Here, we present risk analysis and economical benefit estimates of a water harvesting (WH) system for supplemental irrigation (SI). The focus of the analysis is on reducing investment risk to improve self-sufficiency in staple food production. The analysis suggests a strong mutual dependence between investment in WH for SI and input of fertiliser. The WH system is only economically viable if combined with improved soil fertility management, but the investment in fertiliser inputs may only be viable in the long term when combined with SI.