Assessment of the onset, cessation, and duration of rainfall season over Botswana

Botswana's agricultural viability is affected by climate variability and change, and hence, information on rainfall characteristics is crucial. This study has determined rainfall onset and cessation dates using INSTAT version 3.036 statistical package. It has also determined wet and dry years using the standardized precipitation index (SPI), and has compared soil moisture distribution with that of rainfall and temperature. Mean rainfall onsets occur mainly from the second 10-day to the last 10-day period of November over most parts of the country except over areas in the northwest where they occur relatively late (from the first 10-day to the second 10-day period of December). Mean cessations over most parts occur within the first week of April. The length of the rainfall season varies from 115 to 141 days in the Ngamiland and North East Districts, respectively. The annual mean number of rainy days with at least 5 mm of rainfall range from 18 in Tsabong to 26 in Kasane. In the record considered, Maun experienced the wettest season in 1973/1974 with a total rainfall of 1140 mm and the corresponding SPI of 3.9, whereas Gaborone experienced the driest season in 2006/07 with a rainfall total of only 86 mm and the corresponding SPI of -2.3. Soil moisture patterns over the country match those of rainfall but less so for temperature. The results of the study are crucial mainly for the management of agricultural activities and water.