The efficiency of drip irrigation unpacked

Drip irrigation figures prominently in water policy debates as a possible solution to water scarcity problems, based on the assertion that it will improve water use efficiencies. We use this article to carefully trace the scientific basis of this assertion. Through a systematic review of the literature, we show that the term efficiency means different things to different people, and can refer to different elements in the water balance. Most articles claim that drip irrigation is irrigation water use efficient and crop water use efficient, but different studies use different definitions of these terms. In addition, measured efficiency gains not only refer to different capacities of the technology, but are also based on very specific boundary (scale) assumptions. We conclude that efficiency gains from drip irrigation will only be achieved under narrowly defined operational conditions, and just apply to very specific spatial and temporal scales. Hence, and unlike what generalized statements in policy documents and the overall enthusiasm for drip as a water saving tool suggest, expectations of increased water efficiencies associated with drip will only be realized, and are just realizable, in very specific circumstances.