Data Construction and Spatiotemporal Trend Attribution of Runoff over the African Continent (1981–2016)

Due to global climate change, coupled with the increase in population, growth in water withdrawals, expansion of farmland area and reduction of forest, the surface runoff process in Africa has undergone major changes and extreme hydrological events have been occurred frequently, which has caused greater impact on the production and life of the people. 

In order to systematically understand the response of runoff trends to climate change and human activities, the research team of Researcher Prof. Liu Yuanbo from the Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences (CAS) constructed improved monthly runoff data for the African continent from 1981 to 2016 based on the river discharge data from 535 gauging stations using a revised runoff curve number, downscaling and interpolation statistical methods. Then, monthly and annual runoff data, climate data (precipitation and temperature) and human activities (farmland expansion and water withdrawal) were used to assess runoff trend responses to climate change and human activities in Africa during 1981–2016. Related results were published in the Journal of Hydrometeorology

Analysis of runoff trend responses to climate change and human activities revealed that land cover changes contributed more (72% a-1) to the observed net runoff change (0.30% a-1) than continental climate change (28% a-1). These contributions were results of cropland expansion rate of 0.46% a-1 and precipitation increase of 0.07% a-1. The annual runoff trends were 0.21% a-1 in the tropical region, 0.16% a-1 in the temperate region and 0.91% a-1 in the arid region. The runoff increase in the tropical region was fully caused by human activities, with a contribution to net runoff increase of 160% a-1 due to cropland expansion by 0.53% a-1. Climate change was responsible for an increased runoff in the temperate and arid regions, with contributions of 102% a-1 and 117% a-1, respectively. 

Land cover change was the dominant cause of increased annual runoff, with trends ranging from 0.06% a-1 to 1.38 % a-1 in 7 of the 25 major river basins, including the Africa–Indian Ocean Coast, Limpopo, Shebelle–Juba, Volta, Gulf of Guinea, Africa–East Central Coast and Madagascar due to cropland expansion trends (0.02% a-1 – 1.03% a-1). The Orange, Namibia–Coast, Africa–Red Sea–Gulf of Aden Coast and Zambezi basins experienced runoff reduction (-0.15% – -1.88%) due to the increase in water withdrawal (1.80% a-1 – 3.23% a-1). 

Climate change was the dominant factor that induced annual runoff change in 14 of the 25 major basins, where 11 basins (Africa–South Interior, Africa–West Coast, Nile, Angola–Coast, Rift Valley, Africa–North West Coast, Niger, Mediterranean South Coast, Africa–North Interior, Lake Chad and Senegal) had runoff increase (0.08% a-1 – 1.76% a-1) due to precipitation increase (0.15% a-1 – 0.73% a-1). Three basins (South Africa–West Coast, South Africa–South Coast and Congo) experienced runoff reduction (-0.89% a-1 – -0.02% a-1) due to precipitation decrease (-0.11% a-1 – -0.55% a-1) and temperature rise (0.07% a-1 – 0.17% a-1). 

The performance and simplicity of the statistical methods used in this study could be useful for improving runoff estimations in other regions with limited streamflow data. The results of the current study could be important to natural resource managers and decision makers in terms of raising awareness of climate change adaptation strategies and agricultural land-use policies in Africa.