*3.3. Sensible Heat Flux*

The sensible heat flux was estimated using Equation (4), using high-frequency air temperature measurements of fine-wire thermocouples [34]. A linear regression analysis was performed between the half-hourly datasets of *HSR* and *HEC* under unstable conditions (Figure 5). Overall estimations of the SR method were in better agreement with the EC system with a coefficient of regression *R<sup>2</sup>* = 0.80. This performance was evaluated for the time-lag of 0.5 s, keeping in view the results of previous studies using the same time-lag and frequency for the estimation of surface fluxes using the SR method and sampling frequency (~10 Hz) [10,18]. The uncalibrated *H SR* was corrected using the calibration coefficient of 0.68 obtained from the slope of regression between the uncalibrated and calibrated sensible heat fluxes obtained from the SR and EC systems, respectively, as shown in Table 1 for the same duration under unstable conditions. The comparison was performed to assess the performance of the SR method in tea plants.

**Figure 5.** Half-hourly regression analysis between HSR and HEC.

**Table 1.** The regression statistics between the half-hourly dataset.


The overall results showed good agreement between the estimations by the SR method and the EC system, with *R2* = 0.80, *RMSE* = 27.87 W·m−2, and *RE* = 5.67% (Table 1). Diurnal variation of the half-hourly estimations of *HSR*, *HEC*, and *Rn* were observed for two randomly selected days from the experiment duration, one being a clear day (day of year 125, 2019) (Figure 6a) and one being a day with variable clouds (day of year 101, 2019) (Figure 6b).

(**a**) Clear day (day of year (DOY) 125, 2019). (**b**) Cloudy day (day of year 101, 2019).

**Figure 6.** Diurnal variation of the half-hourly *HSR*, *HEC*, and *Rn* (**a**,**b**).

On the clear day, the estimations were better correlated with the measurements of the EC system in the morning and in the later part of the day (evening). The overall estimation was relatively good throughout the day. On the other hand, the estimation of *HSR* was changing all day, corresponding to the values of *Rn*. In the morning, *HSR* was close to zero and it increased steadily with the increase in *Rn*; the estimation of *HSR* was varied with the measurement of *Rn*, and same trend was observed throughout the day (Figure 6a).

This showed that the air temperature fluctuations present a ramp pattern. The ramps are different for the different climatic conditions and have a direct impact on the measurements of the net radiation.
