weather-based estimates of evapotranspiration from a reference crop such as grass (ET o ) or alfalfa (ET r ) by an empirically derived crop coefficient ( K c ) used to account for specific conditions of the crop ( Allen et al., 1998 ; Snyder et al., 1987a
David R. Bryla, Thomas J. Trout, and James E. Ayars
Giovanni Piccinni, Jonghan Ko, Thomas Marek, and Daniel I. Leskovar
determined. The use of on-site microclimatological data and crop coefficients enable growers the determination of crop water use in a reliable, usable, and affordable format. The concept of “crop coefficient” (K C ) was introduced by Jensen (1968) and
Daniel I. Leskovar, Shinsuke Agehara, Kilsun Yoo, and Nuria Pascual-Seva
in more regions, especially in arid and semiarid climates of the world ( FAO, 2002 ). Furthermore, the application of growth stage-specific crop coefficients (K C ) for irrigation management in onion can provide precise water applications to meet crop
Andrew G. Reynolds, Amal Ehtaiwesh, and Christiane de Savigny
relative humidity, wind speed, solar radiation, and temperature values, which are downloaded from databases such as the Weather Innovations Network in Ontario. ET o values can then be used with a crop coefficient (normally based upon canopy volume) to
U.K. Schuch and D.W. Burger
Twelve species of woody ornamental plants were grown for 2 years in containers at Riverside and Davis, Calif., to determine plant water use (WU) and compare crop coefficients (Kcs). WU was determined gravimetrically in 1993 and 1994, five times each year in Riverside and four times each year in Davis. WU and Kc were affected by significant interactions among species, location, and time of year. WU was primarily influenced by the month, while Kc was most affected by location. Rhaphiolepis and Pittosporum, followed by Juniperus and Photinia, respectively, were the highest water users in Riverside when averaged over the 2 years. Arctostaphylos was the highest water user in Davis, followed by Juniperus, Cercis, and Pittosporum, respectively. Rhamnus, Prunus, and Cercocarpus were among the lowest water users in both locations. Heteromeles, Buxus, and Ceanothus were intermediate water users. The largest difference in species WU between the two locations was found for Arctostaphylos and Cercis, both high water users in Davis, but moderate or low water users in Riverside. The other species ranked similarly in both locations. Kcs of the 12 species, when averaged over the 2-year sampling period, ranked similar to water use. Kcs tended to be artificially high in the winter months and were not correlated to the low WU during that time.
Craig A. Storlie and Paul Eck
Inexpensive weighing lysimeters ($1475/unit) were constructed for measuring evapotranspiration of young highbush blueberries (Vaccinium corymbosum L.). The use of a single load cell and other design characteristics decreased lysimeter measurement accuracy but minimized lysimeter construction costs. Measurement error was within ±3%. Crop coefficient (CC) curves for 5- and 6-year-old `Bluecrop' highbush blueberry plants in their third and fourth year of production were generated using reference evapotranspiration and crop water use data from the 1991 and 1992 growing seasons. The CC increased during leaf expansion and flowering in the spring to its maximum value of about 0.19 in 1991 and 0.27 in 1992 and remained near these values until leaves began senescing in the fall. Water use on sunny days during June, July, and August ranged from (liters/bush each day) 3.5 to 4.0 in 1991 and 4.0 to 4.5 in 1992. During the second year of the study, plants had an average height of 0.9 m, an average diameter of 0.9 m, and covered 18% of the total cultivated area. The maximum calculated CC was equal to 1.5 times the measured canopy cover percentage.
