Abstract

Abstract. Agro-land surface models (agro-LSM) have been developed from the integration of specific crop processes into large-scale generic land surface models that allow calculating the spatial distribution and variability of energy, water and carbon fluxes within the soil–vegetation–atmosphere continuum. When developing agro-LSM models, particular attention must be given to the effects of crop phenology and management on the turbulent fluxes exchanged with the atmosphere, and the underlying water and carbon pools. A part of the uncertainty of agro-LSM models is related to their usually large number of parameters. In this study, we quantify the parameter-values uncertainty in the simulation of sugarcane biomass production with the agro-LSM ORCHIDEE–STICS, using a multi-regional approach with data from sites in Australia, La Réunion and Brazil. In ORCHIDEE–STICS, two models are chained: STICS, an agronomy model that calculates phenology and management, and ORCHIDEE, a land surface model that calculates biomass and other ecosystem variables forced by STICS phenology. First, the parameters that dominate the uncertainty of simulated biomass at harvest date are determined through a screening of 67 different parameters of both STICS and ORCHIDEE on a multi-site basis. Secondly, the uncertainty of harvested biomass attributable to those most sensitive parameters is quantified and specifically attributed to either STICS (phenology, management) or to ORCHIDEE (other ecosystem variables including biomass) through distinct Monte Carlo runs. The uncertainty on parameter values is constrained using observations by calibrating the model independently at seven sites. In a third step, a sensitivity analysis is carried out by varying the most sensitive parameters to investigate their effects at continental scale. A Monte Carlo sampling method associated with the calculation of partial ranked correlation coefficients is used to quantify the sensitivity of harvested biomass to input parameters on a continental scale across the large regions of intensive sugarcane cultivation in Australia and Brazil. The ten parameters driving most of the uncertainty in the ORCHIDEE–STICS modeled biomass at the 7 sites are identified by the screening procedure. We found that the 10 most sensitive parameters control phenology (maximum rate of increase of LAI) and root uptake of water and nitrogen (root profile and root growth rate, nitrogen stress threshold) in STICS, and photosynthesis (optimal temperature of photosynthesis, optimal carboxylation rate), radiation interception (extinction coefficient), and transpiration and respiration (stomatal conductance, growth and maintenance respiration coefficients) in ORCHIDEE. We find that the optimal carboxylation rate and photosynthesis temperature parameters contribute most to the uncertainty in harvested biomass simulations at site scale. The spatial variation of the ranked correlation between input parameters and modeled biomass at harvest is well explained by rain and temperature drivers, suggesting different climate-mediated sensitivities of modeled sugarcane yield to the model parameters, for Australia and Brazil. This study reveals the spatial and temporal patterns of uncertainty variability for a highly parameterized agro-LSM and calls for more systematic uncertainty analyses of such models.

Highlights

  • In recent years, many governments have set targets in terms of biofuels consumption for transportation fuel (Sorda et al, 2010), resulting in a large increase in bioenergy cropping area around the world

  • We first made sure that no parameter with a significant value for μ∗ was above the line σ = 2μ∗, which would imply that non-linearities and/or interactions would be so strong that the uncertainty propagation from the parameter to the model output could not be clearly established

  • The Morris parameters ranks for ORCHIDEE and STICS are respectively shown in Fig. 5a and b, where each radar plot corresponds to one model

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Summary

Introduction

Many governments have set targets in terms of biofuels consumption for transportation fuel (Sorda et al, 2010), resulting in a large increase in bioenergy cropping area around the world. The claimed benefits of biofuels for fossil fuel substitution have been questioned in terms of their net effect on atmospheric CO2 and climate, and even of their economic return (Doornbosch and Steenblik, 2008; Naylor et al, 2007). The conditions of biofuel cultivation, such as the type of crop, practice, previous land use, and local climate, have emerged as key factors that determine the effectiveness of their carbon emissions reduction (Fargione et al, 2008; Hill et al, 2006; Searchinger et al, 2008). Based on recent life cycle analysis studies (de Vries et al, 2010; Schubert, 2006; von Blottnitz and Curran, 2007), ethanol from sugarcane is the most competitive in terms of energy use and net carbon balance, and the energy use projections from the International Energy Agency foresee that by 2050, sugarcane is the only first generation biofuel that that will keep expanding (IEA, 2011)

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