**1. Introduction**

Prolonged drought conditions in California and the associated increased reliance on groundwater resources for irrigation in coastal areas, necessitates a re-examination of agricultural groundwater use in riparian corridors, particularly the impacts of groundwater pumping on instream flows. Minimum flow requirements in coastal creeks are a source of serious concern for riparian forest and land managers, fisheries biologists, and agencies assigned to evaluate sustainable instream flow requirements. Prior works in coastal riparian systems (e.g., [1]) have focused entirely on groundwater pumping for irrigation, with only cursory attention given to consumptive groundwater use by riparian vegetation. An accurate understanding of the impacts of groundwater pumping for irrigation, requires a consideration and characterization of all the components (inputs, outputs, and storage) of watershed-scale water budgets, including the poorly understood consumptive groundwater use by phreatophytic vegetation.

Riparian forests are among the most productive natural ecosystems and perform such ecological functions as filtering agricultural runoff of sediment, nutrients, and other solutes, thereby minimizing non-point source contamination of streams and groundwater. They help maintain the stability of stream banks as well as the quality and quantity of groundwater recharge [2–4]. In addition to ecological functions, riparian forests also play a

**Citation:** Solum, J.; Malama, B. Estimating Canopy-Scale Evapotranspiration from Localized Sap Flow Measurements. *Water* **2022**, *14*, 1812. https://doi.org/10.3390/ w14111812

Academic Editors: Luis Garrote and Alban Kuriqi

Received: 22 April 2022 Accepted: 30 May 2022 Published: 4 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

central role in the earth's strongly coupled energy and hydrologic cycles through consumptive water use from evapotranspiration (ET) and effects on surface roughness and surface reflectivity (albedo). Direct measurement of tree sap flow to better characterize the ET forcing on groundwater flow could lead to improved understanding of the ET component of the hydrologic cycle attributable to consumptive groundwater use by phreatophytic vegetation. Water requirements of riparian vegetation are usually fulfilled by soil moisture and groundwater [5]. However, riparian forests also often contain phreatophytic species, which depend primarily on groundwater for long-term survival [6,7]. The root systems of such species extend to the capillary fringe, the water-table and the underlying saturated zone [7,8]. In groundwater models of diurnal groundwater fluctuations, the water use by phreatophytic vegetation can be characterized by a diurnal ET water-table flux boundary condition (forcing function) or as a volumetric sink within the saturated zone (unconfined aquifer) [9].

Although direct measurement of ET from riparian forests is key to understanding regional and local water and energy balances in hydroclimatological modeling, there remain high uncertainties in seasonal and long-term (decadal scale) riparian forest ET data due to the focus on diurnal fluctuations [10,11]. This limits the ability of models to accurately estimate the groundwater component of water budgets consumed by vegetation in such forests [11]. Riparian zones in semiarid regions often exhibit high rates of ET in spite of low-soil wetness due to the presence of phreatophytic vegetation [12,13], which is reflected in diurnal water-table fluctuations [5,9,12] and can be measured by direct monitoring of vadose zone soil moisture and groundwater levels. In most long-term ET and groundwater studies, the amount of water used by phreatophytes is estimated by empirical formulae that rely on climatic and weather variables or by extrapolation and interpolation of remote sensing measurements. This can be problematic given the uncertainties associated with the subsurface sources of the water [10,11].

Direct ground-based measurements of ET include eddy covariance and sap flow monitoring. There are three common sap flow techniques: (1) thermal dissipation probes (TDP), (2) heat pulse velocity (HPV), and (3) tissue heat balance (THB). Thermal dissipation probes (TDP) proposed by [14] comprise two cylindrical probes that are inserted into the tree stem and separated by a fixed vertical distance. There is some uncertainty on the accuracy of TDP sensing of sap flow taking in fixed position on trees over long periods [15,16]. The workers [17] continuously measured sap flow for 1.2 years, and reported that the mean sap flux density declined by 30% during the second growing season. In a fast-growing tree, the probes become embedded as the vascular cambium produces new phloem and xylem tissue [18,19]. Prior work of [16] reported declines in sap flow as probes became lodged deeper into the sapwood over time, leading to underestimation of the volume sap flow rate.

