*3.4. Comparison with Groundwater Pumping for Irrigation*

Groundwater fluctuations were continuously measured in five piezometers (JFP-1, JFP-2, PHP-1, PHP-4, and VFDP-4 in Figure 1) and the Pump House irrigation well from 18 August 2017 to 27 April 2019. All piezometers are completed in a thin clay/silt aquitard layer that sits atop the underlying leaky confined aquifer [39], and all responded to riparian forest ET as well as to pumping from the Pump House irrigation well. Data from the irrigation well and the most responsive of the piezometers, PHP-1, are shown in Figures 11 and 12 and are also reported in [20]. Piezometer and well data are reported here as changes relative to the respective first water level recorded to facilitate their comparison. Piezometer PHP-1 and the Pump House irrigation well are about 18 m apart. The data show fluctuations that are clearly caused by groundwater pumping, ET, recharge primarily from winter-spring precipitation events, and long-term discharge to the stream and ocean characterized by a period of recession from the end of spring through the summer and into the fall.

**Figure 11.** Groundwater fluctuations observed in piezometer PHP-1 and the Pump House irrigation well from 18 August 2017 through 26 April 2019. Daily precipitation of Swanton Pacific Ranch and stage of Scotts Creek are shown over the same period. The gray boxes represent the two zoomed-in time periods shown in Figure 12.

Diurnal groundwater level fluctuations due to ET forcing are superposed on those due to pumping from the irrigation well. Fluctuations due to ET are more pronounced in the piezometer data and are largely imperceptible in the irrigation well data. They are highlighted in Figure 12, which shows zoomed in plots of the aquifer and aquitard responses for two monitoring periods (08/2017–11/2017 and 08/2018–11/2018). Groundwater response to ET is most perceptible during periods of aquifer and aquitard recovery following a pumping event from the irrigation well. When the pumping frequency is daily, water-level responses to ET are practically indistinguishable from those attributable to pumping. In such cases, the amplitude of the aquitard response is much larger than what can be expected from ET alone. The data clearly show that the riparian corridor

phreatophytes induce measurable fluctuations in the thin clay/silt aquitard that overlies the leaky aquifer.

**Figure 12.** Groundwater fluctuations observed in piezometer PHP-1 and the Pump House irrigation well (**a**) from 18 August 2017 to 1 November 2017, and (**b**) from 1 August 2018 to 1 November 2018.

The diurnal groundwater fluctuations due to ET discussed above are induced by a forest-scale ET flux averaging 3.8 mm/d over the summer seasons. When multiplied by forest area, *A*rf = 9.2 ha, this flux yields a volume flow rate usage by phreatophytes of *Q*et = 350 m3/d = 64.1 gpm. The typical pumping rate from the irrigation at the site is 250 gpm. Hence, ET across the forest amounts to about 25% the pumping rate, which is a substantial proportion.

#### **4. Discussion**

As stated previously, the objective of this work was to estimate riparian forest ET from sap flow measurements collected in a small sample of phreatophytic trees. Sap flow measurements were collected in the same four trees over the two-year monitoring period. For each individual tree, the data were largely repeatable during the growing seasons, with comparable average seasonal amplitudes. The fact that sap flow probes were left in the trees for such a prolonged period but continued to yield meaningful measurements was unexpected because [40] reported that other users of the sap flow probes used the same drill holes for one growing season, at the most. In addition, ref. [18] reported a 30% decrease in daily average sap flux density during the second growing season for red alders. The degradation of data quality over prolonged periods of monitoring has been attributed to tree response to drill-hole wounding by forming tyloses over those vessels, which affects heat exchange with the probes [41]. The good quality data collected over two-year study period may be attributable to the fact that the four instrumented trees were younger, smaller, and of different species than those in other studies. Hence, it may be argued that younger trees are better suited for prolonged monitoring than are older trees as long as their diameters and sapwood areas are corrected for from year to year in the computation of ET.

The ET flux projected across the entire riparian forest correlated strongly with the CIMIS *ETo* and ET computed on the basis of NDVI/meteorological data. The sap flowbased forest ET had consistently lower average magnitudes during the growing seasons with significant departure from the values computed by the other methods over the winter seasons. This divergence in winter may be due to vegetation differences among the methods. Sap flow-based ET was collected on deciduous trees that lost their leaves every winter leading to values that were consistently lower than those from the other methods. The CIMIS *ETo* values are based on a cool-season perennial grass that does not die back during winter and continues to transpire. Additionally, although most trees lost their leaves along lower Scotts Creek in winter, there was still plenty of green understory vegetation and some evergreen overstory vegetation, which were shown by the NDVI data. This may explain why the ET residuals showed seasonal patterns with peaks being highest and smallest during winter and fall seasons, respectively.

