Content uploaded by Mouhamadou Bamba Sylla
Author content
All content in this area was uploaded by Mouhamadou Bamba Sylla on Apr 17, 2019
Content may be subject to copyright.
A preview of the PDF is not available
Regional Climate Change Series: Floods
Abstract and Figures
Climate change is unequivocal. The increase of global temperature since the pre-industrial period has not only intensified the extremes events but also increased their frequency of occurrences. Such events are often translated into climate hazards. These climate hazards have resulted in major disasters with losses of infrastructures, economy, natural resources and human lives. Between 1970 and 2014, a total of 11, 985 disasters have been reported by the UN-ESCAP with storms and floods sharing 64% representing thus an acute increasing trend. However, the magnitudes of the climate hazards and subsequent disasters are not uniformly distributed across the world. The highest death toll, losses and damages are concentrated in developing countries.
In West Africa, floods and droughts are the major climate hazards that cause disasters. In fact often these climate hazards overcome easily local response capacity of the countries and substantially affect the social and economic development. This is particularly true for the year 2017 flagged as the year of climate extremes in West Africa with disasters hitting communities across the region. The frequency of storms and subsequent floods have
substantially increased since 1982 and the year 2017 has seen an unprecedented number of flood events occurring “simultaneously” in many West African countries’ capital cities.
For example on July 2017 Greater Accra, Central Region, Western Region and Eastern Region of Ghana were declared as “flood emergency” areas. On August 2017, Sierra Leone floods kill thousands as mudslides bury houses near Freetown. In Cote d’Ivoire, heavy rainfall in the southwestern areas (i.e. around San Pedro) have flooded the region
threatening the cocoa plantations. In Nigeria, many people were reported dead after intense storms hit Lagos and Port Harcourt occasioning massive floods that washed houses away. Also on August 2017, continuous heavy rainfall in Niger resulted in extensive flooding, destruction of houses and loss of household belongings in several areas including Niamey. Other examples include Republic of Guinea and Mali.
In response to such an increase of disaster occurrences, the international community has agreed on a number climate goals to address this issue. Among them are the Paris Agreement, the Sustainable Development Goals and the Sendai Framework on Disaster Risk Reduction. The Sendai Framework aims at developing actions that prevent new and reduce existing disaster risks. In this regards, many West African countries are implementing their national plan for disaster risk management by strengthening the institutional and political coordination at all levels. The objective is to build functional early warning system and response strategy to be able to face climate hazards such as those that occurred in 2017.
In this context, WASCAL together with its partners initiated this book entitled “Regional Climate Change Series: Floods” to lay ground for such actions. The book comprises of 10 chapters dealing with physical science basis of the climate hazards as well as vulnerability of communities and response strategies. Authors are mainly WASCAL scientists, regional and international partners and alumni from the graduate studies program. This publication is the first volume of an annual series of books on challenges of climate and land use that WASCAL and partners intends to release to assert its contribution to support ECOWAS countries in science-based decision making process and for the achievement of the sustainable development goals.
Figures - uploaded by Mouhamadou Bamba Sylla
Author content
All figure content in this area was uploaded by Mouhamadou Bamba Sylla
Content may be subject to copyright.
This article evaluates the current gaps and describes opportunities for improving flood risk management (FRM) in Ghana, West Africa. A mixed‐method participatory approach comprising questionnaires, workshops, interviews with key stakeholders, and a systematic literature review were employed. Existing problems, discourses, FRM practices, and opportunities to enhance flood resilience were identified. They provided the basis for outlining potential research directions into ways of tracking these challenges. The results show how different actors perceive FRM in Ghana. The stakeholders interviewed have different, and even contradictory perceptions of the effectiveness of FRM, which are embedded in their diverse storylines. The findings show that Ghana's FRM is still reactive rather than preventive and that research in the field of quantitative hazard and risk assessment is still rudimentary. FRM policies and flood early warning systems (FEWS) are in place, but efforts should be directed towards their implementation and monitoring, investigation of social and technical capacity aspects, and enhancement of institutions’ mandates, and coordination. Moreover, the findings illustrate that FRM is moving toward a more constructive engagement of citizens and stakeholders. However, policies and action plans need to consider more inclusive community participation in planning and management to effectively improve their resilience and develop sustainable solutions.
