# Seasonal Variations of Carbon Dioxide, Water Vapor and Energy Fluxes in Tropical Indian Mangroves
## 4 Results and Discussions
### Variations in CO\({}_{2}\) Flux and H\({}_{2}\)O Flux  
The footprint (or fetch) which is attributed to 90% of the total turbulent flux computations was estimated using the [PERSON] _et al._, model [41]. The average footprint during the day was estimated to be about 160 m with a standard deviation of 22 m during the study period (Figure 5). Figure 6 depicts the variations of monthly diurnal averaged carbon dioxide flux over a year (April 2012 to March 2013). Even though mangroves are less prone to phenological changes, there is significant seasonal variation in CO\({}_{2}\) flux when observed annually. Variations in ecosystem respiration during nights (positive CO\({}_{2}\) flux) were minimal with values ranging between 2 to 4 \(\upmu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) throughout the year, whereas CO\({}_{2}\) flux due to photosynthesis during the day (negative CO\({}_{2}\) flux) peaked at \(-6\upmu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) during the summer season (April 2012 to June 2012) and decreased to \(-10\upmu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) during the winter period (October 2012 to December 2012). The half-hourly maximum daytime NEE of \(-11.8\upmu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) was observed during October 2012. The diurnal and seasonal variation of CO\({}_{2}\) fluxes over a year are depicted in Figure 7, which shows the gap-filled NEE (in \(\upmu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)) with respect to time of day and month.  
Figure 4: Total monthly precipitation during April 2012 to March 2013.  
Figure 5: Sundarbans flux tower foot print of 90% of the flux contribution (represented by X 90%) over the study period ranged from 90 m to 300 m.  
Water vapor fluxes (or evapotranspiration (ET) rates), defined as the sum of evaporation and transpiration over an ecosystem, varied between a maximum of 5.5 mmol: m\({}^{-2}\). s\({}^{-1}\) in October 2012 to a minimum of 2.5 mmol: m\({}^{-2}\). s\({}^{-1}\) in July 2013 (Figure 8). The majority of H\({}_{2}\)O fluxes were found to be positive throughout the year with a steady increase during the daytime (photosynthesis and high ET rates), peaking during mid-day (1200 to 1300 h) and decreasing to the lowest during the night (no photosynthesis, lowest ET rates). The mean annual ET during the study period was estimated to be 1.96 \(\pm\) 0.33 mm: day\({}^{-1}\).  
The maximum daytime NEE obtained from the Florida Everglades mangroves is about \(-\)20 to \(-\)25 mmol: m\({}^{-2}\). s\({}^{-1}\)[17]. The NEE values obtained are only one-third of the NEE observed at the Florida Everglades mangroves. The tree heights in the mangroves of the Florida Everglades are 15-20 m [17] which, compared to the mean height of 5 m in the mangroves of Sundarbans, is the possible explanation of the huge variation in NEE.  
Figure 6: Monthly mean diurnal variation of half-hourly CO\({}_{2}\) flux during April 2012 to March 2013. The error bars show one standard deviation of the diurnal average fluxes during respective months.  
Figure 7: Gap-filled net ecosystem exchange (NEE) fingerprint of Sundarbans mangroves showing time of the day (\(x\)-axis) and month-wise (\(y\)-axis) CO\({}_{2}\) fluxes in \(\upmu\)mol: m\({}^{-2}\). s\({}^{-1}\) during the study period.