Long-term passive groundwater monitoring data were analyzed qualitatively to assess the magnitude of fluctuations in water levels from season to season and year to year. On average, at the study site, groundwater levels increased every winter before receding and reaching their lowest levels in the fall. The steady decrease in water levels during the summer and fall is largely attributable to ocean and stream discharge, consumption by phreatophytic vegetation [42], and groundwater withdrawal for crop irrigation. Diurnal groundwater fluctuations attributable to uptake by phreatophytes across the study forest are much smaller than fluctuations due to pumping, with sap flow based estimates of ET over the riparian corridor being about 25% the typical pumping rate for irrigation. Sap flow based estimates of groundwater fluctuations showed appreciable divergence from satellite based measurements, which suggests the importance of the former in calibration of the the latter, especially where site-specific vegetation may have greater control on local ET and consumptive groundwater use. A well characterized ET forcing function is essential modeling for groundwater flow and diurnal fluctuations [5,9,43–45].

There are some limitations in this study that may be addressed in future research, including (1) location and number of instrumented trees, (2) size of instrumented trees, (3) location and number of sap flow probes on tree stems, (4) instrumentation and monitoring of the minor tree species scattered throughout the riparian corridor such as live oak and redwoods, (5) accounting for the effect of understory vegetation on total ET and (6) measurement of sapwood area for individual trees and the entire riparian forest. The sampling design for this study was partly restricted due to the stem-diameter limitation of the probes, which biased monitoring to younger trees that are known to be hydraulically active than older trees. Using a combination of small and large sap flow probes to see the differences in sap flux density could provide more accurate estimates of ET. Additional pairs of probes could be installed on each instrumented tree at different depths and the sap flux densities averaged because the sap flux density is not uniform across sapwood area of a tree [15,16]. The method of using long-term sap flow measurements to estimate the ET of a riparian forest may be replicated on other phreatophytic species for similar or longer periods of time because phreatophytic tree species may react differently to sap flow probes in terms of sap flow behavior and physical intrusion of the probes.

#### **5. Conclusions**

The study presented herein was based on a two-year sapflow monitoring program on a single riparian forest plot where only a small sample of four trees were instrumented. The data were used together with a survey of tree sapwood depth in six plots across the entire forest to upscale the single-plot sapflow measurements to forest canopy-scale evapotranspiration (ET). The upscaled ET results were compared to ET based on NDVIbased estimates and were shown to be in good agreement. This indicates that for expensive ground-based technologies such as sapflow sensing by the heat dissipation models, a instrumentation of a small sample of a forest may yield reasonable estimates of forest canopy-scale ET as long as they are also based on sampling of sapwood depth across the entire forest. The results of the present study, despite the small sample size of sap flow measurements, illustrate the importance of ground-based measurements of sap flow for calibrating satellite based methods and for providing site-specific estimates and to better characterize the ET forcing in groundwater flow models. The small sample size is important because it is necessitated by the high cost of instrumenting individual trees and it suggests the potential usefulness of single-plot monitoring stations for ground-based measurement and estimation of forest ET.

Further research is required to better capture the spatial variability of sapflow across the forest and would include: (1) a larger sample of instrumented trees to better characterize sap flow behavior, (2) a sample of instrumented trees with a greater variety of main stem diameters in order to better characterize the sap flux density for each species, (3) greater spatial distances between instrumented trees, and (4) long-term monitoring of sap flow in additional phreatophytic species across the forest. The need for a greater sample size is clear even from the data from four instrumented trees because they very had different canopy and sapwood areas. Tree with larger sapwood area tend to have higher volumetric sap flow rate, which could bias the results based on a small sample size.

**Author Contributions:** Conceptualization, B.M. and J.S.; methodology, J.S.; software, J.S.; validation, B.M. and J.S.; formal analysis, B.M. and J.S.; investigation, B.M. and J.S.; resources, B.M.; data curation, J.S.; writing—original draft preparation, B.M.; writing—review and editing, B.M. and J.S.; visualization, J.S.; supervision, B.M.; project administration, B.M.; funding acquisition, B.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the USDA NIFA McIntire-Stennis Program Grant No. 16-109.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was facilitated by Swanton Pacific Ranch (SPR) Director Brian Dietterick, Manager of Operations Steve Auten, SPR staff Grant William and Brian Cook, graduate student Devin Pritchard-Peterson, and undergraduate research assistant Aren Abrahamian.

**Conflicts of Interest:** The authors declare no conflict of interest.
