Trend analysis in observed and projected precipitation and mean temperature over the Black Volta Basin, West Africa

Abstract

The study analyzed the trends in observed (1981-2010) and future projected annual precipitation and mean temperature over the Black Volta River Basin using the Mann-Kendall test and the Sen’s slope estimator. Projected changes in precipitation and temperature by multi-model ensemble runs over the Black Volta basin for the late (2051-2075) and end of the 21st century (2076-2100) horizons under two IPCC Representative Concentration Pathways (RCP4.5 and RCP8.5) scenarios was also analyzed. The results showed statistically significant (at the 5% significance level) increase of 111mm in the annual rainfall over the observed period. The future direction of this trend is uncertain as some ensemble members projected positive trends while others gave negative trends. However, both the positive and negative future trends in the rainfall were statistically non-significant. The results also showed that the studied basin has warmed over the observed period, with significant increase of 0.9°C in the mean annual temperature. Similarly, significant increasing trend in the mean annual temperature are projected by the ensemble runs under both RCPs for the late and end of the 21st century. Analyses of the average annual, intra-annual and seasonal precipitation indicated high uncertainty regarding the direction of the future rainfall. Mean annual precipitation change for the late 21st century ranged between -16% and +6% under the RCP4.5 scenario and between -27% and +14% under the RCP8.5 scenario. The end of the 21st century projections showed changes in mean precipitation amounts ranging between -23% and +2% and between -33% and +13% under the RCP4.5 and RCP8.5 scenarios, respectively. With regards to temperature, average annual projections by the ensemble runs showed increases over the basin under both RCP scenarios and for both time periods. Warming over the basin is projected to be higher under the RCP8.5 scenario than under the RCP4.5 scenario, with the end of 21st century period being warmer than the late 21st century. Average annual mean temperature increase across the model run ranged between 2.2oC and 2.6oC under the RCP4.5 scenario and between 3.5oC and 3.7oC under the RCP8.5 scenario for the end of the 21st century.