Using Direct Evapotranspiration Measurements for Comminity-Engaged Education and Extension

The State of California is moving towards sustainable groundwater management and carbon neutrality goals. We aim to answer questions of importance to sustainable agricultural practices, in particular how to: (1) decrease unnecessary water losses from evapotranspiration (ET), (2) increase carbon sequestration, and (3) enhance overall water use efficiency. While implementing our cooperative extension program in Biometeorology, we deployed eddy covariance systems on flux towers placed on numerous farms to directly measure net water and carbon exchange between different agricultural crops and the atmosphere. Currently, we are simultaneously running up to twenty of these flux towers on any day of the year to address the seasonality of carbon and water cycling with innovative agricultural practices. Some of the most urgent questions posed by local communities, commodity boards, state agencies, NGOs and private corporations are mainly related to water use by different agricultural landscapes, but carbon fluxes are increasingly gaining attention as well. In order to explore these questions, our network of towers spans the entire Central Valley from northern California rice fields (that are increasingly being fallowed for water transfers), to the southern part of the state where transferred water is being used for perennial crops. Although some data and measurements are reported on the ET of California crops, the ever-changing adaptation practices in agricultural management are challenging previously established parameters (e.g. crop coefficients) used in irrigation management. For better spatial resolution, we often use semi-direct ET measurements, derived from residuals of observed surface energy budgets. In addition, alternative low-cost measurements, such as the surface renewal method, are actively being evaluated and we are refining different approaches for their independent use. 

This talk will mostly focus on ET measurements and the simultaneous quantification of water budgets of entire agricultural fields. We are designing several of our experiments to encompass a full water budget of the targeted fields. This provides a backup estimate of ET as well as an opportunity to couple soil moisture profiles, runoff, irrigation, and precipitation dynamics to eddy covariance flux tower measurements, and eventually help with the ET partitioning. This information is increasingly being sought by modelers and remote sensing scientists to calibrate their ET estimates as we work together on addressing emerging challenges in California agriculture and could potentially be used to understand the physiological response of crops to changes in plant water status and how this response could affect crop quality and productivity. Evaluation of alternative methods and models will include cross-comparisons of existing techniques with more advanced methods under development, including ACASA. We are especially excited to broaden our extension and farm education to small growers and communities that have been historically overlooked by cooperative extension and funding agencies.