The use of Water Requirement Satisfaction Index for assessing agricultural drought on rain-fed maize, in the Luvuvhu River catchment, South Africa

Analysis of drought on rain-fed maize production in the Luvuvhu River Catchment was measured using the Water Requirement Satisfaction Index (WRSI). Computation of WRSI was performed using a crop water balance model in Instat+ software for a 120-day maturing maize crop using nine different planting dekads from October to December. This study made use of seven weather stations, which adequately represent climatic and geographical environments of the catchment, containing historical rainfall and temperature data from 1974 to 2015. The non-parametric Spearman’s Rank Correlation test was conducted to determine drought trends. Thereafter, probabilities and extreme widespread dry and wet agricultural seasons were analysed using STATISTICA software. Results revealed that since the 1980s, the catchment had been subjected to frequent drought conditions, with an average frequency of once every two to three seasons. Extreme drought episodes (WRSI <50) were commonly identified in the northern drier parts of the catchment as compared to the high rainfall region, whereby 50-93% of the analysed seasons were subjected to extreme drought conditions. Furthermore, there were no drought trends (ρ values close to 0) during the analysed climate period. Water Requirements Satisfaction Index (WRSI) values corresponding to more intense drought conditions were reflected during the December planting date for all stations. Moreover, at the low rainfall region, it was seen that extreme droughts occurred once in five seasons, regardless of the planting date. Thus, the study led to a recommendation of using October-November as the optimum planting date in the catchment. Furthermore, farmers located areas with high probabilities of drought during critical stages can be advised to supplement rain-fed farming with irrigation should they be located nearby to rivers.