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The effect of four different irrigation levels on the marketable yield and economic return of summer-growth lettuce was evaluated during 2005 and 2006 in Eastern Sicily, Italy. The viability of deficit irrigation was evaluated by estimating optimum applied water levels. Actual evapotranspiration ([ET.sub.0]) was estimated by combining pan evaporation measures and the Penman-Monteith approach ([ET.sub.0-PM]). The highest marketable yield of lettuce was recorded for plots receiving 100% [ET.sub.0-PM]. For deficit irrigated plots, reductions in crop production were ascribed to a decrease in lettuce weight. Crop coefficients equal to 1 determined maximum crop production values. Crop water use efficiency was maximum at a 100% [ET.sub.0-PM] level of water applied, corresponding to yield of 0.3 t [ha.sup.-1] [mm.sup.-1]. Irrigation water use efficiency reached its maximum at a 40% [ET.sub.0-PM] level, with values of 0.54 and 0.44 t [ha.sup.-1] [mm.sup.-1] during 2005 and 2006, respectively. Water applied and marketable yield of lettuce showed a significant quadratic relationship. Cost functions had a quadratic form during 2005 and a linear form during 2006. In the land-limiting condition the optimal economic levels fit the agronomic ones well. In the water-limiting condition, ranges of water deficit of 15-44% and 74-94% were as profitable as full irrigation, thus contributing to appreciable water savings. DOI: 10.1061/(ASCE)0733-9437(2008)134:4(437) CE Database subject headings: Economic factors: Evapotranspiration; Irrigation scheduling: Water management; Crops; Italy.

Crop evapotranspiration ([ET.sub.c]) and crop coefficient ([K.sub.co]) values of four clean-cultivated navel-orange orchards that were irrigated with microsprinklers, having different canopy features (e.g., age, height, and canopy cover) were evaluated. Half-hourly values of latent heat flux density were estimated as the residual of the energy balance equation using measured net radiation ([R.sub.n]), soil heat flux density (G), and sensible heat flux density (H) estimated using the surface renewal method. Hourly means of latent heat flux density (LE) were calculated and were divided by the latent heat of vaporization (L) to obtain ETC. Crop coefficients were determined by calculating the ratio [K.sub.co]=[ET.sub.c]/[ET.sub.o], with reference evapotranspiration ([ET.sub.o]) determined using the hourly Penman-Monteith equation for short canopies. The estimated [K.sub.co] values ranged from 0.45 to 0.93 for canopy covers having between 3.5 and 70% ground shading. The [K.sub.co] values were compared with KC values from FAO 24 (reported by Doorenbos and Pruitt in 1975) and FAO 56 (reported by Allen et al. in 1998) and with [K.sub.c] values from research papers that estimated reference ET from pan evaporation data using the FAO 24 method. The observed [K.sub.co] values were slightly higher than [K.sub.c] values for clean-cultivated orchards with high-frequency drip irrigation in Arizona and were slightly lower than for nontilled orchards in Florida. The K,.o values were considerably higher than [K.sub.c] values from FAO 24 and FAO 56 and were higher than [K.sub.c] values from border-irrigated orchards near Valencia, Spain. CE Database subject headings: Evatranspiration; Crops; Coefficients; Irrigation.

Photosynthetically active radiation (PAR) intercepted by orange orchards (Frost Nucellar navel) having different canopy sizes was measured to determine the relationships with crop coefficient ([K.sub.co] and [K.sub.cr]) values and crop evapotranspiration (ET) ([ET.sub.c]). Three separate experiments were carried out near Lindsay, Calif. during the months of July and August 2004 to compute the fraction of light PAR intercepted by mature and immature orange orchards. Periodic readings of PAR data were compared with near simultaneous measurements of net radiation [R.sub.n] (mV), heat transfer through exposed flux plates [F.sub.h] (mV), and incident total solar radiation [R.sub.s] (mV). The PAR data were used to calculate canopy light interception and the results were compared with those computed from the [F.sub.h] and [R.sub.s] data. The other sensors were studied as possible substitutes for the more expensive PAR light bar. Light interception by the different canopies was related to crop coefficient ([K.sub.co] and [K.sub.cr]) values that were determined by micrometeorological measurement of [ET.sub.c] and Penman-Monteith reference evapotranspiration [ET.sub.o] and [ET.sub.r]. CE Database subject headings: Evapotranspiration; Crops; California; Irrigation; Measurement; Light, natural; Radiation.