Harmonic analysis of time-series AVHRR NDVI data

Abstract

Harmonic analysis of a one-year time series (26 periods) of NOAA AVHRR NDVI biweekly composite data was used to characterize seasonal changes for natural and agricultural land uselland cover in Finney County in southwest Kansas. Different crops (corn, soybeans, alfalfa) exhibit distinctive seasonal patterns of NDVI variation that have strong periodic characteristics. Harmonic analysis, also termed spectral analysis or Fourier analysis, decomposes a time-dependent periodic phenomenon into a series of sinusoidal functions, each defined by unique amplitude and phase values. The proportion of variance in the original time-series data set accounted for by each term of the harmonic analysis can also be calculated. Amplitude and phase angle images were produced from analysis of the time-series NDVI data and correlated with information on crop type and extent for the region to develop a methodology for crop-type identification. Crop types occurring in southwest Kansas, including corn, winter wheat, alfalfa, pasture, and native prairie grasslands, were characterized and identified using this technique and biweekly AVHRR composite data for 1992. For crops with a simple phenology, such as corn, the majority of the variance was captured by the first and additive terms of the harmonic analysis, while winter wheat exhibited a bimodal NDvI periodicity with the majority of the variance accounted for by the second harmonic term.