Evaluating strategies for scheduling supplementary irrigation of sugarcane in South Africa

Approximately half of the irrigated crop in South Africa receives supplementary irrigation in regions where rainfall is relatively high. Good management based on relevant information is needed to optimise the use of scarce and expensive irrigation water. The aim of this work was to demonstrate the use of a simple model to identify optimal irrigation scheduling strategies. A test of the model against measurements of crop water use and cane yield showed that it is an appropriate tool to support irrigation planning. Thirty years of daily weather data were then used to simulate different irrigation strategies for different soils, crop cycles and sites. These included different fixed irrigation cycles, different levels of allowable soil water depletion, and making use of rainfall forecasts or not. The study showed that profitability is hugely increased by adopting a water budget approach to scheduling instead of a fixed irrigation cycle. The use of current crop water-usc data instead of long term mean data, and the use of rainfall forecasts had suprisingly little impact on profitability. However, scheduling irrigation using current crop water-use data, rainfall forecasts and an allowable depletion level of 50% consistently produced the highest increase in net operating income due to irrigation.