Towards an integrated soil moisture drought monitor for East Africa

Authors: ANDERSON, W - Johns Hopkins University; ZAITCHIK, B - Johns Hopkins University; HAIN, C - National Oceanic & Atmospheric Administration (NOAA); Anderson, Martha; Yilmaz, Mustafa; MECIKALSKI, J - University Of Alabama; SCHULTZ, L - University Of Alabama

Submitted to: Hydrology and Earth System Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/2012
Publication Date: 8/22/2012
Citation: Anderson, W., Zaitchik, B.F., Hain, C., Anderson, M.C., Yilmaz, M.T., Mecikalski, J., Schultz, L. 2012. Towards an integrated soil moisture drought monitor for East Africa. Hydrology and Earth System Sciences.

Interpretive Summary: Few reliable techniques currently exist for monitoring soil moisture conditions over the African continent for the purposes of monitoring agricultural drought. Rain gauges and soil moisture monitoring stations are sparsely distributed through the region, leading to poor spatial interpolation between stations. Satellite data, in combination with water balance models, can significantly help to address the paucity of ground data available for drought monitoring in many parts of the world. In this paper, we investigate the integration of several types of soil moisture assessments into an improved monitoring product for East Africa. Soil moisture maps generated with satellite images collected in the thermal and microwave bands have complementary strengths and weaknesses in terms of temporal and spatial sampling, and can be used to improve soil moisture output from hydrologic models. A simple data fusion approach was applied to objectively estimate spatial weightings that were used to merge the various maps into a unified soil moisture product that may have utility for operational drought monitoring over the African continent.

Technical Abstract: Drought in East Africa is a recurring phenomenon with significant humanitarian impacts. Given the steep climatic gradients, topographic contrasts, general data scarcity, and, in places, political instability that characterize the region, there is a need for spatially distributed, remotely derived monitoring systems to inform national and international drought response. At the same time, the very diversity and data scarcity that necessitate remote monitoring also make it difficult to evaluate the reliability of these systems. Here we apply a suite of remote monitoring techniques to characterize the temporal and spatial evolution of the 2010–2011 Horn of Africa drought. Diverse satellite observations allow for evaluation of meteorological, agricultural, and hydrological aspects of drought, each of which is of interest to different stakeholders. Focusing on soil moisture, we apply triple collocation analysis (TCA) to three independent methods for estimating soil moisture anomalies to characterize relative error between products and to provide a basis for objective data merging. The three soil moisture methods evaluated include microwave remote sensing using the Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E) sensor, thermal remote sensing using the Atmosphere-Land Exchange Inverse (ALEXI) surface energy balance algorithm, and physically-based land surface modeling using the Noah land surface model. It was found that the three soil moisture monitoring methods yield similar drought anomaly estimates in areas characterized by extremely low or by moderate vegetation cover, particularly during the below-average 2011 long rainy season. Systematic discrepancies were found, however, in regions of moderately low vegetation cover and high vegetation cover, especially during the failed 2010 short rains. The merged, TCA-weighted soil moisture composite product takes advantage of the relative strengths of each method, as judged by the consistency of anomaly estimates across independent methods. This approach holds potential as a remote soil moisture-based drought monitoring system that is robust across the diverse climatic and ecological zones of East Africa.