Debilitating floods in the Sahel are becoming frequent

Despite the long-lasting and widespread drought in the Sahel, flood events did punctuate in the past. The concern about floods remains dwarf on the international research and policy agenda compared to droughts. In this paper, we elucidate that floods in the Sahel are now becoming more frequent, widespread, and more devastating. We analyzed gridded daily rainfall data over the period 1981–2020, used photographs and satellite images to depict flood areas and threats, compiled and studied flood-related statistics over the past two decades, and supported the results with peer-reviewed literature. Our analysis revealed that the timing of the maximum daily rainfall occurs from the last week of July to mid-August in the Eastern Sahel, but from the last week of July to the end of August in the Western Sahel. In 2019 and 2020, flash and riverine floods took their toll in Sudan and elsewhere in the region in terms of the number of affected people, direct deaths, destroyed and damaged houses and croplands, contaminated water resources, and disease outbreaks and deaths. Changes in rainfall intensity, human interventions in the physical environment, and poor urban planning play a major role in driving catastrophic floods. Emphasis should be put on understanding flood causes and impacts on vulnerable societies, controlling water-borne diseases, and recognizing the importance of compiling relevant and reliable flood information. Extreme rainfall in this dry region could be an asset for attenuating the regional water scarcity status if well harvested and managed. We hope this paper will induce the hydroclimate scholars to carry out more flood studies for the Sahel. It is only then encumbered meaningful opportunities for flood risk management can start to unveil

A new and flexible rainy season definition: Validation for the Greater Horn of Africa and application to rainfall trends

Previous studies on observed or projected rainfall trends for the Greater Horn of Africa (GHA) generally focus on calendric 3-month periods, and thus partly neglect the complexity of rainfall seasonality in this topographically heterogeneous region. This study introduces a novel and flexible methodology to identify the rainfall seasonality, the onset, cessation and duration of the rainy seasons and the associated uncertainties from rainfall time series. The definition is applied to the Climate Hazards Group InfraRed Precipitation with Stations (CHIRPS) satellite product and an extensive rain gauge data set. A strong agreement with known seasonal dynamics in the region and the commonly used calendric rainy seasons is demonstrated. Compared to the latter definition, a clear added value is found for the new approach as it captures the local rainfall features (associated with, for example, the sea breeze), thus facilitating evaluations across rainfall seasonality borders. While previously known trends are qualitatively confirmed, trends are amplified in some regions using the flexible definition method. Notably, a drying trend in Tanzania and Democratic Republic of Congo and a wetting trend in central Sudan and parts of eastern Ethiopia and Kenya can be detected. The trends are regionally associated with changes in rainy season cessation. CHIRPS and station trend patterns are consistent over larger regions of the GHA, but differ in regions with known rainfall contributions from warmer cloud tops. Discrepancies are found in coastal and topographically complex areas, and regions with an unstable seasonality of rainfall. As expected, CHIRPS shows spatially more homogeneous trends compared to station data. The more precise definition of the rainy season facilitates the assessment of rainfall characteristics like intensity, rainfall amounts or temporal shifts of rainy seasons. This novel methodology could also provide a more adequate calibration of climate model simulations thus potentially enabling more realistic climate change projections for the GHA

Drought versus flood: What matters more to the performance of Sahel farming systems?

