# Partitioning Evapotranspirationiration in a Cotton Field under
## 3 Results
### ET and GPP  
_ET_ and _GPP_ are important parameters for partitioning \(ET\) components and necessary intermediate variables for estimating T/ET based on the water-carbon flux coupling. Their change processes were similar throughout the whole stage, showing significant characteristics of seasonal variation, as shown in Figure 4. During the fast growth rate stage from the seedling stage to the budding stage, \(ET\) and \(GPP\) showed a significant upward trend. At the blooming and bell stages, the cotton plant morphology was basically unchanged; \(ET\) and \(GPP\) had the same daily maximum values but a large fluctuation; the daily maximum values of \(ET\) in the two years were 3.96 mm\(\cdot\)d\({}^{-1}\) and 4.01 mm\(\cdot\)d\({}^{-1}\), respectively; and the daily maximum values of \(GPP\) in the two years were 12.20 g\(\cdot\)m\({}^{-2}\).d\({}^{-1}\) and 12.38 g\(\cdot\)m\({}^{-2}\).d\({}^{-1}\) respectively. After the end of the blooming and bell stages, the cotton leaves began to wither, the cotton bolls swelled to internal fullness, dehydrated, and finally cracked to achieve the effect of bell opening. During the opening stage, \(ET\) and \(GPP\) gradually decreased until the end of the whole stage. Based on the results of two years of experiments, \(ET\) was 46.49 mm at seedling stage, 92.27 mm at budding stage, 106.11 mm at blooming and bell stage, and 66.03 mm at boll opening stage. \(GPP\) at four growth stages was 165.24 g\(\cdot\)m\({}^{-2}\), 264.60 g\(\cdot\)m\({}^{-2}\), 376.54 g\(\cdot\)m\({}^{-2}\), and 185.12 g\(\cdot\)m\({}^{-2}\), respectively. The size rules of \(ET\) and \(GPP\) at four growth stages were consistent: blooming and bell stage \(>\) budding stage \(>\) boll opening stage \(>\) seedling stage. Figure 3: Day-to-day variation of environmental factors in 2021 (**a**) and 2022 (**b**). During different growth stages over two years, a certain day was selected as a typical day, and the diurnal dynamic change processes of \(ET\) and \(GPP\) were specifically analyzed by taking their mean values. Due to the fact that \(ET\) and photosynthesis mainly occur during the daytime, only the diurnal dynamic changes of \(ET\) and \(GPP\) during 08:00-22:00 were analyzed, as shown in Figure 5.  
For seedling stage, \(ET\) reached the maximum value of 0.13 mm at 16:00, and \(GPP\) reached the maximum value of 0.33 g\(\cdot\)m\({}^{-2}\) at 15:00. The diurnal dynamic change processes of both were unimodal curves. For the budding stage, \(ET\) reached the maximum value of 0.21 mm at 14:00-15:30, and \(GPP\) reached the maximum value of 0.62 g\(\cdot\)m\({}^{-2}\) at 14:00-16:00. The diurnal dynamic change processes of the two showed bimodal curves. For blooming and ball stages, \(ET\) reached the maximum value of 0.27 mm at 14:00-16:00, and \(GPP\) reached the maximum value of 0.80 g\(\cdot\)m\({}^{-2}\) at 14:30-16:30. The diurnal dynamic change processes of both were also bimodal curves. For the ball opening stage, \(ET\) reached the maximum  
Figure 4: Day-to-day variation of the \(ET\) and \(GPP\) in 2021 (**a**) and 2022 (**b**). Figure 5: Diurnal variation of the \(ET\) (**a**) and \(GPP\) (**b**). \(T\) was weak at this stage. \(ET\) was mainly \(E\), so the change curve of \(E\) was slightly higher than that of \(T\). The values of \(T\) in 2021 and 2022 were 17.5 mm and 15.48 mm in seedling stage, respectively, accounting for 37% and 31% of \(ET\) in this growth stage. With the growth of cotton, the cotton plant became higher and higher, and the number and area of leaves and the canopy coverage of soil begun to increase. Therefore, at the budding stage and the blooming and boll stage, \(T\) became stronger and stronger, and its proportion in \(ET\) became larger and larger. \(ET\) was dominated by \(T\), and the change curve of \(T\) was much higher than that of \(E\). During the whole stage in two years, the values of \(T\) in the budding stage were 54.52 mm and 54.49 mm, accounting for 62% and 59%, respectively.