Two Years of Sap Flow Data for Evapotranspiration Characterization in
Riparian Vegetation
Abstract
To close the water use budget in irrigated agricultural fields in flood
plains with substantial riparian corridors, it is necessary to
understand groundwater usage by dominant phreatophyte vegetation,
particularly when the primary source of the water for irrigation comes
from groundwater abstraction. We report here results of measurements of
sap flow in phreatophyte vegetation in a riparian corridor, which is
part of a watershed located along the coast in Santa Cruz County,
California. The riparian corridor is within a study area of 75 to 140
meters wide in the lower portion of Scotts Creek watershed, which is
bounded to the west by the Pacific Ocean. Canopy cover in the study area
often approaches 100 percent, with dominant trees being red alder (Alnus
rubra), arroyo willow (Salix lasiolepis), and pacific willow (Salix
lasiandra var. lasiandra). Other trees include boxelder (Acer negundo),
big leaf maple (Acer macrophyllum), and California bay laurel
(Umbellularia californica). Common understory vegetation includes
California blackberry (Rubus ursinus), stinging nettle (Urtica dioica
subsp. gracilis), poison hemlock (Conium maculatum), Cape ivy (Delairea
odorata), and Italian thistle (Carduus pycnocephalus). For the study
reported here, only the two most dominant phreatophyte species, namely
red alders and arroyo willows, were instrumented with sap flow sensors.
In addition to diurnal fluctuations, sap flow data collected hitherto
also shows expected seasonal variation with summer maxima and winter
minima, with transition fall and spring periods. Sap flow measurements
from the study area are projected across the entire riparian forest
using sampled tree sapwood area and used to estimate forest
evapotranspiration (ET). The ET is then used in a groundwater flow model
to more accurately predict observed groundwater fluctuations and usage
by riparian vegetation.