Ursula K. Schuch and David W. Burger
Twelve species of woody ornamentals were grown in containers in Riverside and Davis, Calif., to determine plant water use and compare crop coefficients (Kc) calculated with reference evapotranspiration (ET) from local weather stations (ETcim) or atmometers (ETatm). Water use, Kcatm, and Kccim differed by species, location, and month of the year. Raphiolepis indica (L.) Lindl., Pittosporum tobira (Thunb.) Ait., Juniperus sabina L., and Photinia ×fraseri Dress. were the highest water users in Riverside and Arctostaphylos densiflora M.S. Bak., Juniperus, Cercis occidentalis Torr., and Pittosporum used the highest amount of water in Davis, when averaged over the 20-month study period. Rhamnus californica Eschsch., Prunus ilicifolia (Nutt.) Walp., and Cercocarpus minutiflorus Abrams. were among the lowest water users in both locations. Although plant water use fluctuated considerably between individual sampling dates, the relative ranking of species water use in each location changed very little over the study period. During periods of high winds, ETcim may not provide an accurate reference for container crops. Kc values fluctuated seasonally from as much as 1 to 4.7 for high water users, while values were stable for low water users and also for Buxus microphylla japonica Rehd. & E.H. Wils., an intermediate water user. During periods of low ET, especially in fall and winter, Kc values were artificially high and failed to correspond to the plants' low water use. Kc values for low water users seem to be useful to estimate water requirements over an extended period of time, whereas general Kc values seem to have limited value for plants with high water demand and need to be modified for different growth stages and growing locations.
Giovanni Piccinni, Daniel I. Leskovar, and Thomas Marek
Lysimeters are used to measure real-time water use during the growing season. By relating the water use of a specific crop to a well-watered reference crop, such as alfalfa or grass, crop coefficients (Kc) can be developed to assist in predicting accurate crop needs using available meteorological data. Reference evapotranspiration can be obtained from several weather networks; however, without crop coefficients for specific crops, this information is only useful for grass. Three weighing lysimeters, consisting of undisturbed 1.5 m × 2.0 m area by 2.2-m depth cores of soil, comprise the Texas A&M Research and Extension Center–Uvalde lysimeter facility. Two lysimeters, weighing around 15,000 kg, have been placed beneath a linear LEPA irrigation system and used in field production. A third lysimeter measures reference ET values (ETo) and is located in a grassed area near the field lysimeters irrigated by subsurface drip irrigation system. Spinach was grown in one of the two crop lysimeters while onion was grown in the second lysimeter. Daily water use was measured on 5-min intervals. Results show the possibility of saving ≈61 to 74 million m3 of water per year in the irrigated farms of the Edwards aquifer region if proper irrigation management techniques are implemented in conjunction with the newly developed crop coefficients. Crop water requirements, Kc determination, and comparison to existing FAO Kc values will be discussed.
Genhua Niu, Denise S. Rodriguez, Raul I. Cabrera, Cynthia McKenney, and Wayne Mackay
Relatively little work has been done to determine the water requirements of ornamental plants. To meet this need, five woody ornamental species including Abelia grandiflora `Edward Goucher', Buddleia davidii `Burgundy', Ilex vomitoria `Pride of Houston', Euonymus japonica, and Nerium oleander `Hardy Pink' were investigated to determine their water use and crop coefficients. Parallel experiments were conducted by growing the shrubs both in 56-L (15 gal) drainage lysimeters and in aboveground 10-L containers. Water use per plant, crop coefficients, and overall growth parameters differed by species and culture system. Of the five species tested, Buddleia and Nerium had higher water use per plant in the lysimeters than in the containers. There was no significant difference in water use per plant for Abelia, Euonymus, and Ilex between the two culture systems. Crop coefficients and growth indices of Abelia, Euonymus, and Ilex were statistically similar between the two systems. The growth index of Buddleia and Nerium was much higher in the lysimeters than in the containers. Abelia and Euonymus had more growth in the containers than in the lysimeters while Ilex had slightly larger leaf area in the lysimeters than in the containers. The culture system did not affect the water use per unit leaf area of all species. Therefore, our results indicated that by quantifying the leaf area, the plant water use in the two culture systems is convertible.
Doudou Guo, Ziyi Chen, Danfeng Huang, and Jingjin Zhang
the arid climate without water vapor transfer limitation, and the simplified model was implemented in some commercial greenhouse irrigation software. The equation is given as: E T 0 = R n λ ,  Single crop coefficient (K c ) method was used