ASHS 2024 Conference

Accurate estimation of crop water requirements (CWR) is essential to optimize water use efficiency and develop efficient irrigation scheduling practices. This is particularly important in California where frequent droughts have accentuated the need to conserve water and improve on-farm water management. The most accurate method to determine CWR is with precision weighing lysimeters, which measure actual crop evapotranspiration (ETa). Thus, the objectives of this study were to determine ETa data, develop new crop coefficients (Kc), and evaluate the relationship between Kc and crop fractional ground cover (Fc) for forage sorghum grown under drip irrigation. Being drought and salt tolerant, sorghum has been considered as an alternative crop to corn in regions of water scarcity and marginal soils. Daily ETa measurements were collected over three years on a clay loam soil using the precision weighing lysimeter available at the University of California Westside Research and Extension Center in Five Points, CA. Crop coefficient (Kc) estimates were derived from the ETa measurements and reference ETo data were obtained from a CIMIS weather station located at the study site. Weekly Fc measurements were also performed to derive relationships between Kc and fractional ground cover. Results from our study indicated that the seasonal ETc ranged from 665 to 870 mm, with average peak ET of 7.0 mm/day and midseason Kc spanning from 0.90 to 1.1. A strong correlation (r2 > 0.95) was also observed between crop Kc and Fc. This study represents the first experiment determining ETc and Kc for forage sorghum grown under drip irrigation in California. Findings also demonstrated the variability of CWR due to seasonal differences.