Return to List of Research Summaries
TITLE:
Estimating Water Vapor Flux From Riparian Vegetation Using LIDAR Measurements of Humidity Fields
INVESTIGATORS:
Lawrence Hipps (Utah State University), Dan Cooper (LANL)
APPROACH
The narrow width and relatively tall nature of the riparian vegetation at the study site, presents a 2 dimensional problem for estimation of water vapor flux. Diffusion of water vapor from the vegetation will include upward turbulent fluxes above the canopy, and lateral and upward flux at the downstream side of the canopy. The lateral flux from the side will likely be a combination of turbulent diffusion and the mean transport due to horizontal mean wind passing through the canopy.
As a result, typical micrometeorological approaches designed for extensive patches of a surface type, cannot likely be directly applied. There may not be enough horizontal fetch for flux measurements over the top of the canopy, and accommodation must somehow be made for the flux out the leeward side.
One approach is to measure the fluxes of water vapor at all sides of a control volume in which air is modified by the evaporation of the vegetation zone. Ideally, this would be measured with a series of eddy covariance systems with 3-D sonics at several heights at the upwind and downwind sides, as well as above the canopy. This would allow estimation of the fluxes in and out of the volume, yielding the source term of the vegetation zone. However, this does not appear to be feasible for logistical reasons. A total of 3 towers over 30 m would be needed, along with 5 to 7 eddy covariance systems with 3-D sonics!
Hence, another approach is posed involving measuring the water vapor field above and leeward of the vegetation canopy. This spatial humidity field can be quantified with the LIDAR. A control volume can be defined from the LIDAR images, which includes the region of air where the humidity is modified by the evaporation. A simple expression can be formed which describes the difference between mean horizontal flux of water vapor at the upwind and downwind sides of the volume. This requires estimates of humidity and horizontal wind vs. height. Sodar will provide winds at canopy top and above, while some other approach must be considered to estimate mean wind at the upwind and downwind sides.
Return to List of
Research Summaries
homepage