Micrometeorological Measurement of Riparian Vegetation Evapotranspiration

At two different sites along the Cosumnes River in Central California, this study was initiated to provide evapotranspiration data to an existing hydrologic and ecologic study. This research contributes to understanding the feedback between groundwater and vegetation and riparian micrometeorology in general. Using two different techniques we calculate the amount of water lost from the hydrological budget to evapotranspiration and likewise observe the effect of groundwater availability on the riparian ecosystem.

Two towers, each approximately twenty meters tall, at two different sites along the river are used in this study. Both towers include measurements of ground heat flux, net radiation, surface temperature, wind velocity, temperature and relative humidity. After estimating the aerodynamic resistance of the areas using two independent techniques, these measurements are used to calculate latent energy and evapotranspiration at half-hour and one-hour intervals.

The expanse and biometeorological uniformity of only one of the two sites is sufficient to support the use of eddy-covariance. We estimate both carbon and water flux at this site, but the main focus is on the exchange of water. Using this more direct measure of evapotranspiration at the more ideal site we can provide some degree of validation of the energy balance/infrared thermometry technique we use to estimate aerodynamic resistance and evapotranspiration at both sites.

The eddy-covariance tower became operational in January 2004 and the other tower has been collecting data since July 2003. Preliminary analysis of the July site, which has very poor fetch, places the aerodynamic resistance estimate as a function of wind speed to the power of -.07. This is encouraging, and approximately what we expected as the trees below the tower could be classified aerodynamically as somewhere between a uniform canopy and an isolated object.