Agroclimatic classification of the semi-arid tropics I. A method for the computation of classificatory variables

A simple method based on rainfall (R) and potential evapotranspiration (PE) for deriving variables to classify the semi-arid tropics into relevant agronomically homogeneous zones is suggested. A term ‘available effective rainy period’ is introduced for this purpose. The available effective rainy period is defined as the number of consecutive weeks in which the 14-week moving average of R/PE is ⩾ 0.75, but for the initial week the value of R/PE is ⩾ 0.50. The preceding week is taken to be the week of commencement of the sowing rains. The method permits estimation of wet and dry spells during the available effective rainy period and an estimate of the likely percentage of crop-failure years. These selected parameters allow a more relevant and realistic assessment of the agroclimatic environment and agricultural production potential of a selected location or region.