Should seasonal rainfall forecasts be used for flood preparedness?

Erin Coughlan De Perez,Simon Mason,Maarten Van Aalst,Hannah Nissan,Florian Pappenberger,Bart Van Den Hurk,Konstantinos Bischiniotis,Elisabeth Stephens

doi:10.5194/hess-21-4517-2017

Abstract

Abstract. In light of strong encouragement for disaster managers to use climate services for flood preparation, we question whether seasonal rainfall forecasts should indeed be used as indicators of the likelihood of flooding. Here, we investigate the primary indicators of flooding at the seasonal timescale across sub-Saharan Africa. Given the sparsity of hydrological observations, we input bias-corrected reanalysis rainfall into the Global Flood Awareness System to identify seasonal indicators of floodiness. Results demonstrate that in some regions of western, central, and eastern Africa with typically wet climates, even a perfect tercile forecast of seasonal total rainfall would provide little to no indication of the seasonal likelihood of flooding. The number of extreme events within a season shows the highest correlations with floodiness consistently across regions. Otherwise, results vary across climate regimes: floodiness in arid regions in southern and eastern Africa shows the strongest correlations with seasonal average soil moisture and seasonal total rainfall. Floodiness in wetter climates of western and central Africa and Madagascar shows the strongest relationship with measures of the intensity of seasonal rainfall. Measures of rainfall patterns, such as the length of dry spells, are least related to seasonal floodiness across the continent. Ultimately, identifying the drivers of seasonal flooding can be used to improve forecast information for flood preparedness and to avoid misleading decision-makers.

Highlights

Humanitarians have been investing significant attention and resources in the uptake and use of climate services to inform their work in disaster risk management
When the rainfall and floodiness are aggregated by food producing units (FPUs) and correlated, the correlations improve in almost all locations, suggesting that seasonal total rainfall forecasts for FPUs (Fig. 1c and d) might be of greater use than grid-box forecasts (Fig. 1a and b) as a predictor of flood hazard
We calculate the probability of flooding conditional on seasonal rainfall being in the top tercile of the distribution, which is the focus of many seasonal forecasts

Summary

Introduction

Humanitarians have been investing significant attention and resources in the uptake and use of climate services to inform their work in disaster risk management. While many early warning systems focus on short-term hydrological flood warnings, these climate service initiatives promote the use of forecasts of seasonal total rainfall. The use of such forecasts has yielded mixed results when used to prepare for heightened flood risk in Africa, such as prepositioning flood relief items (Braman et al, 2013) and evacuating vulnerable people (Anon, 2016). To clarify whether seasonal total rainfall forecasts indicate increased risk of flooding, we identify the dominant indicators of seasonal flooding in different locations of subSaharan Africa. It is likely that total rainfall is not the dominant driver, and other seasonal descriptors would give a better indication of the risk of flood hazards. Cumulative rainfall is not the dominant flood-generating process for floods in most river basins in the United States (Berghuijs et al, 2016), and monthly total rainfall has not been shown to be a good indicator of regional river “floodiness”, or the percentage of regional rivers with extreme

Methods

Results

Conclusion