The three state-of-the-art global atmospheric reanalysis models—namely, ECMWF Interim Re-Analysis (ERA-Interim), Modern-Era Retrospective Analysis for Research and Applications (MERRA; NASA), and Climate Forecast System Reanalysis (CFSR; NCEP)—are analyzed and compared with independent ob- servations in the period between 1989 and 2006. Comparison of precipitation and temperature estimates from the three models with gridded observations reveals large differences between the reanalyses and also of the observation datasets. A major source of uncertainty in the observations is the spatial distribution and change of the number of gauges over time. In South America, active measuring stations were reduced from 4267 to 390. The quality of precipitation estimates from the reanalyses strongly depends on the geographic location, as there are significant differences especially in tropical regions. The closure of the water cycle in the three reanalyses is analyzed by estimating long-term mean values for precipitation, evapotranspiration, surface runoff, and moisture flux divergence. Major shortcomings in the moisture budgets of the datasets are mainly due to inconsistencies of the net precipitation minus evaporation and evapotranspiration, respectively, (P 2 E) estimates over the oceans and landmasses. This imbalance largely originates from the assimilation of radiance sounding data from the NOAA-15 satellite, which results in an unrealistic increase of oceanic P 2 E in the MERRA and CFSR budgets. Overall, ERA-Interim shows both a comparatively reasonable closure of the terrestrial and atmospheric water balance and a reasonable agreement with the observation datasets. The limited performance of the three state-of-the-art reanalyses in reproducing the hydrological cycle, however, puts the use of these models for climate trend analyses and long-term water budget studies into question.
Understanding of hydroclimatic processes in Africa has been hindered by the lack of in situ precipitation measurements. Satellite-based observations, in particular, the TRMM Multisatellite Precipitation Analysis (TMPA) have been pivotal to filling this void. The recently released Integrated Multisatellite Retrievals for GPM (IMERG) project aims to continue the legacy of its predecessor, TMPA, and provide higher-resolution data. Here, IMERG-V04A precipitation data are validated using in situ observations from the Trans-African Hydro-Meteorological Observatory (TAHMO) project. Various evaluation measures are examined over a select number of stations in West and East Africa. In addition, continent-wide comparisons are made between IMERG and TMPA. The results show that the performance of the satellite-based products varies by season, region, and the evaluation statistics. The precipitation diurnal cycle is relatively better captured by IMERG than TMPA. Both products exhibit a better agreement with gauge data in East Africa and humid West Africa than in the southern Sahel. However, a clear advantage for IMERG is not apparent in detecting the annual cycle. Although all gridded products used here reasonably capture the annual cycle, some differences are evident during the short rains in East Africa. Direct comparison between IMERG and TMPA over the entire continent reveals that the similarity between the two products is also regionally heterogeneous. Except for Zimbabwe and Madagascar, where both satellite-based observations present a good agreement, the two products generally have their largest differences over mountainous regions. IMERG seems to have achieved a reduction in the positive bias evident in TMPA over Lake Victoria.
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.
