Estimating Evapotranspiration of a Riparian Forest Using Sap Flow Measurements

<p>To close the water use budget of irrigated agricultural fields in floodplains with substantial riparian corridors, it is necessary to understand groundwater usage by dominant phreatophytic vegetation, particularly when the primary source of water for irrigation comes from groundwater abstraction. We report here results of estimated evapotranspiration (ET) of a riparian forest, which were based on measurements of sap flow in phreatophytic vegetation within a riparian corridor. The riparian corridor was within a study area 75 to 140 meters wide in the lower portion of the Scotts Creek watershed, which is bounded to the west by the Pacific Ocean in Santa Cruz County, California. Canopy coverage in the study area often approaches 100% during the growing season, with dominant trees being red alder (<em>Alnus rubra</em> Bong.), arroyo willow (<em>Salix lasiolepis</em> Benth.), and pacific willow (<em>Salix lasiandra</em> Benth. var. <em>lasiandra</em>). Other trees include boxelder (<em>Acer negundo</em> L.), bigleaf maple (<em>Acer macrophyllum</em> Pursh.), California bay laurel (<em>Umbellularia californica</em> (Hook. & Arn.) Nutt.), and coastal redwoods (<em>Sequoia sempervirens</em> (D. Don) Endl.). Common understory vegetation includes California blackberry (<em>Rubus ursinus</em> Cham. and Schlecht.), stinging nettle (<em>Urtica dioica</em> subsp. <em>gracilis</em> L.), poison hemlock (<em>Conium maculatum</em> L.), Cape ivy (<em>Delairea odorata</em> Lem.), Italian thistle (<em>Carduus pycnocephalus</em> L. subsp. <em>pycnocephalus</em>), and western poison oak (<em>Toxicodendron diversilobum</em> (Torr. & A. Gray) Greene). We hypothesized that the ET of a riparian forest could be estimated by measuring the sap flow of riparian phreatophytic trees. For the study reported here, only the two most dominant phreatophytic species, namely red alders and arroyo willows, were instrumented with thermal dissipation probes. In addition to diurnal fluctuations, sap flow data collected hitherto also showed expected seasonal variation with summer maxima and winter minima, with transition fall and spring periods. Sap flow measurements from the study area were used to estimate riparian forest ET by projecting them across the canopy areal extent of the riparian forest using sampled tree sapwood areas from six sample plots. The sap flow-based ET results were then compared to ET results reported by two other methods. Additional research, including increased number of trees with thermal dissipation probes, further analysis of sap flow behavior, and continued long-term measurement of sap flow, is needed to further improve the method of using long-term sap flow measurements to estimate the ET of a riparian forest.</p>

Name thesis:degree_name

MS in Forestry Sciences

Discipline thesis:degree_discipline

Natural Resources Management

Year dc:date.available

2020

Author dc:creator

• Solum, James R

Contributors dc:contributor

• Bwalya Malama
• Natural Resources Management
• College of Agriculture, Food, and Environmental Sciences

• Riparian Forest
• Phreatophyte Vegetation
• Evapotranspiration
• Sap Flow
• Thermal Dissipation Probes
• Groundwater
• Hydrology

Identifier

10.15368/theses.2020.63

OAI identifier oai:identifier

oai:digitalcommons.calpoly.edu:theses-3663