In the present study, sap flow was measured using thermal dissipation probes in four trees, continuously, for two years with the objectives of (1) comparing up-scaled ground-based sap flow estimates of ET to satellite-based measurements and (2) evaluating groundwater usage by phreatophytes in comparison to pumping for irrigation. The approach involved installation of thermal dissipation probes (sap flow probes) in select phreatophytes, vegetation surveys focusing on phreatophytes, measurement of sapwood area, and up-scaling of plot-scale sap flow measurements to forest-scale ET estimates.

#### **2. Materials and Methods**

#### *2.1. Study Site*

The study was conducted at Swanton Pacific Ranch, located along the Pacific coast in Santa Cruz County, California, about 84 km south-southeast of San Francisco. A map of the watershed and study area is shown in Figure 1. The climate of the region is Mediterranean, with warm, mostly dry summers and cool, wet winters. The mean summer air temperature highs are 24 ◦C and mean winter air temperature lows are 5 ◦C. The rainy season is typically from October to April, with an average yearly precipitation of 975 mm, with an average of

193 mm occurring in January. Even during the recent prolonged drought in California from December 2011 to March 2019, the average yearly precipitation was 945 mm. The average yearly precipitation over the duration of this study (August 2017 through August 2019) was 855 mm. Streamflow in main stream in the watershed, Scotts Creek, is typically very low in the summer (≤0.1 m3/s). During the winter, peak flows typically are 20–70 m3/s, based on data from a Scotts Creek stream gauge.

**Figure 1.** A map (adapted from [20]) of the Scotts Creek watershed, Swanton Pacific Ranch, and the riparian forest study area. The map shows the location of the instrumented phreatophytes, survey plots, and piezometers.

The riparian corridor within the study area is about 70–140 m wide with a canopy cover that often approaches 100% during the growing season [21]. The dominant trees along the lower portion of the Scotts Creek watershed are red alders (*Alnus rubra* Bong.), arroyo willows (*Salix lasiolepis* Benth.), and pacific willows (*Salix lasiandra* Benth. var. *lasiandra*). Other trees include box elder (*Acer negundo* L.), bigleaf maple (*Acer macrophyllum* Pursh.), California bay laurel (*Umbellularia californica* (Hook. & Arn.) Nutt.), and coastal redwoods (*Sequoia sempervirens* (D. Don) Endl.) Common understory vegetation includes California blackberry (*Rubus ursinus* Cham. & Schltdl.), stinging nettle (*Urtica dioica* subsp. *gracilis* L.), poison hemlock (*Conium maculatum* L.), Cape ivy (*Delairea odorata* Lem.), and Italian thistle (*Carduus pycnocephalus* L. subsp. *pycnocephalus*) [21–23]. The phreatophytes documented within the study area, including red alders, arroyo willows, pacific willows, box elders, and bigleaf maples, are all deciduous. They typically lose their leaves in November/December and their leaf buds burst in early March. They maintain maximum leafage for most of the spring, summer, and fall growing seasons. **The typical site vegetation is shown in Figure 2, which shows (a) the dominant phreatophytic trees (b) understory vegetation, and (c) deciduous vegetation during winter dormancy.** Red alders are fast-growing, relatively short-lived, shade intolerant, and tend to favor sites with bare mineral soil and high sun exposure that were disturbed by floods, windthrows, logging, or fires.

**Figure 2.** Typical vegetation, including (**a**) phreatophytic trees (**b**) understory vegetation, and (**c**) deciduous vegetation during winter dormancy in the study area within the lower Scotts Creek riparian corridor in June 2017, June 2018, and January 2019, respectively.
