calculate daily and weekly grass reference evapotranspiration (ETo) using the FAO Penman- Monteith method (Allen et al.,1998): 900 0.408A(R, G) + 90 (e, -e) ET =-- T+273 (3-1) A A+ (1+0.34u2) where ETo reference evapotranspiration (mm/d), Rn net radiation at the crop surface (MJ/mm/d) G soil heat flux density (MJ/m2/d) T air temperature at 2 m height (C), U2 wind speed at 2 m height (m/s), es saturation vapour pressure (kPa), ea actual vapor pressure (kPa), es-ea saturation vapour pressure deficit (kPa), A slope vapour pressure curve (kPa/C), 7 psychrometric constant (kPa/C), Crop ET, is calculated by multiplying grass reference ETo by a theoretical or field estimated crop coefficient, Kc. Kc values According to Allen et al. (1998), crop coefficients can account for specific crop characteristics, variations in climate, and modified management practices to provide an accurate representation of crop water demand, ETc. Although recommended Kc values exist for a wide variety of crops and regions throughout the country, the estimation of Kc values should reflect the local microclimate and growing practices. To investigate differences between Kc values, three different published Kc values were compared to a field estimated K,. The published values used in the analysis are given in Table 3-2. The first Kc value, KCIFAS, was taken from the Vegetable Production Guide for Florida (Maynard and Olsen, 2001). The second value, KCFAO, was based on recommendations given in FAO Irrigation and Drainage Paper No. 56 (Allen et. al. 1998) for crops grown on plastic mulch.