Crop yield response to water - Cra

almond had its highest Gl of close to 1 cm/s at 12:00 hours, and pistachio reached an evengreater Gl value of 1.15 cm/s at 10:00 hours, more than twice the maximum citrus Gl. Bothpistachio and almond maintained similar high values throughout most of the day.Since citrus is an evergreen plant, many water use studies report a single crop coefficient(K c ) value. These include 0.62 for Valencia in Sunraysia (Grieve, 1989), 0.44 for clementines inMazagon, Spain (Villalobos et al., 2009), and 0.52 for lemons in Ventura, California (Grismer,2000). Others have divided the season into winter and summer and suggested that the K c was0.70 and 0.65, respectively. They suggested increasing these values by 0.1 or 0.2 for humid andsemi humid regions (Allen et al., 1998).Many studies indicate that, compared to the summer, the citrus K c is slightly higher in thewinter and early spring and appreciably higher in the autumn. This is generally attributedto the citrus stomata being sensitive to evaporative demand (the air vapour pressure deficit,VPD), closing under dry, hot, windy conditions and opening under the opposite conditions.Thus, the K c in the mild Mediterranean and coastal climates should be higher than those ofmore arid, inland valleys. In addition, in Mediterranean environments, high K c values in winterreflect high soil evaporation rates from frequent rainfall during that part of the season. Notall studies found that the K c was minimum in the summer. One study for cv. Valencia (Hoffmanet al., 1982) and another for navels (Chartzoulakis et al., 1999) reported just the opposite.The monthly K c values published in six studies are summarized in Table 1. These include the cvs.Salustiana and Valencia under a variety of different climatic conditions. Spring K c ranged from0.49 in Valencia, Spain (navel) to 0.77 (Valencia) in Kiyú, Uruguay with a mean value of 0.63.For the summer, minimum and maximum values were 0.52 for cv. Shamouti in Rehovot, Israeland 0.87 for cv. Valencia in Kiyú, Uruguay, respectively, with a mean value of 0.66. Autumnminimum and maximum values were 0.58 for Shamouti in Rehovot, Israel and 0.85 for navelsin Tempe, Arizona, respectively, with a mean value of 0.73.Working with drip irrigated clementines in a precise weighing lysimeter in Valencia, Spain,Castel found that the annual K c was linearly related to ground cover and reported thefollowing relationship:K c = 0.006 ground cover + 0.272 (R 2 = 0.96)He also emphasized that the K c will depend on the frequency of wetting of the orchard floor;he found that without rain, surface evaporation for young drip irrigated trees varied between8 to 30 percent of total ET c while after a rain, it reached 30 to 50 percent of total (Castel, 1997).Actual annual ET c values will, of course, depend not only on the K c but on evaporativedemand and, to some degree, irrigation frequency and the amount of wetted surface area.The range of reported ET c worldwide include 820-1 200 mm in Florida and 1 080-1 500 mmin Arizona, Unites States, 1 300 mm in South Africa, and 800- 850 mm for the coastal areasof East Spain and Israel.322crop yield response to water