Crop coefficients (K(c)) are a useful means of predicting how much water is needed for irrigating a crop. The crop water stress index (CWSI), on the other hand, is a means of knowing when to irrigate. Two field experiments were conducted during the summers of 1990 and 1991 at Maricopa Agricultural Center and Marana Agricultural Center, respectively, to evaluate water use (evapotranspiration, ET) of different cotton varieties, to develop crop coefficients for cotton grown in the state of Arizona, and to evaluate empirical and theoretical crop water stress indices under field conditions. For the 1990 experiment, ET from the cotton variety DPL 77 was obtained using soil water balance (SWB) and steady state heat balance (SSHB) techniques. For the 1991 experiment, ET from two cotton varieties (DPL 20 and Pima S-6) was estimated using the Bowen ratio energy balance (BREB) method and the steady state heat balance method. Reference evapotranspiration (ETᵣ) was obtained from weather stations located close to the experimental plots. Average daily ET from the SSHB measurements ranged from 8.24 to 15.13 mm and from 10.34 to 12.12 mm for the 1990 and 1991 experiments, respectively. Total ET from the SWB was approximately 19% less than the total ET estimated by the SSHB. Total ET from individual plants was well correlated with average stem area over the evaluation periods. Daily ET from the two cotton varieties (DPL20 and Pima S-6) was approximately similar when irrigation conditions were the same, but differed later by as much as 48.4% as irrigation continued for the variety Pima S-6 only. Daily ET from the BREB measurements and ETᵣ were used to develop a crop coefficient curve for cotton grown at Marana, Arizona, which had a maximum smoothed value of 1.21. A critical value of CWSI equal to 0.3 was obtained by observing the pattern of the CWSI values over well-watered and drier conditions, and from previous research. Using the developed crop coefficient curve and the CWSI should provide a useful means of scheduling irrigation for cotton grown under climatic conditions similar to those at Marana, Arizona.

Reprinted August 1968

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.