*4.4. ET-weather Relationships*

The SWAT- and MODIS-models produced markedly different relationships between monthly ET and weather. Monthly ET from SWAT, as well as flux towers, showed a significant positive relationship to air temperature (Figure 6, left and middle columns). At the flux towers, slopes of linear regression between ET and temperature ranged from 3.8 to 4.0 mm mo<sup>−</sup><sup>1</sup> (◦C)−1. These slopes for SWAT ranged from 3.0 to 5.2 mm mo<sup>−</sup><sup>1</sup> (◦C)−1. In contrast, monthly ET from MODIS showed either no significant relationship to temperature (*p*-value > 0.05) or a significant negative relationship (Figure 6, right column). At the lower and middle sites where no significant relationship to temperature occurred, there were distinct mid-temperature peaks in MODIS ET. At the lower site, MODIS ET reached a peak value at an intermediate air temperature of 16.1 ◦C (Figure 6i). At the middle site, this peak ET occurred at an intermediate temperature of 7.9 ◦C (Figure 6f). Mid-temperature

ET peaks such as these did not occur in the flux tower observations or SWAT predictions. Vapor pressure deficit, noted earlier to be strongly correlated to temperature, showed a significant positive relationship to ET from both flux towers and SWAT model (Supplement Figure S6). In contrast, vapor pressure deficit showed either no significant relationship (*p*-value > 0.05) or a significant negative relationship to ET from MODIS (Supplement Figure S6).

**Figure 5.** Monthly averages in air temperature (T), vapor pressure deficit (VPD), ET, and potential ET (PET) at the (**<sup>a</sup>**–**<sup>c</sup>**) upper site, (**d**–**f**) middle site, and (**g**–**i**) lower site. TWR = flux tower observation, MOD = MODIS value, SWT = SWAT value. MODIS PET (MOD PET) values have been multipled by 0.5.

**Figure 6.** Monthly ET versus air temperature from different data sources at the (**<sup>a</sup>**–**<sup>c</sup>**) upper site, (**d**–**f**) middle site, and (**g**–**i**) lower site. Dashed lines are best fits from linear regression. *m* = slope of regression best-fit with asterisk where *p*-value < 0.05, *R*<sup>2</sup> = coefficient of determination of best-fit. In center column, SWAT modeled ET supplied primarily by aquifer water ("revap" flux) are plotted as blue-colored diamond symbols.

Scatterplots of monthly ET from SWAT exhibited kink-like features at the middle and upper sites, and a bifurcation pattern at the lower site (Figure 6, middle column). Data points within the elbows of these kinks and lower arms of bifurcation occurred during times when SWAT ET was supplied primarily by aquifer water (blue diamond symbols in Figure 6, middle column). At other times, SWAT ET was supplied primarily by water in soil overlying the aquifer. The transition from soil- to aquifer-sourced ET was observed (not shown) to coincide with the relatively abrupt declines in SWAT ET during late summer and fall (Figure 4), and may have contributed to the excessively narrow shapes of ET annual waveforms mentioned in Section 4.2.
