Alfalfa agriculture is a natural laboratory for studying land-atmosphere interactions due to its homogeneity and fluctuating leaf area index from periodic cuttings throughout the year. However, a few problems exist, which are the objectives of this thesis: 1) measuring energy and water fluxes in-situ can be very expensive (~$50K); 2) spatiotemporal heterogeneity could bring in unexpected quantities (e.g., heat and moisture) into the field, distorting the fluxes of energy and water; and 3) long term carbon and water budgets remain largely unknown for irrigated alfalfa in California. In the following, I used a combination of eddy covariance measurements and satellite remote sensing to investigate these problems.
The first chapter is on the development of cost-effective measurements for sensible (H) and latent heat fluxes (λE). I deployed the variance-Bowen ratio technique in an irrigated alfalfa field, and measured H and λE using only a sonic anemometer and an air temperature and relative humidity sensor (T-RH). Measured H and λE were validated against eddy covariance measurements, where H showed strong agreement (slope = 0.98, R2 = 0.96, n = 3726), while λE showed good agreement (slope = 0.89, R2 = 0.91, n = 3773). Thermal remote sensing observations from The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) and existing tower array showed that the uncertainty in λE was attributed to a product of horizontal heat and moisture advection from upwind fields. Based on our results, the variance-Bowen ratio technique is a robust, inexpensive, yet user-friendly approach to measure sensible and latent heat flux. The utility of this approach could extend to measure horizontal heat advection and provide sensor networks of energy and water fluxes for calibrating/validating remote sensing models.
The second chapter re-evaluated the theoretical limitation of eddy covariance given the spatiotemporal heterogeneity at the site region. Specifically, I focused on the assumed negligible horizontal heat and moisture advection and measured them with tower arrays and profile measurements. Results showed local and non-local processes affected land-atmosphere interactions. Locally, competing process was observed between atmospheric demand and stomatal regulation. As a result of the upwind λE, advection humidified the atmosphere and increased stomatal opening, but λE was suppressed with a lowered atmospheric demand. Non-locally, spectral analysis revealed that low frequency (i.e., large) eddies contributed high heat and moisture advection. Thermal imagery from ECOSTRESS and Landsat 8/9 showed that these large eddies were generated over the upwind surface, and they were independent of the local boundary layer conditions. Hence, λE was enhanced through this non-local transport of heat and moisture. Lastly, by conditionally including the advective fluxes in the turbulence budget, the energy balance closure improved from 89% to 97% (r2 = 0.97, p<0.001) over 37 days.
The third chapter explored how water scarcity affects alfalfa’s ability to consistently provide high yields and serve as a robust carbon sink. Long-term eddy covariance data of energy, water, and carbon fluxes were used over the course of 7 years. In 2022, net ecosystem exchange (-175 g C m-2 y-1) and evaporation (722 mm y-1) suddenly declined, compared to the average value of net ecosystem exchange at -544 g C m-2 y-1 and evaporation at 861 mm y-1. This result showed that water stress greatly impacted carbon and water budgets during the active summer growing season in 2022. Specifically, limited water supply from record-low springtime precipitation and irrigation curtailment impeded crop growth, leading to higher stomatal closure and a consequent decrease in carbon sink strength and evaporation throughout this year.
This thesis addressed key challenges in understanding the land-atmosphere interactions in alfalfa agriculture. The simple and cost-alternative sensors developed in Chapter 1 allow researchers to measure fluxes everywhere, all the time across key ecosystems. Missing fluxes quantified in Chapter 2 highlights the significance of advection in land-atmosphere interactions. Lastly, long-term flux budgets assessed in Chapter 3 underscores the vulnerability of agricultural systems.
