2014-05-15_REVISION - M1987064 - Laserfiche WebLink

Home Browse Search

286 D.C. Goodrich et al. /Agricultural and Forest Meteorology 105 (2000) 281 -309 endorsement) were also acquired over a section of the riparian corridor from approximately Hereford to Fairbank in 1997 during four of the synoptic mea- surement periods (21 April, 13 July, 12 August, 14 October, corresponding to DOY 111, 194, 224, and 287, respectively). To aid in estimation of seasonal riparian ET over the entire corridor, the single - channel thermal re- motely sensed data were examined to assess whether they could be used to discern stream reaches with differentially stressed riparian vegetation due to sig- nificant hydrologic changes from image to image. For example, during the April overflight (DOY 111), the entire reach from Lewis Springs to Fairbank had surface water present in the channel. During the July overflight (DOY 194), reaches roughly 6 km down- stream of the Charleston USGS gage dried up. In this area, the cottonwood trees were observed to be drop- ping leaves and yellowing, presumably from water stress. By the time of the August overflight (DOY 224), monsoon floods had reinundated the entire San Pedro River reach under study. 3.3. Ground measurements Three primary vegetation communities in the San Pedro riparian corridor are capable of utilizing sig- nificant quantities of groundwater including the cot - tonwood/willow (C /W) forest gallery, mesquite, and phreatophytic sacaton grasslands. An initial objective is to estimate the amount of riparian ET originat- ing from groundwater for each of these groups on a per unit area of vegetation basis. To address this objective, a variety of ground -based measurements were made. These measurements are described in more detail in the preface (Goodrich et al., 2000) as well as in a number of the other papers in this issue. Measurements used to directly address the ob- jectives of this study are briefly described herein. Other measurements are noted with appropriate references. The majority of ground -based measurements were made at the Lewis Springs site. Continuous measure- ments of water levels in a transect of six deep wells, meteorology and fluxes (Bowen ratio) over the ripar- ian grass and mesquite thicket (Scott et al., 2000), and stream stage were made during all of 1997. Measure- ments taken during the synoptic runs included hourly stream stage and water levels from five river sections and the piezometer network within the Lewis Springs reach, stream discharge measurements determined by current metering, dye - dilution, and an in- stream flume for the June campaign (see summary by Mad- dock et al., 1998). A continuous stage recorder was maintained by the USGS at cross - section three near Lewis Springs. Throughout 1997, a series of 18 cur- rent meter discharge measurements were made at this location to establish a stage – discharge relation- ship for low flows. Tree sap flow, water potential, stomatal conductance, and water sources using stable isotopes were determined for cottonwood and willow during each of the synoptic campaigns to capture variations in transpiration demand as a function of atmospheric demand and water availability (Schaef- fer et al., 2000). The sap flux measurements were made on a small sample of trees at the perennial Lewis Springs and ephemeral Escapule Wash site. We were not able to measure ET from the understory of the C/W forest gallery with this data collection methodology. Water sources using stable oxygen and hydrogen isotope ratios were determined for mesquite, cot- tonwood, willow and several other species at Lewis Springs site as well as an intermittent and ephemeral riparian stream reach (at Boquillas and Escapule locations, respectively — see Fig. 1). These data al- lowed the proportion and magnitude of surface water use by the gallery trees as a function of groundwater availability to be evaluated (Snyder and Williams, 2000). During aircraft overflights, ground -based re- motely sensed data as well as an array of vegetation characteristics were collected for calibration purposes (Moran et al., 2000). During the August campaign, additional instrumen- tation was deployed at the Lewis Springs site from 8 to 19 August. This included an array of eddy correla- tion flux instrumentation (see ET summary by Hipps et al., 1998), a scintillometer, and the Las Alamos National Laboratory Raman LIDAR system (Cooper et al., 2000). During the first portion of the August campaign, the LIDAR was deployed on the east bank of the San Pedro River at Lewis Springs. After sev- eral days, the LIDAR was moved to the west bank and the flux instruments were redeployed across the highly heterogeneous vegetation types under the path of the scintillometer.