Characterizing the Response of Vegetation Cover to Water Limitation in Africa Using Geostationary Satellites

Abstract

Hydrological interactions between vegetation, soil, and topography are complex, and heterogeneous in semi‐arid landscapes. This along with data scarcity poses challenges for large‐scale modeling of vegetation‐water interactions. Here, we exploit metrics derived from daily Meteosat data over Africa at ca. 5 km spatial resolution for ecohydrological analysis. Their spatial patterns are based on Fractional Vegetation Cover (FVC) time series and emphasize limiting conditions of the seasonal wet to dry transition: the minimum and maximum FVC of temporal record, the FVC decay rate and the FVC integral over the decay period. We investigate the relevance of these metrics for large scale ecohydrological studies by assessing their co‐variation with soil moisture, and with topographic, soil, and vegetation factors. Consistent with our initial hypothesis, FVC minimum and maximum increase with soil moisture, while the FVC integral and decay rate peak at intermediate soil moisture. We find evidence for the relevance of topographic moisture variations in arid regions, which, counter‐intuitively, is detectable in the maximum but not in the minimum FVC. We find no clear evidence for wide‐spread occurrence of the “inverse texture effect” on FVC. The FVC integral over the decay period correlates with independent data sets of plant water storage capacity or rooting depth while correlations increase with aridity. In arid regions, the FVC decay rate decreases with canopy height and tree cover fraction as expected for ecosystems with a more conservative water‐use strategy. Thus, our observation‐based products have large potential for better understanding complex vegetation‐water interactions from regional to continental scales.

Observation-based assessment of secondary water effects on seasonal vegetation decay across Africa

“…The asymptotic decay function quantifying the decay rate, independent of amplitude and timing of the event, allows comparing the rate of decay of vegetation across different climate zones, thus understanding the driving factors behind the spatial variation of λ. Initial analysis of λ showed that λ corroborates the rate of decrease in plant available water use under water limited conditions (Küçük et al, 2022). For a given level of aridity, a taller canopy decays more slowly, thus larger λ values, than a shorter one, which agrees with the previous field-based studies (Teuling et al, 2006;Boese et al, 2019).…”

“…For instance, the impact of secondary water on vegetation can be expected to be the largest in periods of progressive water limitation. Küçük et al (2022) showed that vegetation cover decay is controlled by water availability to the first order over most of Africa, consistent with previous literature, and with theoretical expectations of dryland ecohydrology (Rodriguez-Iturbe and Porporato, 2005). The secondary moisture effects essentially act to keep water longer in the system and fuel plant accessible soil moisture for a prolonged period, which results in a delayed and buffered decay of the vegetation cover.…”

“…In this study, we investigate the drivers of the spatial variations of the seasonal decay rate of vegetation cover, λ, which is estimated using an asymptotic exponential decay function across Africa using daily geostationary satellite retrievals of 16year long Fractional Vegetation Cover data at ca. Five kilometer spatial resolution (Küçük et al, 2022). The asymptotic decay function quantifying the decay rate, independent of amplitude and timing of the event, allows comparing the rate of decay of vegetation across different climate zones, thus understanding the driving factors behind the spatial variation of λ.…”

Analysis of Short-Term Drought Episodes Using Sentinel-3 SLSTR Data under a Semi-Arid Climate in Lower Eastern Kenya

“…The was a strong correlation between April and September in both years (Figure 9c,d), whi can be attributed to differences in moisture content between the two periods, hydrothe mal characteristics, and consistency of precipitation in April and a contrary situation September. Studies indicate that FVC can increase and decrease abruptly for a short p riod, and it is positively correlated with precipitation and moisture index [59,121,123].…”

Multi-fold increase in rainforests tipping risk beyond 1.5-2⁰C warming

“…3 confidence about the possible tipping of the Amazon forest by the end of the 21 st century 37 . However, with mounting empirical evidence on how climate change influences rainforest ecosystems 6,13,38,39 , the research on rainforest resilience loss has accelerated considerably in the recent decade 40,41 . Yet, forest resilience is often assessed based on changes in forest carbon stocks 41,42 or precipitation 23,43,44 ; and rarely on actual moisture storage capacity in the root zone 13 .…”