Power-Spectrum Behavior of Yearly Mean Grain Yields
Abstract
Advanced statistical approaches have been applied to determine the temporal scaling-invariance laws in annual mean yields registered in Mexico during the period 1897–1999. When the maize, bean, wheat and rice series are treated as self-affine profiles through power spectrum analysis, the temporal fluctuation f-β is detected in each case for time scales of 2 to 103 years. β values (p < 0.0001) of -1.84 ± 0.15, -1.71 ± 0.218, -1.62 ± 0.194 and -1.7 ± 0.139 for maize, bean, wheat and rice profiles, respectively, suggest that short-term variation is important and are influencing the dynamic evolution of the yearly mean yields. Thus, in addition to the linear trends driven by endogenous factors of agricultural systems, grain yield variation may be due to external forcings such as climatic oscillations and/or solar signals. Through the spectral analysis using the maximum entropy method, the influence of external forcings on the behavior of the annual mean yields in all four crops was evidenced. The detected main external forcings are: a 6- to 8-year political oscillation, the magnetic and sunspot cycles of the Sun, and El Niño Southern Oscillation (ENSO). Taking into account the number of significant periodicities, the order of annual mean yield series' sensitivity to external forcings influencing their short-term variation is as follows: maize < wheat < rice < bean.
