Using Water Supply Forecasts and Water Shortage Triggers to Manage Droughts

Authors:

  • Reed Palmer - Hazen and Sawyer

The City of Raleigh, NC is one of the fastest growing cities in the US and is located in a region subject to increasing water scarcity. This presentation describes the evolution of Raleigh’s drought preparedness since the NC legislature passed a statute requiring each utility to prepare a Water Shortage Response Plan (WSRP) following the severe drought of 2007-08. 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 DWR had sophisticated water basin models developed that provided stakeholders like Raleigh with new planning and forecasting tools. When the WSRP was put into practice over 3 consecutive dry years from 2010-2012 it was determined that the conservation triggers in the WSRP were sub-optimal.

The short-term solution was to utilize the basin model to generate a water supply forecast that predicts the probability of being below a target reservoir level at any date up to a year into the future. This knowledge informed the decision to enact mandatory conservation measures and also helped reduce the probability of shortage by demonstrating the optimal proportion of demand to withdraw from the City’s two reservoir systems, thereby minimizing overall risk.

The longer-term solution was to improve the drought triggers. 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 sound understanding of the supply system dynamics and, in particular, identifying factors 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 mitigate shortages can be accurately modeled. The reservoir dynamics, in particular the drawdown-refill cycle, were evaluated using the basin model. Estimates of water use reduction by drought stage were developed with knowledge of 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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