Effect of Microclimates on Evapotranspiration Rates, Energy Balance, and Water Use Estimation in the Sacramento-San Joaquin Delta, California, USA

Research involving the atmospheric-surface exchange of greenhouse gases in California's Sacramento-San Joaquin Delta (Delta) has primarily focused on peat oxidation and resulting subsidence from over a century of agricultural land management practices. Currently there is a network of flux towers used to investigate management plans to mitigate subsidence and, in some cases, increase land elevation. Nevertheless, Delta land elevations have decreased by over 10m and water resources are largely allocated to maintain levee stability and prevent salt-water intrusion into the Delta, the source of fresh water to over to 22 million Southern Californians. These water allocations are potentially modeled using outdated evapotranspiration (ET) rates. The network of flux towers in the Delta has provided researchers the ability to calculate the atmospheric exchange of water vapor from a variety of land surfaces. From these results, ET rates are found to be reduced compared to the same land surface measurements outside the Delta region and are most likely due to the Delta's unique microclimate. In the summertime, this area is an oasis of cool, moist air (Delta Breeze) when compared to other areas in the Sacramento and San Joaquin Valleys, where daytime high temperatures are often 5 to 10°C higher. The air mass that influences the delta region is formed from a complex interaction between the sub-tropical Pacific High pressure system, upwelling along the California coast, upper atmospheric westerlies, and the unique break in the California Coastal range (i.e. the San Francisco Bay). In general, ET rates are lower than the surrounding geography, as the onset of the "Delta Breeze" occurs in the afternoons, increasing the sensible heat exchange and reducing the energy available for latent heat. Current ET rates were calculated using eddy covariance flux systems for a variety of land uses within the Delta: agricultural crops (corn, rice, alfalfa, and irrigated pasture), a newly restored wetland, and a 14-year old restored wetland. ET rates from the various Delta locations were compared with concurrent eddy covariance measurements of corn, rice, and alfalfa outside the Delta's microclimate. For comparing wetland ET rates, we use historical ET rates from the literature, under varying climate conditions, and current water-use models to determine the change in water use, as conversion from agricultural land to wetlands may be considered a viable alternative to mitigating subsidence in the Delta. This researches points towards the possible inaccuracies of using one general ET rate for each land use over large regions, as microclimates may cause an increase or, in the case of the Delta, a decrease in total water consumption.