In just the past 5 years, the field of Earth observation has progressed beyond the offerings of conventional space-agency-based platforms to include a plethora of sensing opportunities afforded by CubeSats, unmanned aerial vehicles (UAVs), and smartphone technologies that are being embraced by both for-profit companies and individual researchers. Over the previous decades, space agency efforts have brought forth well-known and immensely useful satellites such as the Landsat series and the Gravity Research and Climate Experiment (GRACE) system, with costs typically of the order of 1 billion dollars per satellite and with concept-to-launch timelines of the order of 2 decades (for new missions). More recently, the proliferation of smart-phones has helped to miniaturize sensors and energy requirements , facilitating advances in the use of CubeSats that can be launched by the dozens, while providing ultra-high (3– 5 m) resolution sensing of the Earth on a daily basis. Start-up companies that did not exist a decade ago now operate more satellites in orbit than any space agency, and at costs that are a mere fraction of traditional satellite missions. With these advances come new space-borne measurements, such as real-time high-definition video for tracking air pollution , storm-cell development, flood propagation, precipitation monitoring, or even for constructing digital surfaces using structure-from-motion techniques. Closer to the surface , measurements from small unmanned drones and tethered balloons have mapped snow depths, floods, and estimated evaporation at sub-metre resolutions, pushing back on spatio-temporal constraints and delivering new process insights. At ground level, precipitation has been measured using signal attenuation between antennae mounted on cell phone towers, while the proliferation of mobile devices has enabled citizen scientists to catalogue photos of environmental conditions, estimate daily average temperatures from battery state, and sense other hydrologically important variables such as channel depths using commercially available wireless devices. Global internet access is being pursued via high-altitude balloons, solar planes, and hundreds of planned satellite launches, providing a means to exploit the " internet of things " as an entirely new measurement domain. Such global Published by Copernicus Publications on behalf of the European Geosciences Union. 3880 M. F. McCabe et al.: The future of Earth observation in hydrology access will enable real-time collection of data from billions of smartphones or from remote research platforms. This future will produce petabytes of data that can only be accessed via cloud storage and will require new analytical approaches to interpret. The extent to which today's hydrologic models can usefully ingest such massive data volumes is unclear. Nor is it clear whether this deluge of data will be usefully exploited, either because the measurements are superfluous, inconsistent, not accurate enough, or simply because we lack the capacity to process and analyse them. What is apparent is that the tools and techniques afforded by this array of novel and game-changing sensing platforms present our community with a unique opportunity to develop new insights that advance fundamental aspects of the hydrological sciences. To accomplish this will require more than just an application of the technology: in some cases, it will demand a radical rethink on how we utilize and exploit these new observing systems.
Even though the significance of indigenous knowledge in agriculture is internationally recognised, the role thereof in disaster risk reduction in South Africa is not well documented. This article determined the influence of indigenous knowledge in drought risk reduction in O.R. Tambo district of the Eastern Cape province (South Africa). Primary data were collected from 87 communal farmers through purposive sampling using a structured questionnaire. Focus group discussions were also held with the target group (farmers and extension officers) to gain more information and clarification on indigenous knowledge. The finding indicated that the majority of respondents (64.4%) relied on indigenous knowledge in their farming practice and drought risk reduction. Two-thirds (66.7%) of the respondents revealed that indigenous knowledge contributed to the resilience of farmers towards drought. The respondents unanimously agreed that indigenous knowledge is losing its significance in farming and drought risk reduction, because the younger generation did not value it anymore. Lack of documentation and deterioration of its application by the younger generation were found to be the main challenge for most respondents. The study concluded that indigenous knowledge was still an integral part of agricultural practices, applied drought risk reduction strategies and contributed to resilience against disasters. Based on the findings, the study recommends that indigenous knowledge be compiled, documented and published so that all farmers can learn of effective farming practices, passed on from generation to generation. Community holders of such information are encouraged to make younger generations aware of the benefits of indigenous knowledge to promote its usage.
