Characterization of rainfall over the Limpopo province, South Africa, for the period 1990 to 2020.

Abstract

The Limpopo Province is home to a large rural population which is highly dependent on rainfed agriculture. Although extensive research has been undertaken to understand rainfall variability over South Africa, a better understanding of the localized rainfall characteristics and variability remains crucial for decision-makers and the livelihoods of the local community. The study aims to investigate rainfall variability and trends over the Limpopo province, understand the distribution (both spatial and temporal) of seasonal rainfall characteristics, and also establish the relationship between seasonal rainfall characteristics with larger modes of climate variability (i.e. El Niño Southern Oscillation (ENSO), and Southern Indian Ocean Dipole (SIOD)). High-resolution Climate Hazard Group Infrared Precipitation with Station Data (CHIRPS) 0.05° gridded data spanning the duration 1990-2020 was employed to analyze the spatial distribution of rainfall over the Limpopo province. In this study dry spells (pentads with < 5 mm rainfall), moderate (rainfall ranging from 10-30 mm per day) and heavy wet days (rainfall > 30 mm per day) were analyzed. Standardized Anomaly Index (SAI) was used to understand the relationship between rainfall characteristics and anomalies. The Mann-Kendall test was also used to determine the trends of seasonal rainfall characteristics over the province. The Pearson correlation was used to establish the association between seasonal rainfall characteristics (dry spells, moderate and heavy wet days) and large modes of variability (ENSO and SIOD). The results of this study show that seasonal rainfall exhibits high spatial and temporal variability over the study period. Throughout the extended summer season (October-March (ONDJFM)), dry spells migrate from the north of Vhembe and Capricorn to the northeast of Mopani, with their frequency and extent increasing from early summer (October-November) to late summer (February-March). The distribution of these rainfall characteristics follows that of mean annual rainfall. Of all periods, December-January (DJ) receives the highest frequency of moderate wet days with a larger spatial extend ranging from 6-13 days in the high-lying escarpment of Vhembe, west of Mopani, south-east of Capricorn, Waterberg, and Greater Sekhukhune. The highest heavy wet day frequency is also observed in the DJ period, over the high-lying escarpment of Vhembe, west of Mopani, south-east Capricorn and north of Greater Sekhukhune records heavy wet days ranging from 3-7 days. The results of the Mann-Kendall trend test revealed a statistically significant decreasing trend in dry spells during DJ and February-March (FM) over the entire Limpopo province. Statistically significant moderate wet day trends were observed during ON over north and east of Mopani, south-east of Capricorn, and west of Mopani district, whereas during the DJ periods, statistically significant increasing trends are recorded over the south-east of Vhembe and northwest of Mopani. During DJ, statistically significant increasing heavy wet day trends are observed over Vhembe, Greater Sekhukhune, and west of Waterberg. The relationship between seasonal rainfall characteristics and rainfall anomalies was observed. The results show that the inter-annual variability of seasonal rainfall characteristics does not always reflect in seasonal rainfall totals/anomalies. This shows that anomalies overlook the isolated impact of seasonal rainfall characteristics. The relationship between seasonal rainfall characteristics and large modes of variability was observed. A strong negative correlation with moderate wet days over the high-lying escarpment in the Vhembe district and south of the Mopani district. However, a complex relationship was observed between the inter-annual rainfall characteristics and large modes of variability. The results showed that not all La Nina years or positive phase SIOD phase equate to wet seasons. Furthermore, years with neutral ENSO and SIOD phases still exhibited above-average wet days.