poster - International Conference of Agricultural Engineering

indicates that ET o will increase/decrease as the variable increases. The larger the sensitivity
coefficient, the larger effect a given variable has on ET o . In graphical form, the sensitivity
coefficient is the slope of the tangent at the origin of the sensitivity curve. A sensitivity
coefficient of 0.1 for a variable would in this case mean that a 10% increase of that variable,
while all other variables are held constant, may increase ET o by 1%. Many researchers have
used this methodology to characterize sensitivity models (Saxton, 1975; Coleman and
DeCoursey, 1976; Piper, 1989; Meyer et al., 1989; Rana and Katerji, 1998) using different
variables and equations. In the present study, sensitivity coefficients for each station were
calculated on a daily basis for air temperature, wind speed, relative humidity and solar
radiation. Monthly and yearly average sensitivity coefficients were obtained by averaging
daily values.
3. Results and Discussion
Global warming due to greenhouse effect is expected to cause major changes in climate of
some areas (Goyal, 2004). The increase in temperature affects ET o primarily by increasing
the capacity of air to hold water vapor. Daily sensitivity coefficients exhibit large fluctuations
during the year at all locations. The same feature has also been reported by Hupet and
Vanclooster (2001) and Gong et al. (2006). In general, net radiation (R n ) had large sensitivity
coefficients thought the year at all cities. At all locations, the coefficients for temperature are
systematically and significantly lower very close from zero to 0.15 than all other variables,
but all variables exhibited large fluctuations during the year. Daily variation patterns of
sensitivity coefficients for air temperature (S T ) agree with those of air temperature. ET o is
insensitive to temperature in winter and the sensitivity gradually increases and achieves its
maximum value in summer (Fig. 1). The similar patterns of ST and air temperature indicate
that air temperature determines the extent of the seasonal variation of ST. Fig. 1 shows that
ET o is most sensitive to wind in winter season in Baft, Bam and Kerman, but, the fluctuations
of the sensitivity coefficients of wind speed (S U2 ) is large in all of the day in Jiroft (Fig. 1d). In
general, temperature and wind speed were less influential to ET o and their sensitivities were
similar to each other in warm months. Sensitivity coefficients of vapor pressure deficit (S VPD )
shows a pronounced seasonal cycle with a maximum value in January, February, March,
October, November and December, similar to the seasonal cycle of the S U2 . The effects of
VPD, which increases the evaporation, reached higher levels during the winter and cold
months. As it shown in Figs. 1a, b, c, S U2 and S VPD had a similar pattern in all cities. In
winter, 10% change in VPD could cause approximately a 5%, 3.7%, 3.8% and 1.8% change
in ET o in Kerman, Baft, Bam and Jiroft, respectively. But in summer months, 10% change in
VPD could cause a 3.5%, 2.7%, 2.5% and 1.7% change in ET o in these cities. There was
also a considerable difference Between Jiroft from others. The sensitivity coefficients of U 2 ,
VPD and T are changes between zero to 0.4 in Jiroft. The sensitivity of evapotranspiration
rates to changes in temperature and wind speed is less in Jiroft than in other study areas. In
general, R n was the most sensitive variable at the daily scale in all of cities. Strong positive
sensitivity coefficients indicated that increases in Rn greatly induce the potential
evapotranspiration. In summer, 10% change in Rn could cause approximately a 6.5%, 7.4%,
7.5% and 8.3% change in ET o in Kerman, Baft, Bam and Jiroft, respectively. In other words,
The ET o estimates were very sensitive to R n , with an annual mean value of 0.58, 0.68, 0.70
and 0.83 in Kerman, Bam, Baft and Jiroft, respectively (Table 2). This confirms that the
radiation term in Penman-Monteith equation is generally dominant over the aerodynamic
term in the prediction equation as reported by Beven (1979).
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