How Bad Will It Get? Using Water Supply Forecasts and Water Shortage Triggers to Manage Droughts

Authors:

  • Reed Palmer - Hazen and Sawyer

After the City of Raleigh made it through a drought in 2007-08 with just 56 days of storage remaining, the NC legislature passed a statute requiring each utility to prepare and submit a Water Shortage Response Plan.

To develop better drought triggers, there needs to be an understanding of the natural drawdown/refill cycle and an estimate of how much conservation measures will reduce demand.

Where the area of concern is set depends on demand, effectiveness of conservation measures, confidence level of inflows in refill season, and risk tolerance.

Seasonally adjusted storage triggers help catch drought emergencies early while reducing false alarms.

The City of Raleigh, NC is one of the fastest growing cities in the US, and like many growing cities in the southeast, is located in a region subject to increasing water scarcity. Following the 2007-08 drought, the NC legislature passed a statute requiring each utility to prepare and submit a Water Shortage Response Plan (WSRP). The law stipulates that each utility’s WSRP be set up with several stages of increasingly stringent conservation requirements with quantifiable triggers defining the conditions for implementation. Concurrent to these events, the NC Division of Water Resources (DWR) developed sophisticated water basin models that provide stakeholders like Raleigh with new planning and forecasting tools.

When the City’s WSRP was put into practice over 3 consecutive dry years from 2010-2012 it became evident that the conservation triggers in the WSRP were sub-optimal. The short-term solution was to generate a water supply forecast with DWR’s basin model that better informed the decision to enact mandatory conservation measures. The forecasts estimate the probability of being below a target reservoir level at key future dates and were also used to reduce the probability of shortage by optimizing the proportion of demand to withdraw from the City’s two reservoir systems. The longer-term solution was to improve the drought triggers in the WSRP. Effective drought response triggers facilitate a utility’s ability to manage emerging droughts promptly while simultaneously minimizing false alerts. False alerts (mandating conservation when unnecessary) aggravate customers, erode conservation compliance during future droughts, and disrupt the utility’s revenue stream.

Creating effective triggering mechanisms for a WSRP requires two important types of information. The first is a detailed understanding of the water supply system and, in particular, the dynamics that distinguish normal hydrologic cycles from droughts. The second is an estimate of expected water use reduction at each WSRP stage so the WSRP’s ability to manage shortages can be accurately modeled. Estimates of water use reduction by drought stage were developed with data on reductions achieved during prior droughts coupled with additional demand sector study. Using an iterative process with the basin models, a new set of triggers was developed that are expected to reduce the frequency of WSRP activation by 40-50% without increasing the risk of exhausting the City’s water supply during the worst droughts on record.

For more information, please contact the author at rpalmer@hazenandsawyer.com.

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