title: Modelling carbon and water exchange of a grazed pasture in New Zealand constrained by eddy covariance measurements.
abstract: We used two years of eddy covariance (EC) measurements collected over an intensively grazed dairy pasture to better understand the key drivers of changes in soil organic carbon stocks. Analysing grazing systems with EC measurements poses significant challenges as the respiration from grazing animals can result in large short-term CO2 fluxes. As paddocks are grazed only periodically, EC observations derive from a mosaic of paddocks with very different exchange rates. This violates the assumptions implicit in the use of EC methodology. To test whether these challenges could be overcome, and to develop a tool for wider scenario testing, we compared EC measurements with simulation runs with the detailed ecosystem model CenW 4.1. Simulations were run separately for 26 paddocks around the EC tower and coupled to a footprint analysis to estimate net fluxes at the EC tower. Overall, we obtained good agreement between modelled and measured fluxes, especially for the comparison of evapotranspiration rates, with model efficiency of 0.96 for weekly averaged values of the validation data. For net ecosystem productivity (NEP) comparisons, observations were omitted when cattle grazed the paddocks immediately around the tower. With those points omitted, model efficiencies for weekly averaged values of the validation data were 0.78, 0.67 and 0.54 for daytime, night-time and 24-hour NEP, respectively. While not included for model parameterisation, simulated gross primary production also agreed closely with values inferred from eddy covariance measurements (model efficiency of 0.84 for weekly averages). The study confirmed that CenW simulations could adequately model carbon and water exchange in grazed pastures. It highlighted the critical role of animal respiration for net CO2 fluxes, and showed that EC studies of grazed pastures need to consider the best approach of accounting for this important flux to avoid unbalanced accounting.
authors: Landcare Research, Private Bag 11052, Palmerston North, New Zealand. Electronic address: KirschbaumM@LandcareResearch.co.nz.
authors: Miko U F
authors: MUF
authors: Kirschbaum
authors: School of Science, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand.
authors: Susanna
authors: S
authors: Rutledge
authors: Landcare Research, Private Bag 11052, Palmerston North, New Zealand.
authors: Isoude A
authors: IA
authors: Kuijper
authors: School of Science, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand; Landcare Research, Private Bag 3127, Hamilton 3240, New Zealand.
authors: Paul L
authors: PL
authors: Mudge
authors: Landcare Research, Private Bag 11052, Palmerston North, New Zealand.
authors: Nicolas
authors: N
authors: Puche
authors: School of Science, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand.
authors: Aaron M
authors: AM
authors: Wall
authors: DairyNZ, Private Bag 3221, Hamilton 3240, New Zealand.
authors: Chris G
authors: CG
authors: Roach
authors: School of Science, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand.
authors: Louis A
authors: LA
authors: Schipper
authors: School of Science, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand.
authors: David I
authors: DI
authors: Campbell
keywords: CenW; Evapotranspiration; Grazing; Net ecosystem exchange; Net ecosystem productivity; Photosynthesis; Respiration