This study examines the impacts of climate change on characteristics of extreme precipitation events over four African coastal cities (Cape Town, Maputo, Lagos and Port Said) under two future climate scenarios (RCP4.5 and RCP8.5). Fourteen indices were used to characterise extreme precipitation and 16 multi-model simulation datasets from the Coordinated Regional Climate Downscaling Experiment (CORDEX) were analysed. The capability of the models to reproduce past characteristics of extreme precipitation over the cities was evaluated against four satellite datasets after quantifying the observation uncertainties over the cities. The models give realistic simulation of extreme precipitation characteristics over the cities, and in most cases, the magnitudes of the simulation biases are within the observation uncertainties. For both the RCP4.5 and RCP8.5 scenarios, the models project a decrease in wet days and an increase in dry spells over the four cities in the future. More intense daily precipitation is projected over Maputo, Lagos and Port Said. The intensity and frequency of extreme precipitation events are projected to increase over Lagos, but decrease over the other cities. A decrease in annual precipitation is projected over Cape Town, Maputo and Port Said, whilst an increase is projected over Lagos, where the water surplus from the more extreme precipitation events exceeds the deficit from the less wet days. A decrease in the number of widespread extreme events is indicated over all the cities. Wet-day percentile and all-day percentile methods signal opposite future changes in the extreme precipitation thresholds over the cities (except over Lagos). The results of this study may have application in managing the vulnerabilities of these coastal cities to extreme precipitation events under climate change.
Western and Central Equatorial Africa (WCEA), home to the Congo rainforests, is the green heart of the otherwise dry continent of Africa. Despite its crucial role in the Earth system, WCEA’s climate variability has received little attention compared to the rest of Africa. Climate variability in the region is a result of complex interactions among various features acting on local and global scales. The mesoscale convective systems (MCSs) that have a preferentially westward propagation and present a distinct diurnal cycle are the main source of rainfall in the region. As a result of strong MCS activity, WCEA stands out as a convective anomaly within the tropics and experiences the world’s most intense thunderstorms as well as the highest lightning flash rates. The moisture of the region is supplied primarily from the Atlantic Ocean, with additional contributions from local recycling and East Africa. WCEA, in turn, serves as a moisture source for other parts of the continent.
One striking characteristic of WCEA is its intrinsic heterogeneity with respect to interannual variability of rainfall, resulting in delineation of the region primarily in the zonal direction. This is in contrast to the meridionally oriented spatial variability of the annual cycle and underlines the fact that driving factors of the two can be quite different. The annual cycle is mainly determined by the seasonal excursion of the sun. However, the interannual and intraseasonal variability of the region are modulated by remote forcings from all three oceans, reflected via zonal atmospheric cells and equatorial wave dynamics. The local atmospheric jets and regional Walker-like circulations also contribute to WCEA’s climate variability by modulating the moisture transport and vertical motion.
The region has experienced an increasing rate of deforestation in recent decades and has made a significant contribution to the global biomass burning emissions that can alter regional and global circulation, along with energy and water cycles. The mean annual temperature of the region has increased by about 1°C in the past 70 years. The annual rainfall over the same period presents a negative trend, though that is quite negligible in the eastern sector of the region.
Flood risk occurrence is very often related to heavy precipitation. The availability of analysis of weather data is a potential source for long term flood risk prediction and management. The aim of this paper was to determine and analyse trends of observed and future rainfall indices from 1961 to 2010 and 2011 to 2100 using rclimdex model in Abidjan District. This work was based on the integration of daily weather data within rclimdex model throughout quality control test, homogeneity test and indices calculation of ten (10) rainfall indices. The results showed an overall decrease trend of the rainfall indices namely through a negative trend in the annual total rainfall, maximum number of consecutive wet days, and number of extremely wet days during the period from 1961 to 2010. Exception was made from 1995 to 2010 where the same indices showed a positive trend. However, the results have showed also an increase trend of consecutive wet days (CWD), Simple daily intensity index (SDII) and Number of heavy precipitation days (R10) indices from 2011 to 2100. Thus these findings explain the nowadays flood occurrence and indicate that rainfall extreme under flood risk events will continue in the future. Therefore it call decision makers for preparedness and mitigation strategies in Abidjan District.
Keywords: Climate events, Rainfall indices, Rclimdex, Abidjan, Côte d’Ivoire