AbstractRecent climate change has brought new patterns of extreme events in terms of both drought and heavy rainfall to the drought‐prone African Sahel. The effects of these recent extreme events on the performance of the Sahel farming systems are still weakly investigated. This study aims at assessing effects of droughts versus floods on crop yield levels and losses, focusing on the so‐called recovery period, particularly 2001–2020. A newly developed productivity‐drought condition index (PDCI) is utilized to assess agricultural productivity as related to drought or flood in a highly vulnerable region, that is, the Sudanese Sahel. Four farming systems, namely traditional rainfed, mechanized rainfed, gravity irrigated and spate irrigated systems, with sorghum and millet as staple food crops, are considered. The PDCI is defined as a function of the integrated normalized difference vegetation index (iNDVI) over the growing season. To address temporal and spatial variabilities, scaling of the PDCI is done in two dimensions: space and time. Crop statistics are used to derive yield losses. Our results show that both drought and flood episodes (seven and six episodes, respectively) can be captured using the PDCI. Drought remains the most relevant risk to Sahel's crop productivity. Some recent large‐scale floods led to yield loss. However, floods cause smaller risks to agricultural productivity compared to droughts. Floods may even result in enhanced crop yields. Based upon scaling in the time or space domain, ranking the severity of drought impacts on crop yield for individual years from 2001 to 2020 reveals least to slightly different results. Vulnerability to drought depends on the crop type and farming system. Drought effect on crop yield from the irrigated sector is clear on individual years but not as a general statistical relationship. The parameter ‘percentage area under drought’ explains around one‐third of the variation in the rainfed crop yield. The spate irrigation scheme, the gravity irrigated system and the rainfed farmlands experienced respectively 87%, 57% and 46% of area under drought on average. Irrigated systems produce much higher crop yields than rainfed systems. The mechanized system is more drought‐vulnerable than the traditional system. These results call for identifying agricultural management pathways that recognize the combined implications of both hydrological extremes for the region's food security.A newly devised productivity‐drought condition index (PDCI) based on integrated normalized difference vegetation index (iNDVI) data is used to capture the performance of different Sahel farming systems. The performance is evaluated spatially and temporally in a comparative study of effects of droughts versus floods on crop yield levels and losses during 2001–2020. Our research shows that: Crop productivity of all farming systems is severely affected by drought; Flood events can also lead to a decline in productivity, but usually to a much lesser extent; The vulnerability to droughts and floods depends upon the farming system and crop type. Our analysis shows that the farming systems in the Sudanese Sahel have not reverted to conditions that could be described as a Sahel recovery. This study calls for agricultural management decisions, which are specific for the different farming systems, in response to climate variability. Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659https://doi.org/10.1594/PANGAEA.921846https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/africa/east/dekadal/emodis/ndvi_c6/https://earlywarning.usgs.gov/fews/datadownloads/East 20Africa/eMODIS 20NDVI 20C

Filling Africa’s largest hydropower dam should consider engineering realities

The Grand Ethiopian Renaissance Dam (GERD) on the Nile River will double Ethiopia’s electricity generation and reduce the Nile flow to Sudan and Egypt during reservoir filling. We argue that multi-country negotiations over the initial filling and long-term operation of the GERD reservoir should not overlook key dam engineering features

Debilitating floods in the Sahel are becoming frequent

Despite the long-lasting and widespread drought in the Sahel, flood events did punctuate in the past. The concern about floods remains dwarf on the international research and policy agenda compared to droughts. In this paper, we elucidate that floods in the Sahel are now becoming more frequent, widespread, and more devastating. We analyzed gridded daily rainfall data over the period 1981-2020, used photographs and satellite images to depict flood areas and threats, compiled and studied flood-related statistics over the past two decades, and supported the results with peer-reviewed literature. Our analysis revealed that the timing of the maximum daily rainfall occurs from the last week of July to mid-August in the Eastern Sahel, but from the last week of July to the end of August in the Western Sahel. In 2019 and 2020, flash and riverine floods took their toll in Sudan and elsewhere in the region in terms of the number of affected people, direct deaths, destroyed and damaged houses and croplands, contaminated water resources, and disease outbreaks and deaths. Changes in rainfall intensity, human interventions in the physical environment, and poor urban planning play a major role in driving catastrophic floods. Emphasis should be put on understanding flood causes and impacts on vulnerable societies, controlling water-borne diseases, and recognizing the importance of compiling relevant and reliable flood information. Extreme rainfall in this dry region could be an asset for attenuating the regional water scarcity status if well harvested and managed. We hope this paper will induce the hydroclimate scholars to carry out more flood studies for the Sahel. It is only then encumbered meaningful opportunities for flood risk management can start to unveil