Temperature Adjustment for Reference Evapotranspiration Calculation in Central Arizona

A key component in the calculation of reference crop evapotranspiration $\left({\mathrm{ET}}_r\right)$ is the weather data. If the weather data have been collected from a station under nonreference conditions, the data itself may contain errors, which will in turn yield inaccurate ${\mathrm{ET}}_r$ estimates. It was proposed by Allen in 1996 that data used for evapotranspiration be scrutinized by comparing daily minimum temperature $\left(T_{\min }\right)$ and the daily average dew point temperature $\left(T_{\mathrm{dew}}\right)$. If the difference between $T_{\min }$ and $T_{\mathrm{dew}}$ is greater than $3°C$, then the site is considered to be arid (nonreference) and adjustments are recommended for temperature and dew point data. In Arizona, normal weather conditions often occur where $T_{\min }$ and $T_{\mathrm{dew}}$ do not approach each other. This study examined the appropriateness of applying the conditions set forth by Allen to temperature data collected in central Arizona. Two weather stations were set up in a $35.5\mathrm{ha}$ alfalfa field in central Arizona to measure dry bulb and wet bulb temperatures. Additionally, plant temperature data were collected to verify field conditions. Daily data were taken for 1.5 years at the University of Arizona’s Maricopa Agricultural Center. Of the 611 days of data collected, the difference between $T_{\min }$ and $T_{\mathrm{dew}}$ was greater than $3°C$ on 329 days, indicating that these data were not taken under reference conditions. Among these data, 178 days were verified as nonreference but 151 were verified as actually being under reference conditions. Making adjustments for these days (151 days) resulted in a $47\mathrm{mm}$ decrease in ${\mathrm{ET}}_r$ estimation, which mostly occurred during the summer.