Lysimetric Measurements of Evapotranspiration for Drip-Irrigated Crops.

Despite its arid climate, California has been able to maintain its leading position as the top agricultural state in the U.S. due to its innovative agronomic and irrigation practices.  Major changes in crop production and irrigation systems have occurred over the past decade throughout the state.   In the Central Valley, many producers have transitioned from low-value crops grown under flood irrigation to higher value crops produced with low-volume drip irrigation.  More recently, this trend has also been observed for row crops and has been mostly attributed to the current drought that has severely affected the availability of water resources.  Thus, water conservation has become a top priority in California and has required growers to adopt management practices that optimize irrigation and water use efficiency.  One approach to conserve water consists in optimizing irrigation scheduling through the development of new crop evapotranspiration (ET) and crop coefficient (Kc) estimates that better reflect the current agricultural and irrigation management practices. The most accurate approach for determining ET and K_c values involves the use of weighing lysimeters located in fields with adequate fetch, such as the lysimeter facility located in the western San Joaquin Valley (WSJV) of California.  For the last four years, our research has focused on developing these estimates for crops grown on a clay loam soil at the WSJV facility under drip irrigation.  The first study was conducted for processing tomato under sub-surface drip irrigation from 2011 to 2013. Average peak crop ET and basal Kc for tomato at midseason averaged 9 mm/day and 1.1, respectively.  Preliminary results for the second study, initiated in 2014 on sugarbeet grown under surface drip, indicated that midseason ET was 8 mm/day and Kc peaked at 1.25.  An excellent relationship (R² > 0.90) between Kc and crop ground cover was observed for each crop.