Latent heat flux over a furrow-irrigated tomato crop using Penman–Monteith equation with a variable surface canopy resistance

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The Penman–Monteith (P–M) model with a variable surface canopy resistance ( r c) was evaluated to estimate latent heat flux (LE) or crop evapotranspiration (ET) over a furrow-irrigated tomato crop under different soil water status and atmospheric conditions. The hourly values of r c were computed as a function of environmental variables (air temperature, vapor pressure deficit, net radiation, and soil heat flux) and a normalized soil water factor ( F), which varies between 0 (wilting point, θ WP) and 1 (field capacity, θ FC). The Food and Agricultural Organization (FAO-56) method was also evaluated to calculate daily ET based on the reference evapotranspiration, crop coefficient and water stress coefficient. The performance of the P–M model and FAO-56 method were evaluated using LE values obtained from the Bowen ratio system. On a 20 min time interval, the P–M model estimated daytime variation of LE with a standard error of the estimate (SEE) of 46 Wm −2 and an absolute relative error (ARE) of 3.6%. Thus, daily performance of the P–M model was good under soil water content ranging from 118 to 83 mm ( θ FC and θ WP being 125 and 69 mm, respectively) and LAI ranging from 1.3 to 3.0. For this validation period, the calculated values of r c and F ranged between 20 and 114 s m −1 and between 0.87 and 0.25, respectively. In this case, the P–M model was able to predict daily ET with a SEE of 0.44 mm h −1 (1.1 MJ m −2 d −1) and an ARE of 3.9%. Furthermore, the FAO-PM model computed daily ET with SEE and ARE values of 1.1 mm h −1 (2.8 MJ m −2 d −1) and 5.2%, respectively.