Developing an Integrated Understanding of the Relationship Between Urban Wastewater Flows and Downstream Reuse in Irrigated Agriculture: A Global Perspective

Abstract:

Globally, less than ten percent of collected wastewater receives any form of treatment. This untreated wastewater is discharged to surface waters where it is diluted and reused by farmers and municipalities downstream. Without proper safeguards, the use of these waters can present health risks. However, these same waters also provide a reliable and nutrient rich water source for farmers, often in regions where water is already physically or economically scarce. Case studies show the prevalence and diversity of motivations for indirect reuse, but are difficult to interpret in aggregate at the global scale.

This study quantifies the global extent and characteristics of the reuse of wastewater in irrigated agriculture through three main components:

1. Quantifying the global extent of urban and peri-urban irrigated and rainfed croplands;
2. Evaluating the contribution of urban wastewater production to available blue water at the catchment scale;
3. Developing an irrigation water quality indicator and classifying irrigated croplands downstream of cities on the basis of this indicator.

Each of these components integrates several global scale spatial datasets including MIRCA2000 (irrigated croplands); GDBD (stream channels and catchments); and compilations of water use, sewerage and wastewater treatment data. All analyses were conducted using spatial analysis tools in ArcGIS and Python.

This analysis found that 60 percent of all irrigated croplands (130 Mha) were within 20 km of cities. Urban irrigated croplands were found to be farmed with greater cropping intensity (1.48) as compared to non-urban irrigated croplands. Ten percent of the global catchment area is in catchments where domestic wastewater constitutes greater than five percent of available blue water. In contrast, 25 percent of irrigated croplands are located in catchments where domestic wastewater exceeds five percent of available blue water. Particularly in the water scarce regions of North Africa and East Asia, a strong correlation between the volume of urban wastewater production and the area of peri-urban irrigated croplands was found. A better understanding of global reliance on the indirect reuse of untreated wastewater in irrigated agriculture can provide valuable insights for large-scale water allocation planning and risk mitigation efforts.