Abstract
Abstract. One of the more frequently applied methods for integrating controls on primary production through satellite data is the Light Use Efficiency (LUE) approach. Satellite indices such as the Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI) and the Shortwave Infrared Water Stress Index (SIWSI) have previously shown promise as predictors of primary production in several different environments. In this study, we evaluate NDVI, EVI and SIWSI derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite sensor against in-situ measurements from central Sudan in order to asses their applicability in LUE-based primary production modeling within a water limited environment. Results show a strong correlation between vegetation indices and gross primary production (GPP), demonstrating the significance of vegetation indices for deriving information on primary production with relatively high accuracy at similar areas. Evaluation of SIWSI however, reveal that the fraction of vegetation apparently is to low for the index to provide accurate information on canopy water content, indicating that the use of SIWSI as a predictor of water stress in satellite data-driven primary production modeling in similar semi-arid ecosystems is limited.
Highlights
With the emergence of international environmental treaties such as United Nations Framework Convention on Climate Change (UNFCCC) and its Kyoto Protocol, there is an urgent need to quantify the global carbon budget and its temporal and spatial variations (IPCC, 2007)
Correlations showed that the relationship between Enhanced Vegetation Index (EVI) and gross primary production (GPP) was highly linear (Fig. 3a), there are still a number of factors that influence the vegetation signal recorded at the sensor that in turn can greatly influence the 1:1 relationship assumed between fPAR and EVI in the model
Even though TIMESAT minimizes negatively biased noise due to the interference of clouds and atmospheric constituents, effects of varying solar zenith angles on satellite vegetation index data has previously been shown to be considerable at intermediate Leaf Area Index (LAI) values between 0.25 and 2 (Goward and Huemmrich, 1992)
Summary
With the emergence of international environmental treaties such as United Nations Framework Convention on Climate Change (UNFCCC) and its Kyoto Protocol, there is an urgent need to quantify the global carbon budget and its temporal and spatial variations (IPCC, 2007). One of the least wellcovered regions by studies on carbon dynamics and climate change is Africa, a continent with widespread poverty and slow economic development (Giles, 2007). The region has recently been flagged as a hotspot for climatic change as findings from polar orbiting satellites reveal a widespread increase in vegetation greenness (Eklundh and Olsson, 2003; Herrmann et al, 2005; Seaquist et al, 2006). This observed greening has partly been explained by variations in rainfall (Hickler et al, 2005) and could represent part of the residual terrestrial carbon sink (Houghton, 2003). Knowledge on primary production in this region is of key importance, both in the light of the climatic fluctuations that have occurred in this region over the last decades (Hulme, 2001) and considering the predicted effects of climate change (IPCC, 2007)
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