Effects of climatic data low-pass filtering on the ICBM temperature- and moisture-based soil biological activity factors in a cool and humid climate

The air temperature (Tₐᵢᵣ), total precipitation (TP) and potential evapotranspiration (PET) are standard input data for soil carbon dynamic models, i.e., for calculating temperature and moisture effects on soil biological activity. The resolution needed depends on objectives, the complexity of models and inbuilt pedotransfer functions. The Introductory Carbon Balance Model (ICBM) soil climate front end model calculates a multiplicative soil-temperature (rₑ_ₜₑₘₚ) and -moisture (rₑ_wₐₜ) factor with a daily time-step to estimate soil biological activity, i.e., rₑ_cᵣₒₚ=rₑ_ₜₑₘₚ×rₑ_wₐₜ. Our objective was to determine how much rₑ_ₜₑₘₚ, rₑ_wₐₜ and rₑ_cᵣₒₚ are affected by low-pass filtering of the climatic input data for a cool, humid temperate region. To achieve this we conducted spectral analysis on Tₐᵢᵣ, TP, PET and rₑ_cᵣₒₚ in the frequency domain. Thereafter we applied Fourier low-pass filters of 5, 15, 30, 60 and 180 days on Tₐᵢᵣ, TP, PET and tracked their effects through the soil climate model's state variables and outputs. This was done using a sandy and a heavy clay soil and an 89-year daily time-series from a meteorological station in Quebec (Canada). The Fourier spectra showed that the variance for Tₐᵢᵣ, PET and rₑ_cᵣₒₚ was dominated by an annual cycle, as could be expected. There was no yearly cycle for TP. The variation in rₑ_ₜₑₘₚ explained most of the variance in rₑ_cᵣₒₚ. The soil climate module outputs were not sensitive to low-pass filtering of PET. A daily time-step was needed to avoid overestimating rₑ_cᵣₒₚ for the sandy soil. Using a weekly time-step for TP and Tₐᵢᵣ allowed us to explain about 80% of the variance in rₑ_cᵣₒₚ for the heavy clay soil. This study also indicates that the standard leaf (and green) area index functions for calculating transpiration should receive more attention, since they have significant effects on the state and output variables.