ABSTRACT

The San Joaquin Valley produces upwards of 100,000 ha or 85 to 90 percent of the U.S. Pima cotton, Gossypium barbadense, crop. Although it typically produces lower yields than the Upland Gossypium hirsutum cultivar, the Pima cultivar has extra-long staple cotton, which is prized for its high quality, and the cultivar is adapted for most cotton producing regions of the world. The active vegetative growth period of commonly grown Pima cultivars exceeds that of Upland by 10 to 21 days, while a similar extension of the fruit maturity period has been observed. We studied the extended vegetative growth period to observe any influence on seasonal crop water use patterns. During this three-year study we made plant canopy measurements and monitored crop evapotranspiration (ET_c) throughout the growing season in grower fields. While similar water management approaches were employed between fields each year of the study, year-to-year differences in grower water management practices appear to have influenced annual differences in ET_c. Increases in ET_c were observed for Pima cotton both early and again late in the season; however, midseason water use rates did not differ. The frequency and volume of irrigation water applied was at least as important to seasonal crop water use as climate variation.