Investigating flash floods potential areas using ASCAT and TRMM satellites in the Western Cape Province, South Africa

Abstract

The aim of the study was to evaluate flash flood potential areas in the Western Cape Province of South Africa, by integrating remote sensing products of high rainfall intensity, antecedent soil moisture and topographic wetness index (TWI). Rainfall has high spatial and temporal variability, thus needs to be quantified at an area in real time from remote sensing techniques unlike from sparsely distributed, point gauge network measurements. Western Cape Province has high spatial variation in topography which results in major differences in received rainfall within areas not far from each other. Although high rainfall was considered as the major cause of flash flood, also other contributing factors such as topography and antecedent soil moisture were considered. Areas of high flash flood potential were found to be associated with high rainfall, antecedent precipitation and TWI. Although TRMM 3B42 was found to have better rainfall intensity accuracy, the product is not available in near real time but rather at a rolling archive of three months; therefore, Multi- sensor precipitation estimate rainfall estimates available in near real time are opted for flash flood events. Advanced Scatterometer (ASCAT) soil moisture observations were found to have a reasonable r value of 0.58 and relatively low MAE of 3.8 when validated with in situ soil moisture measurements. The results of this study underscore the importance of ASCAT and TRMM satellite datasets in mapping areas at risk of flooding.

Keywords:

• topographic wetness index
• satellite rainfall estimates
• antecedent soil moisture
• flash flood

Acknowledgments

Authors would like to thank the South African Weather Services (SAWS) and Agricultural Research Council of South Africa (ARC) for providing hourly rainfall data. Many thanks also go to the City of Cape Town Rapid Response Disaster Risk Management for providing maps of flash flooding risk areas. South Africa’s Department of Water Affairs is acknowledged for providing hourly stream gauge measurements for the Breede and Berg rivers. Finally, the University of the Western Cape, Department of Environmental & Water Science for being resourceful during data collection.

Disclosure statement

No potential conflict of interest was reported by the authors.