Spatio-temporal trends in daily precipitation extremes over the Enkangala escarpment of South Africa: 1961–2021

Abstract

The Intergovernmental Panel on Climate Change in its Sixth Assessment Report reported that increases in extreme weather events can already be detected in every region in the world. For eastern southern Africa the report provides evidence of an increasing trend in extreme precipitation events, but more research is needed to understand how this change is manifesting over regions as diverse as South Africa’s eastern escarpment and northern Mozambique. This study evaluates the trends in extreme precipitation over the Enkangala Escarpment, part of South Africa’s eastern escarpment, over the period 1961 to 2021. Daily precipitation data covering 22 stations with fewer than 10% missing values over 61 years were obtained from the South African Weather Service. A total of twelve extreme events precipitation indices were computed using the ClimPACT2 package, and a time series trend analysis was performed using the Trend Free Whitening Mann Kendal test and a field significance test. The results indicate significant increasing trends in nine out of the twelve extreme events indices that were investigated. The three indices which have shown significant decreasing trends include Consecutive Wet days (CWD) at (0.03 day/year total annual rainfall (PRCPTOT) (0.61 mm/decade) and the annual count of days when precipitation≥10 mm (R10mm) of 0.32 days/decade. Indices such Rx1 days, Rx3 days and Rx5 days exhibited increasing trends of 0.2 days/decade each. Other indices with an increasing trend are CDD (3.6 day/decade), R20mm (0.2 days/decade) and R30mm (0.2 days/decade). The R95p (11.2 mm/decade) and R99p (6.0 mm/decade) indices, and Simple Daily Intensity Index(0.4 mm/decade) also exhibit positive trends over the study period. Overall the analysis is indicative of a change in the nature of rainfall over the Enkangala escarpment of South Africa, with annual rainfall totals, the number of consecutive wet days and the frequency of events with smaller thresholds such as 10 mm/day decreasing, but with extreme events of larger thresholds occurring more frequently. That is, when it rains, it tends to rain more intensely than in the past. The use of a field significance test was useful to formalise when relatively small but spatially homogeneous trends detected at separate weather stations yield statistical significance. The findings of the paper are relevant to decision and policy in multiple of socio-economic sectors active in South Africa’s eastern escarpment.