Prediction of modified class a pan evaporation using radiation, temperature and wind speed data
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Date
1993
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Sokoine University of Agriculture
Abstract
A simple evaporation model relating pan evaporation (Ep) to shortwave solar
radiation (RJ, mean daily temperature (T) and wind speed (W) developed for
different climatic conditions was calibrated for the Malawi conditions. The objectives
of the study were to examine the possibility of using the model to estimate pan
evaporation where such data are missing either because no such readings are
recorded or a very short record is available whose extension is sort for various
purposes. The model was also examined for use in areas where no shortwave solar
radiation is measured and also for the possibility of using the model to estimate
reference evapotranspiration (ETO) for such areas.
Three stations, Bvumbwe, Chitedze and Ngabu were used in this study. The stations
represent different climatic conditions within Malawi. Five-day averages of pan
evaporation, shortwave radiation, mean daily temperature and wind speed were
computed from data collected by the Meteorological Department between 1985 and
1988 inclusive. Data for 1985 and 1986 were used to calibrate the models and the
rest were used for validation. A statistical software package (MSTATC) was used
to calibrate the models using regression techniques while a climate version of
INSTANT package was used for computing Rs and ETO by the modified Penman
equation as presented by Doorenbos and Pruitt (1977). The models calibrated from
measured Rs were found to be adequate for the three stations used in the study. All
the models estimated pan evaporation to within 6%. Models developed from R, computed from tables were found to be slightly superior to models developed from
measured solar radiation in that the variables included accounted for an average of
75% and 66% of the variability in the response for Ngabu and Chitedze respectively,
as opposed to 74% and 56% for measured R,, despite the fact that only one year of
data was available for their calibration. Correlation coefficients between observed
pan evaporation and computed evaporation were high. High correlation coefficients
(r=95%) were also observed between reference evapotranspiration and evaporation
computed from models derived from tabulated solar radiation indicating that
computed evaporation represents ETO well.
These results indicate that the model is suitable for the Malawian climatic conditions
and can be used to estimate evaporation where no such measurements are made and
also to estimate both missing pan evaporation and ET
Description
Dissertation
Keywords
Temperature, Radiation, Wind, Wind speed