Puche NJB, Kirschbaum MUF, Rutledge S, Schipper LA, Mudge PL, Wall AM, Campbell DI, Hedley, MJ (2019). Detailed gas-exchange modelling of a grazed New Zealand dairy pasture with critical assessment of the standard eddy-covariance gap-filling algorithm. Agricultural and Forest Meteorology (Submitted)

Abstract. Pasture-based livestock production is important for New Zealandís economy, but it is also a main contributor to the countryís net greenhouse gas emissions. In our work, we aimed to better understand the effect of dairy farming on net carbon balances and contribute towards the development of alternative management options to maximise soil carbon stocks. We used half-hourly eddy-covariance (EC) data, previously collected over 2 consecutive years from a grazed dairy pasture in the Waikato region in New Zealand. To match the time step of the observations, we developed a new half-hourly version of the physiologically based CenW ecosystem model. To deal with the heterogeneity of surface fluxes caused by rotational grazing, e ran the model independently for each of 26 individual paddocks surrounding the EC tower and used a footprint model to estimate resultant net fluxes at the EC tower. Once parameterised with best-quality observations, modelled half-hourly fluxes agreed well with observed fluxes with Nash-Sutcliffe model efficiencies of 0.81 for net ecosystem productivity, 0.75 for gross primary production and 0.70 for ecosystem respiration. The model was then applied to test the reliability of the conventional Reichstein gap-filling algorithm to fill gaps in net carbon and water fluxes under different climatic conditions. While fluxes modelled by the two models were highly correlated, there were systematic discrepancies between average model estimates, especially for respiration rates, which were then also reflected in net ecosystem exchange rates. On average, the Reichstein model estimated net ecosystem productivity that was about 0.4 umol m-2 s-1 lower than CenW estimates.

Keywords: CenW, net ecosystem productivity, carbon balance, grazing, respiration, gap filling, eddy-covariance, model.  

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