A global, spatially-explicit assessment of irrigated croplands influenced by urban wastewater flows

Abstract

When urban areas expand without concomitant increases in wastewater treatment capacity, vast
\nquantities of wastewater are released to surface waters with little or no treatment. Downstream of many
\nurban areas are large areas of irrigated croplands reliant on these same surface water sources. Case studies
\ndocument the widespread use of untreated wastewater in irrigated agriculture, but due to the practical and
\npolitical challenges of conducting a true census of this practice, its global extent is not well known except
\nwhere reuse has been planned. This study used GIS-based modeling methods to develop the first spatially
\nexplicit estimate of the global extent of irrigated croplands influenced by urban wastewater flows, including
\nindirect wastewater use. These croplands were further classified by their likelihood of using poor quality
\nwater based on the spatial proximity of croplands to urban areas, urban wastewater return flow ratios, and
\nproportion of wastewater treated. This study found that 65 percent (35.9 Mha) of downstream irrigated
\ncroplands were located in catchments with high levels of dependence on urban wastewater flows. These
\nsame catchments were home to 1.37 billion urban residents. Of these croplands, 29.3 Mha were located in
\ncountries with low levels of wastewater treatment and home to 885 million urban residents. These figures
\nprovide insight into the key role that water reuse plays in meeting the water and food needs of people
\naround the world, and the need to invest in wastewater treatment to protect public health.