Ecosystem Productivity and Water Stress in Tropical East Africa: A Case Study of the 2010-2011 Drought

Characterizing the spatiotemporal patterns of ecosystem responses to drought is important in understanding the impact of water stress on tropical ecosystems and projecting future land cover transitions in the East African tropics. Through the analysis of satellite measurements of solar-induced chlorophyll fluorescence (SIF) and the normalized difference vegetation index (NDVI), soil moisture, rainfall, and reanalysis data, here we characterize the 2010-2011 drought in tropical East Africa. The 2010-2011 drought included the consecutive failure of rainy seasons in October-November-December 2010 and March-April-May 2011 and extended further east and south compared with previous regional droughts. During 2010-2011, SIF, a proxy of ecosystem productivity, showed a concomitant decline (similar to 32% lower gross primary productivity, or GPP, based on an empirical SIF-GPP relationship, as compared to the long-term average) with water stress, expressed by lower precipitation and soil moisture. Both SIF and NDVI showed a negative response to drought, and SIF captured the response to soil moisture with a lag of 16 days, even if it had lower spatial resolution and much smaller energy compared with NDVI, suggesting that SIF can also serve as an early indicator of drought in the future. This work demonstrates the unique characteristics of the 2010-2011 East African drought and the ability of SIF and NDVI to track the levels of water stress during the drought.