Abstract
We investigate the CO2 flux calculated by the ISBA
soil-vegetation-atmosphere transfer model (Noilhan and Planton, 1989)by
comparing three different formulations for the plant (dark) respiration
scheme applied to a soybean culture. The model includes CO2
flux/photosynthesis based on Jacobs (1994) in a manner similar to Calvet
et al. (1998) (ISBA-A-gs). The first respiration scheme (M0) computed
the autotrophic respiration Rd similarly to Jacobs (1994) but with an
ad-hoc temperature correction calibrated by statistical parameter
fitting using measured data. For the second model (M1), we implemented
the respiration proposed by Joetzjer et al. (2015). Finally we
implemented a third respiration scheme (M2) as in Wang (1996). The three
models were calibrated and CO2 fluxes were compared with measurements
made over a soybean culture using eddy covariance method between
December, 2008 and March, 2009, at a farm near Buenos Aires, Argentina.
The total CO2 maximum, minimum and mean measured flux values were
respectively 0.9890, -0.2479 and 0.3087 mg m-2 s-1. For the sake of
comparison, statistics were computed for the full daily cycle flux
(total) and also for nighttime flux, as a means to avoid masking of the
results due to the much larger daytime photosynthetic flux. We here
present the Nash-Sutcliffe efficiency (NSE) coefficient for each model.
M0 gave the best overall performance with 0.7568 for the total daily CO2
flux and 0.0795 for the dark flux. M1 gave similar predictions for the
daily CO2 flux with 0.7582, butthe worst result for the nighttime period
with -0.4965. M2 gave 0.7424 for the full daily flux and 0.0119 for the
night CO2 flux. The results show a seemingly better performance of the
models in predicting the total CO2 flux compared to the dark CO2 flux.
This is due to several facts such as: respiration is less understood and
harder to predict than photosynthesis; measurements are more difficult
at nighttime due to the limitations of the eddy-covariance technique in
low turbulent activity; in the measured data, it is difficult to
identify and separate the portions of CO2 fluxes as soil respiration,
autotrophic respiration and photosynthetic flux, without many auxiliary
measurements. We also conclude that there is a clear influence of the
temperature on the respiration, which can be suitably incorporated in
the models.