Field estimates of alfalfa evapotranspiration by energy balance-resistance modelling

Abstract

Estimates of mean daily evapotranspiration using the aerodynamic resistance-surface energy balance (ARSEB) model of Monteith were obtained for three periods during 3 September (day of year 246) to 15 November (day 319) 1983 in an alfalfa (Medicago sativa L.) field in central Arizona near Coolidge. These estimates were compared with Penman method estimates, within situ soil water balance (SWB) method estimates, and with long-term soil water depletion-based estimates of evapotranspiration for central Arizona. The SWB method estimates were obtained from soil moisture data collected to 1.5 m depth at 60 field sites, and to 1.8 m depth at 16 of the 60 sites. During relatively wet field conditions in September, daily estimates from the ARSEB method were on average about 19% to 23% lower than estimates from the Penman and SWB methods, respectively. However, for these wetter conditions, soil moisture data were only collected to 1.5 m depth. Thus, the SWB method may have overestimated evapotranspiration because drainage below the crop root zone was not measured. During drier conditions later in the season, ARSEB estimates of daily evapotranspiration were on average about 5% greater than Penman estimates. Also, ARSEB estimates were on the average about 24% to 35% greater than SWB method estimates. Agreement was good between ARSEB method estimates and long-term alfalfa evapotranspiration estimates by Erie et al. (1981) for central Arizona.