Differences in the surface energy balances of rice grown in the Sacramento-San Joaquin Delta and the Sacramento Valley from 2009 and 2010

Traditionally rice grown in California was mostly limited to the hot and dry summer conditions of the Sacramento Valley. Until recently, the Sacramento-San Joaquin Delta was thought to have water and air temperatures too low to match the production of rice in the Sacramento Valley. With the increasing interest of mitigating subsidence through the reduction of oxidation of peat soils and the development of new rice varieties for cooler climates, several Delta islands are being used to successfully grow 3000 acres of rice. Current research on Twitchell Island is investigating the western regions of the Delta for rice cultivation where water and air temperatures are the coolest and water and soil salinities begin to impact agricultural production. Included in this research is estimating water losses by evapotranspiration for land management decisions, as water becomes an increasingly valuable resource in California. Energy balance measurements of latent and sensible heat fluxes were collected at 10 Hz and averaged over thirty minutes using the eddy covariance method. Net radiation and soil heat flux terms were recorded at lower frequency and averaged for the same thirty minute period as the eddy covariance terms. In the 2009 growing season, the cooler water in the Delta looks to be a net sink for sensible heat flux, contributing more energy to latent heat flux, soil heat flux, and storage terms compared to pre-flood conditions. Latent heat flux in July reached values above 500 Wm-2 and total daily evaporation rates were close to 7.5 mm. Sensible heat flux remains negative throughout the day with the highest magnitudes occurring in the afternoon and evening when strong breezes draw cooler air from the Bay area and the Pacific Ocean. For rice grown in the Sacramento Valley, latent and sensible heat fluxes were measured using the surface renewal technique, while net radiation and soil heat fluxes were measured directly for 2007 through 2009. Results from 2007 and 2008 show peak values of latent heat flux to be higher than in the Delta, close to 600 Wm-2, however, total daily evaporation rates are around 6 mm. Daily sensible heat flux values remain positive through the growing season, indicating total energy lost to this term may reduce overall daily latent heat flux values. These differences are most likely due to the windy conditions and cooler water temperatures found in the Delta region, but the differences in agricultural management practices may also play a role. This presentation will discuss the energy balances from rice grown in both regions for 2009 and preliminary results from 2010.