## Results
### Seasonal and Interannual Variabilities of Modeled and Observed Carbon Dioxide and Water Fluxes  
#### 3.2.1 Day-to-Day and Seasonal CO\({}_{2}\) and Water Fluxes Variability  
For managed grassland ecosystems, important drivers of day-to-day and seasonal variabilities are management practices, particularly the timing and intensity of moving and grazing that combine with the natural temporal climate variability to drive the behavior of ecosystems and strongly affect the CO\({}_{2}\) dynamic and C balance of managed grasslands [8]. Depending on a number of climatic factors (solar radiation, temperature, and precipitation), ecological factors (leaf area and water, nutrient, and temperature stresses), and management practices (nitrogen fertilization, moving and grazing timing, duration, and intensity), modeled and observed CO\({}_{2}\) and water fluxes demonstrate pronounced temporal dynamics over several years [79], as exemplified by the time series presented in Figures 6 and 7. The apparent day-to-day and seasonal variabilities of observation based GPP was well captured by the CenW model for both of the managed grassland sites (Figures 6a and 7a). The highest assimilation rates occurred during spring and summer, with GPP values up to 160 kgC ha\({}^{-1}\) d\({}^{-1}\) when growth conditions were the most favorable. Over the summer months, both modeled and observed GPP were reduced through water limitations, which occurred over most summer months but varied in intensity from year to year. Lower CO\({}_{2}\) assimilations rates were found during the winter months as temperature and radiation were low and limited photosynthesis and vegetation growth, but some gas exchange continued throughout even the coldest winters. After moving events, during the peak growing season, GPP was strongly reduced down to wintertime levels. GPP typically dropped from preharvest values in the range of 120 to 160 kgC ha\({}^{-1}\) d\({}^{-1}\) to postharvest values between 20 and 50 kg C ha\({}^{-1}\) d\({}^{-1}\) (Figures 6a and 7a). These reductions of GPP by 2/3 are important and even if, on average, only three harvests were carried out each year, they have significant and long-lasting effects on ecosystem behavior and gas exchange. CenW managed to simulate accurately the recovery of the ecosystem gas exchanges rates (Figure 6, Figure 7, and Figure A1) after cutting and grazing events. The day-to-day variability of observation-derived ER was also reasonably well captured in the model simulation and the seasonal pattern was well reproduced, in particular, displaying ongoing reasonably high respiration rates throughout the winter months (Figure 6b). Harvesting did not affect ER as strongly as GPP (Figure 6a,b) since autotrophic respiration from above ground vegetation is only part of the total ecosystem respiration, and (belowground) heterotrophic respiration was mostly unaffected by harvests. Because NEE is the difference between the two large fluxes of C assimilation through photosynthesis (GPP) and ecosystem respiration (ER), it was also affected by vegetation harvests (Figure 6c). Mowing changed the pasture CO\({}_{2}\) status from a net carbon sink to a net source, and it usually took a few days to a week for NEE to become a sink again, and a few more weeks to return to preharvest carbon fixation levels. On average, over the five years of the study, the GPP recovery from moving took 20 to 25 days. The seasonal and day-to-day variability of GPP and ER, controlled by the variability of climate conditions and timing of harvests causing the sharp reduction of photosynthesisizing and respiring biomass, were well captured by the CenW model for the moved grassland area. Evapotranspiration flux (ET) was highest during spring and summer when climatic conditions were the most favorable for water losses and vegetation was the most active (Figure 6d).