In order to calculate how much water is guzzled by city dwellers' crops, Mark Johnson from the University of British Columbia, Canada, and his colleagues developed a model called CityCrop. Using climate measurements and LiDAR data, the model estimates the amount of shade that trees and buildings provide, in order to calculate plant evapotranspiration rates and hence the water requirements of urban crops. The researchers applied CityCrop to one square kilometre of a typical residential neighbourhood in Vancouver.

Compared to crops growing in full sunlight, the shade provided by buildings and trees reduced plant evapotranspiration and water requirements by around 10%, the scientists found. However, this extra shade had very little effect on the crop yield. "This was a surprise for us – crop growth with the median level of shade experienced a yield drop of only about 3%," said Johnson.

The team also found that over the 75-day cropping cycle, the average crop water requirements were around 17% less than for a well-watered lawn. "However, crops that require a lot of water for washing post-harvest, such as root crops, may use more water overall than a lawn," said Johnson.

If all the lawns in this particular neighbourhood were replaced with crops, Johnson and his colleagues estimated that around 37% of the local population would be provided with all their vegetable needs for the year, assuming a 150-day growing season and a density of around 5000 people per square kilometre. However, replacing lawns with potatoes and beans would have a significant impact on urban water supplies, particularly in regions that are already water-stressed.

"We estimated that the water demand could increase by more than 50% if urban agriculture were scaled to a significant degree," said Johnson, who published the findings in Environmental Research Letters (ERL). “Water-smart agriculture – drip irrigation, rain-water harvesting and the like – would help manage the additional water demand and should be encouraged, particularly in regions experiencing water stress."

Nonetheless, the results indicate that if carefully planned and managed, urban agriculture can play an important role in food production, often in neglected and sub-optimal spaces. "Partial shade is fine for many crops or orchards, and the bonus is that water requirements are a bit less in these areas," said Johnson.

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