Abstract
Abstract. The JULES-crop model (Osborne et al., 2015) is a parametrisation of crops within the Joint UK Land Environment Simulator (JULES), which aims to simulate both the impact of weather and climate on crop productivity and the impact of croplands on weather and climate. In this evaluation paper, observations of maize at three FLUXNET sites in Nebraska (US-Ne1, US-Ne2 and US-Ne3) are used to test model assumptions and make appropriate input parameter choices. JULES runs are performed for the irrigated sites (US-Ne1 and US-Ne2) both with the crop model switched off (prescribing leaf area index (LAI) and canopy height) and with the crop model switched on. These are compared against GPP and carbon pool FLUXNET observations. We use the results to point to future priorities for model development and describe how our methodology can be adapted to set up model runs for other sites and crop varieties.
Highlights
The Joint UK Land Environment Simulator (JULES) (Best et al, 2011; Clark et al, 2011) is a process-based model that simulates the fluxes of carbon, water, energy and momentum between the land surface and the atmosphere
Even when the scatter in the FAPAR observations is taken into account, the decrease in gross primary productivity (GPP) for lower diffuse radiation fractions does not appear to be as large in the model as in the GPP observations, and this is the source of the overestimation of GPP we saw in the model output in Fig. 18 before the onset of senescence
Fitting γ and δ to the specific leaf area (SLA) observations in just these site and year combinations (2006, 2010, 2011 for US-Ne1 and 2011 for US-Ne2) gives 18.0 and −0.45 respectively. Using these parameters in JULES runs with the crop model gives much better agreement with leaf area index (LAI) observations (Fig. 24)
Summary
The Joint UK Land Environment Simulator (JULES) (Best et al, 2011; Clark et al, 2011) is a process-based model that simulates the fluxes of carbon, water, energy and momentum between the land surface and the atmosphere. No attempt was made to find more appropriate crop parameter values In this model evaluation paper, we use the observations available at the Mead FLUXNET sites US-Ne1, US-Ne2 and US-Ne3 to investigate how well each individual component of JULES performs for maize and how much of an improvement can be achieved by using more appropriate parameter values, taking into account advances in the JULES code since the Osborne et al (2015) study. This investigation splits into three distinct parts.
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