Using Water Supply Forecasts and Water Shortage Triggers to Manage Droughts
- 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 2007, a year in which severe drought gripped much of the Southeast.
The next year, the NC legislature passed a statute requiring each utility to prepare a Water Shortage Response Plan (WSRP) and file it with the NC Division of Water Resources (DWR). 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 better informed the decision to enact mandatory conservation measures. The forecasts were developed using position analysis, a technique that uses current water supply conditions, basin operations, and past hydrologic records. The forecasts predict the probability of being below a target reservoir level at any date up to a year into the future. They also helped reduce the probability of shortage by demonstrating the optimal proportion of demand to withdraw from the City’s two reservoir systems to minimize 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 detailed understanding of the water supply system and, in particular, the inter-annual and intra-annual 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 forestall more severe 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.
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