The delusive accuracy of global irrigation water withdrawal estimates

Abstract

Miscalculating the volumes of water withdrawn for irrigation, the largest consumer of freshwater in the world, jeopardizes sustainable water management. Hydrological models quantify water withdrawals, but their estimates are unduly precise. Model imperfections need to be appreciated to avoid policy misjudgements.

Models with higher effective dimensions tend to produce more uncertain estimates

“…We illustrate this exercise with a basic formula to estimate the total amount of water withdrawn to irrigate a crop. Such a formula requires the calculation of the evapotranspiration of a reference crop (usually grass or alfalfa), the calculation of the crop evapotranspiration (wheat in this case), and lastly the estimation of the water withdrawn for irrigation ( 40 ). However, there are c. 50 different equations available to calculate the reference evapotranspiration and no agreement as to which method might work best ( 41 ).…”

“…However, there are c. 50 different equations available to calculate the reference evapotranspiration and no agreement as to which method might work best ( 41 ). Some of the most used are the Priestley-Taylor and the FAO-56 Penman-Monteith ( 42 ), known to provide results that can differ by a factor of 2 or more ( 40 ). We examine the impact that this source of uncertainty has on the model’s effective dimensions and uncertainty with a trigger ( 43 ), a parameter that randomly decides which evapotranspiration equation should be used in each model simulation (see the Supplementary Materials).…”

“…What is undesirable is the coupling of excess complexity with the illusory accuracy that results from scarce attention to the propagation of uncertainties onto the prediction of the model. This practice may miss on the unanticipated behaviors, foster tunnel vision, and underestimate serious socioenvironmental risks such as the effects of climate change (47), the impact of insecticides on bees (48) or the environmental impact of irrigated agriculture (40,43). Among practitioners, this pathology is known as "garbage in, garbage out" ("where uncertainties in inputs must be suppressed lest outputs become indeterminate") (49).…”

“…For instance, how many readers outside the circle of practitioners know that the 1.5 degree climate target is achieved in integrated assessment models thanks to the adoption of so-called negative emissions, a technology still to be fully developed ( 56 )? Or that global hydrological models assume that irrigation optimizes crop production and water use (a premise at odds with the practices of traditional irrigators) ( 40 )?…”

Spatiotemporal variation in irrigation water requirements in the China–Pakistan Economic Corridor

“…Irrigation water use is essential for improving productivity of existing farmland 9 and quantification of catchment water budgets is also fundamental to sustainably manage agricultural water resources 10 . The water consumption for crop growth and development depends on the meteorological conditions, crop types, and the crop planting area.…”