Comparing Three Respiration Models for the ISBA SVAT

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 R_d 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, but
the 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.