# A Networked Sensor System for the Analysis of Plot-Scale Hydrology
## 3 Results and Discussion
### Sap Flow Data and Transpiration Estimation
#### 3.3.2 Transpiration Estimates from Sap Flow Measurements  
Table 2 shows a comparison between the \(A_{s}\) estimates of monitored silver maple trees in site 2 (Figure 5 for locations) and modeled sapwood area, \(\hat{A}_{s}\), based on Equation (6), for which the same values of \(d\) were used from the field survey. The coefficients \(B_{0}\) and \(B_{1}\) were selected according to the tree species [61]. It is observed that the values of \(\hat{A}_{s}\) are similar to the field estimates (RMSE = 78.6 cm\({}^{2}\)); therefore, Equation (6) was used to calculate the sapwood area of the trees located inside the 1300 m\({}^{2}\) survey area. Table 3 shows the results for the \(A_{S}/A_{G}\) ratios for the three sap flow nodes based on their regions of interest within the survey area (Figure 6). Figure 10 shows the transpiration (\(\tau\)) calculations using Equation (5) for the sap flow measurements from nodes 2014, 2084 and 2134 based on a mean-weighted \(A_{S}/A_{G}\) ratio of 12.06 m\({}^{2}\)-ha\({}^{-1}\) from Table 3. The selected time period (i.e., from July to October) represents the time of year when, on average, most of the evapotranspiration occurs around the study site [87]. Figure 10a shows \(\tau\), in mm/h, every 10 min, which corresponds to the sap flow sensor's sampling  
Figure 9: Sap flow results for node 2084 between 20 and 27 July 2016. (**a**) Raw voltages (i.e., ADC in mV) from the HP (red scatter plot) and TP (blue scatter plot) between 0 and 1000, and smoothed plot for the HP and TP in red and blue, respectively; (**b**) Filtered and smoothed temperatures for the HP (red) and TP (blue) from the raw voltages ADC0 and ADC1 respectively, difference in temperature (HP-TP) in Celsius degrees (magenta); (**c**) Sap flow time series (mm/h). interval. The peaks in Figure 10a represent a time close to noon on each day. Despite having few noticeable high peaks, \(\tau\) is mostly within the range 0.2-0.4 mm/h. Integrating the hourly \(\tau\) rates in Figure 10a to monthly totals yields 40.8 mm from 11 to 31 July, 55.2 mm from 1 to 31 August, 65.4 mm from 1 to 30 September, and, 30.7 mm from 1 to 11 October. According to the NRCC, the monthly average potential evapotranspiration (PET) estimates for the greater Pittsburgh area are 110.7 mm for July, 96.3 mm for August, 66.3 mm for September and 39.12 for October. Considering the average values from NRCC as a reference, these results suggest that there could be an overestimation of the \(\tau\) during September and October, since monthly \(\tau\) calculated for September is very close to the estimated average PET (i.e., 65.4 mm compared to 66.3 mm) and the monthly \(\tau\) calculated for 11 days of October (i.e., 30.7 mm) are projected to be higher than the estimated average PET (i.e., 39.12 mm). Besides two noticeable peaks in \(\tau\) during these two months (i.e., 7 September and 10 October in Figure 10a), the remaining values are consistently higher than in the previous months (i.e., July and August). This suggests that for these two months, the \(\tau\) rates were higher than average and there is not an overestimation of the transpiration. However, since the \(A_{S}/A_{G}\) ratio is a major factor controlling \(\tau\) rates, an extension of the survey area can be considered for a better estimation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Node** & \(\mathbf{d}\) **(cm) a** & \(A_{S}\) **(\%) b** & \(\mathbf{A_{total}}\) **(cm2) c** & \(\hat{A}_{S}\) **(cm2) d** & \(\hat{A}_{S}\) **(cm2) e** \\ \hline
2045 & 34 & 77.4 & 839 & 650 & 700 \\
2055 & 30.7 & 70 & 682 & 478 & 601 \\
2065 & 31.5 & 76.7 & 719 & 552 & 633 \\
2095 & 41.2 & 81.7 & 1250 & 1020 & 954 \\
2115 & 24.3 & 83.1 & 415 & 345 & 394 \\ \hline \hline \end{tabular} a  
\end{table} 
Table 2: Comparison of silver map (Acer saccharinum) sapwood area from [66] measurements and Equation (6) estimations in site 2 of the ASWP network.