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Earth Sciences & Geography - Earth System Sciences | Historical Variability of Rainfall in the African East Sahel of Sudan - Implications for Development

Historical Variability of Rainfall in the African East Sahel of Sudan

Implications for Development

Hermance, John F.

2014, XI, 123 p. 48 illus., 13 illus. in color.

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The northward migration of the African monsoon rains in summer, associated with the seasonal march of the Intertropical Convergence Zone (ITCZ) across the plains south of the Sahara, is the most critical asset for the livelihoods of indigenous peoples and local economies of the Sahel.  It is essential that climate science (and its publicly available database) play a key role in characterizing the variabilities of these rainfall patterns in space and time if sustainable life styles are to accommodate the expanding populations of the region.  This study turns to the East Sahel of Sudan by analyzing over 100 years of historical rainfall data from three of the few long term standard WMO rain gauge stations in substantially different rainfall settings.  From north to south, transecting the Sahel, the stations with their annual rainfall are Khartoum (130 mm); Kassala (280 mm); and Gedaref (600 mm).  The conclusions challenge a popular notion that changing climate, drought and desertification in the East Sahel may have already accelerated the deterioration of its water resources. However, any evidence of a persistent and coherent regional trend of diminishing rainfall is obscure.  Quite the contrary, the evidence demonstrates that the fluctuations of climate and weather patterns over the ensuing decades of the past century - at all temporal scales from days to years to decades - profoundly overwhelm any suggestion of a large-scale, coherent decrease (or increase) in rainfall.  The implication is that, it is not long term change, but the highly localized interseasonal, interannual and multiannual variability of rainfall that poses the greatest and most immediate societal threat from naturally-induced causes; a process constantly destabilizing an agrarian economy struggling to survive in a climate that irregularly vacillates between years of drought and years of flooding.  While this report may have some interest for climate scientists, it is primarily directed to a general readership (including students in public policy and anthropology) concerned with the availability of water in the Sahel, particularly the long term sustainability of local small-scale farms and transhumant pastoralism.

Content Level » Research

Keywords » Africa - Climate Change - Floods and Droughts - Hazard and Risk Management - Rainfall - Rainfall Harvesting - Sahel Sudan - Sustainability and Agriculture - Water Policy

Related subjects » Earth System Sciences - Environmental Management - Geography - Global Change - Climate Change - Life Sciences - Meteorology

Table of contents 

1       Introduction

         1.1     Water Availability in Sub-Saharan Africa

         1.2     Refocusing Climate and Water Policy from Regional to Local Scales

         1.3     Scope of the Discussion

         1.4     Long Term Precipitation Patterns South of the Sahara

         1.5     Hydroclimate Setting of the Study Area

         1.6     Interseasonal Variability of Rainfall

         1.7     Precipitation Patterns for the Sahel of East Sudan: Annual Expected Totals

           1.8       Annual Precipitation Patterns in the Study Area



2         Analysis of Long Term (100 yr) Patterns of Rainfall Variability

           2.1       Study Area in East Sahel

           2.2       Constructing Time Series of Annual Precipitation Totals

           2.3       Spatial Differences in Annual Precipitation Totals

           2.4       Expected Range of Annual Precipitation Totals

                        2.4.1      Gedaref (Median Annual Prcp = 600 mm)

                        2.4.2      Kassala (Median Annual Prcp = 280 mm)

                        2.4.3      Khartoum (Median Annual Prcp = 130 mm)

           2.5       Normalized Precipitation Metrics to describe Interannual Variability

                        2.5.1      The Standardized Precipitation Index (SPI)

                        2.5.2      Percent (%) of Expected Annual Precipitation

                        2.5.3      Percent (%) Departure from Expected Annual Total

           2.6       Variability and Synchrony of Annual Rainfall in Space & Time



3         Interannual and Interseasonal Variations in Monthly Rainfall

           3.1       General Patterns

           3.2       Longer Term Patterns of Interannual Variations in Monthly Precipitation

           3.3       Statistics of Seasonal Behaviors of Monthly Precipitation

           3.4       Statistics for the Month of Annual Maximum Monthly Totals

           3.5       Statistics for the Month of Maximum Variability



4         Intra-Seasonal Patterns of Rainfall from Daily Values

           4.1       General Patterns of Daily Precipitation

           4.2       Daily Precipitation Records of Opportunity

           4.3       An Overview of Daily Precipitation Patterns

           4.4       Statistics of Daily Rainfall Intensities

           4.5       Extreme Daily Precipitation

           4.6       Seasonal (Annual) Patterns of Daily Precipitation

           4.7       Seasonal Accumulation of Annual Precipitation

           4.8       Seasonal Models for Percent of Annual Precipitation based on Daily Data



5         Analysis of Storm Events (& Interstorm Dry Periods)

           5.1       Statistics of Storm Events

                        5.1.1      Defining Storm "Events"

                        5.1.2      Partitioning Storm Event Totals as a Percentage of Occurrence

                        5.1.3      Partitioning Storm Event Totals by Duration

                        5.1.4      Partitioning Storm Event Totals as a Percentage of Annual Rainfall

                        5.1.5      Essential Characteristics of Storm Events in the East Sahel

           5.2       Time Interval between Storms: Dry Periods

                        5.2.1      Statistics of interstorm intervals

                        5.2.2      Classification of Interstorm Intervals by Duration

                        5.2.3      Statistics for Dry Periods in the Wet Season

                        5.2.4      Statistics on the Interannual Dry Period



6         Overview and Conclusions

           6.1       General Patterns of Spatial Variability

           6.2       Multi-Station versus Single Station Metrics

                        6.2.1      Un-normalized versus Normalized Metrics

                        6.2.2      Interannual Patterns of Rainfall Variability for the East Sahel

           6.3       Interannual and Interseasonal Variations in Monthly Rainfall

           6.4       Months of Maximum Precipitation and Greatest Variability

           6.5       Statistics of Storms: Duration, Frequency and Total Rainfall

                        6.5.1      Importance of Daily Rainfall Data for Water Management

                        6.5.2      Spatial and Temporal Character of Storm Days

                        6.5.3      Duration and Frequency of Storm Events

           6.6       Interannual Variability in Seasonal Rainfall Patterns

           6.7       A View of Climate Change

           6.8       Conclusions


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