Monitoring Sahelian floodplains using Fourier analysis of MODIS time‐series data and artificial neural networks

Westra, Toon; Wulf, Robert R. De

doi:10.1080/01431160600887698

Cited by 28 publications

(12 citation statements)

References 16 publications

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“…Wetland areas (CLC41 and CLC42) showed a bimodal pattern (more clearly distinguishable for CLC42) with an early peak in February-March and a long period of high values between June-July to October-November with two sub-peaks for this second half year. Westra and De Wulf [25] also observed this typical pattern of a bimodal temporal distribution for flooded land, resulting in relatively high amplitude values for the second and third harmonic terms. For wetland areas, significant phase variations are expected due to the start and end of the flooding cycle and the density of the flooded vegetation [25].…”

Section: Fourier Transform Parametermentioning

confidence: 95%

“…Westra and De Wulf [25] also observed this typical pattern of a bimodal temporal distribution for flooded land, resulting in relatively high amplitude values for the second and third harmonic terms. For wetland areas, significant phase variations are expected due to the start and end of the flooding cycle and the density of the flooded vegetation [25]. This phenomenon provides an explanation for the different behaviors (vegetation re-growth after winter) of wetland areas.…”

Section: Fourier Transform Parametermentioning

confidence: 95%

“…The phenological behavior of this kind of land-cover classes was comparable for the test areas in south Alicante and the Moody and Johnson's Californian. In relation to wetland areas, Westra and De Wulf [25] observed for Sahelian floodplains a bimodal temporal distribution for EVI time series, that it is also observed in Alicante (CLC41 and CLC42) by the relative importance of the second and third harmonic terms.…”

Section: Fourier Transform Parametermentioning

confidence: 99%

“…In addition, the amplitude and phase values of the harmonic terms for the different frequencies inform about the relative importance of periodic climate processes [21]. Harmonic analysis of time-series data offers a valuable tool for monitoring land surface phenologies [16,22], for mapping land-uses [20,23], for modeling bioclimatic indices [24], or monitoring flooding extent [25].…”

mentioning

confidence: 99%

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Land-Cover Phenologies and Their Relation to Climatic Variables in an Anthropogenically Impacted Mediterranean Coastal Area

et al. 2010

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Section: Fourier Transform Parametermentioning

confidence: 95%

Section: Fourier Transform Parametermentioning

confidence: 95%

Section: Fourier Transform Parametermentioning

confidence: 99%

mentioning

confidence: 99%

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Land-Cover Phenologies and Their Relation to Climatic Variables in an Anthropogenically Impacted Mediterranean Coastal Area

et al. 2010

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“…Each individual wave is called a harmonic term and is characterized by its amplitude (height of the maximum), frequency (number of cycles), and phase (delay from time zero). Previous studies have used this method successfully to characterize seasonal changes in natural land-cover and land-use types (Jakubauskas, Legates, and Kastens 2001;Jakubauskas, Legates, and Kastens 2002;Canisius, Turral, and Molden 2007;Westra and De Wulf 2007). Jung and Chang (2015) assessed land-cover change using harmonic analysis.…”

Section: Introductionmentioning

confidence: 99%

Estimating fractional cover of plant functional types in African savannah from harmonic analysis of MODIS time-series data

Ibrahim

Balzter

Tansey

et al. 2018

International Journal of Remote Sensing

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Assessments of tree/grass fractional cover in savannahs using remote sensing are challenging due to the heterogeneous mixture of the two plant functional types. Time-series decomposition models can be used to characterize vegetation phenology from satellite data, but have rarely been used for attributing phenological signal components to different plant functional types. Here, tree/ grass dynamics are assessed in savannah ecosystems using timeseries decomposition of 14 years of Moderate Resolution Imaging Spectroradiometer (MODIS) normalized difference vegetation index data acquired from 2002 to 2015. The decomposition method uses harmonic analysis and tests the individual harmonic terms for statistical significance. Field data of fractional cover of trees and grasses were collected for 28 plots in Kruger National Park, South Africa. Matching MODIS pixels were analysed for their tree/grass phenological signals. Tree/grass annual and interannual variability were then assessed based on the harmonic models. In most harmonic cycles, grass-dominated sites had higher amplitudes than tree-dominated sites, while the tree green-up started earlier than grasses, before the start of the wet season. While changes in tree phenology are gradual, grasses present higher variability over time. Tree cover showed a significant correlation with the amplitude (r (correlation coefficient) = −0.59, p = 0.001) and phase of the first harmonic term (r = −0.73, p = 0.0001) and the number of cycles of the second harmonic term (r = 0. 56, p = 0.002). Grass cover was also significantly correlated with the amplitude (r = 0. 51, p = 0.005) and phase of the first harmonic term (r = 0.55, p = 0.002) and the number of cycles of the second harmonic term (r = −0.52, p = 0.005). The positive correlation of grass cover with phase and negative correlation with number of cycles is indicating a late greening period and higher variability, respectively. Tree cover estimated from the phase of the strongest harmonic term showed a positive correlation with field-measured tree cover (R 2 (coefficient of determination) = 0.55, p < 0.01, slope = 0.93, root mean square error = 13.26%). The estimated tree cover also had a strong correlation with the woody cover map (r = 0.78, p < 0.01) produced by Bucini. The results show that MODIS time-series data can be used to estimate the fractional tree cover in heterogeneous savannahs from the phase of the plant functional type's phenological behaviour. This study shows that harmonic analysis is able to discriminate between fractional cover by trees and grasses in savannahs. The quantitative analysis of tree/grass phenology from satellite time-series data enables a better understanding of the dynamics of the tree/grass competition and coexistence.

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An Integrated Approach to Modeling Grazing Pressure in Pastoral Systems: The Case of the Logone Floodplain (Cameroon)

Moritz

Soma

Scholte³

et al. 2010

Hum Ecol

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The discussion about the impact of pastoral systems on ecosystems has been profoundly shaped by Hardin's "tragedy of the commons" argument that held pastoralists responsible for overgrazing the range. Recent studies have shown that grazing ecosystems are much more complex and dynamic than was previously assumed and that pastoralists adaptively manage these systems. However, we still have little understanding how everyday herding affects ecosystems at the landscape level. We conducted a study of daily herd movements and grazing strategies in a mobile pastoral system in the Logone floodplain, Cameroon. We integrated GPS/GIS technology, video recordings of animal behavior, and ethnographic methods to develop a more accurate measurement of grazing pressure that takes into account both livestock densities and grazing behavior. We used the resulting grazing pressure data to evaluate existing conceptual models of grazing pressure at a landscape level. We found that models that predict that grazing pressure is skewed towards the direction of water most accurately reflect the situation in the Logone floodplain in the dry season. However, we found that the higher grazing pressure is not only the result of a higher density of cattle but also a change in the grazing behavior of animals after watering. Finally, we caution that the models of grazing pressure in the dry season cannot simply be extrapolated to the landscape level because mobile pastoralists do not remain in one central place.

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Monitoring Sahelian floodplains using Fourier analysis of MODIS time‐series data and artificial neural networks

Cited by 28 publications

References 16 publications

Land-Cover Phenologies and Their Relation to Climatic Variables in an Anthropogenically Impacted Mediterranean Coastal Area

Land-Cover Phenologies and Their Relation to Climatic Variables in an Anthropogenically Impacted Mediterranean Coastal Area

Estimating fractional cover of plant functional types in African savannah from harmonic analysis of MODIS time-series data

An Integrated Approach to Modeling Grazing Pressure in Pastoral Systems: The Case of the Logone Floodplain (Cameroon)

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