Rainfall and runoff trend analysis in the Limpopo river basin using the Mann Kendall statistic

Abstract

An in-depth understanding of rainfall and runoff patterns of a catchment is central to sustainable water resources management and enhanced disaster risk reduction. This makes it important for studies in rainfall and runoff analysis to be carried out in developing countries like Zimbabwe. This study endeavoured to understand the rainfall patterns and the resultant runoff downstream along the Limpopo River. The study focused on the lower part of Mzingwane Catchment; which is a sub basin of the Limpopo River. Monthly observed data for four rainfall and two runoff stations were used for the study. Trend analysis of the data for the period 1949–2015 was conducted using Mann Kendal and Sen Slope Tests. The Seasonal Kendal test was applied to observe the influence of seasonal variations in rainfall and runoff. Trend analysis of rainfall patterns carried out for West Nicholson, Beitbridge and Chiredzi rainfall stations revealed an insignificant decreasing trend in rainfall activity at p < 0.05. However, Gwanda recorded an increasing rainfall trend though insignificant. Runoff activity at B62 Doddieburn station upstream of the catchment showed a declining trend while the B35 Limpopo River station exhibited an increase in river runoff. The overall p-value (of 0.03) for B35 station was below 0.05 thereby suggesting that there was a significant trend observed in the dataset. The increase in runoff at B35 could be attributed to managed releases from dams upstream. The study recommends measurement of all dam water releases and enhanced environmental management upstream and within the catchment in general.