Will Your City Run Dry?

Growing Blue

A new report issued by the Columbia University Water Center, in conjunction with Veolia Water and Growing Blue, raises an additional concern to add to future projections of water scarcity: Drought.

According to Upmanu Lall, the director of Columbia’s Water Center, many of the tools which are currently developed to help businesses assess water risk “actually understate the risk of climate variations.” Lall noted that in addition to water scarcity projections, certain areas should also consider their exposure to drought, since droughts will magnify any problems created by water scarcity.

In developing this study, Columbia researchers first developed a new water research metric that they call the Normalized Deficit Cumulated (NDC) index. The index pulls from more than 60 years of precipitation data and the current water use pattern for the United States in order to better depict the discrepancy between water use and availability.

The study clearly shows that decision-makers need to be thinking beyond the problems of water scarcity, to the way drought will affect regions that are already facing problems. Said Lall, “Droughts will create an additional impact that needs to be understood, because drought magnifies the effects of scarcity.”

Areas with the Highest NDC:

  • Washington DC metro area
  • New York metro area
  • California area, from San Diego to Santa Barbara and inland
  • Agricultural belt: Dakotas
  • Agricultural belt: Nebraska
  • Illinois
  • Lower Mississippi belt: Arkansas area
  • Agricultural belt: North Texas
  • Agricultural regions in Ohio
  • Agricultural regions in Minnesota

The risk metric used here considers only locally renewable supply through rainfall or snow in the county. As a result, it exposes dependence on water from outside the county. This is the case for New York and Washington, which rely on water from the Delaware and Potomac, respectively.

Helpful Links

Explanation of the Normalized Deficit Index (NDC)
Two risk metrics were developed to capture the influence of within year dry periods (Normalized Deficit Index – NDI) and of drought across years (Normalized Deficit Cumulated – NDC). The NDI is computed as one number for each year using historical daily rainfall data for the area and current daily water needs. It measures the maximum cumulated water shortage each year during the dry period that needs to be provided for from ground water or from surface water storage or transfers from other areas.

The NDC is computed as one number over the historical climate record. It represents the largest cumulative deficit between renewable supply and water use over the entire period. Consequently, it reflects the stress associated with multi-year and within-year drought impacts at a location. Given that 60 years of historical climate data were used, the maximum of the NDI (i.e. the worst single year), and the NDC (i.e., the effect of a string of bad years) may have an average recurrence interval of approximately 60 years. The NDI data provides insights into other recurrence intervals as well.