ESTIMA TION OF MONTHLY REFERENCE EVAPOTRANSPIRA TION (ETO) FOR AREAS WITH IN ADEQUATE METEOROLOGICAL DATA IN ETHIOPIA

Abstract

Often long-term measured meteorological data are
not available or in adequate in water resources
development areas such as dam and irrigation
sites. Under this condition, design estimates of
monthly evaporation from resenJoirs and monthly
evapotrampiration from crops are unreliable.
Estimation of mean 11I0nth~vevaporation, in this
case, can he obtained using regional regression
methods that relate evaporation with readi(v
available data such as altitude.
In this paper month~v reference evapotranspiration
(£1'0)- altitude regression equations are developed
for the country usingI25 stations meteorological
data. ETo are calculated based on radiation
(sunshine hours), temperature, relative humidity,
and wind speed data. FA O-Penman Monteith
formula is applied to calculate monthly ETo for
125 stations. The developed relationships are valid
over the altitude range from 100 to 3000 meters
above mean see level.
The developed equations can he used in areas
(irrigation and dam sites) where meteorological
data such as radiation (sunshine hours),
temperature, relative humidi~v, and wind speed are
not available. A{ean month~v evaporation from
reservoirs, ponds, and lakes are also estimated by
multip~ving the month~v ETo by 1.2. Estimate of
monthly crop evapotranspiration is made by
multip~ying month~y ETo by corresponding monthly
crop coefficients.