddle method
was found to produce large errors relative to weighing lysimeter data indicat-
ing overestimation on average by 17% in humid regions and underestimation
on average by 16% in arid regions [14].
In a study that compared seasonal consumptive use for pumpkin and onion
at two locations in Puerto Rico, Harmsen et al. [15] reported large differenc-
es between the SCS Blaney-Criddle method [estimates obtained from Goyal
1989, 11] and the Penman-Monteith method. The Penman-Monteith ap-
proach used crop coeffi cients as determined by the FAO procedure [10]. Crop
stage durations, used to construct the crop coeffi cient curves, were based on
crop growth curve data presented by Goyal [16]. The maximum observed dif-
ferences in the estimated seasonal consumptive use were on the order of 100
mm per season. The study concluded that large potential differences can be
expected between the SCS Blaney-Criddle and the Penman-Monteith meth-
ods, with under-estimations in some months and overestimations in other
months.
Because of inherent errors associated with the SCS Blaney-Criddle meth-
od, the published values of K p for Puerto Rico may not be accurate. The United
Nations Food and Agriculture Organization (FAO) currently recommends
using the ratio of pan evaporation divided by the Penman-Monteith-estimat-
ed reference ET for calculating the pan coeffi cient [10]. The Penman-Monteith
based reference ET was found to have a high degree of accuracy in the above-
mentioned ASCE study [14], with errors not exceeding ±4%.
This chapter indicates how to update pan coeffi cient values for the seven
substations in Puerto Rico using the Penman-Monteith reference ET, and to
incorporate 20 years of additional pan evaporation data. As part of the study,
long-term trends in pan evaporation data were evaluated.

8.2 MATERIALS AND METHODS
8.2.1 PAN EVAPORATION DATA
Historical pan evaporation data were evaluated to determine whether de-
creasing or increasing trends existed in the data. Roderick and Farquhar [17]
and Ohmura and Wild [18] have reported that pan evaporation rates have
been decreasing globally. The cause of the decrease has been attributed to the
decrease in solar irradiance (during the last decade) and changes in diurnal
temperature range and vapor pressure deficit [17]. If in fact pan evaporation