Water Managers' Response to La Niña: Two Case Studies

El Niño and La Niña produce well-known effects on winter climate in the Southeastern United States that can be described using statistical approaches. For La Niña, these impacts include the likelihood of warmer than normal temperatures and drier than normal precipitation over the winter months, particularly in Florida and southern Georgia. Knowledge of the developing La Niña in fall 2010 provided water managers with probabilistic information that was incorporated into decisions on water storage and distribution based on past climatic variability and specific system operating guidelines. In this presentation we highlight two case studies outlining how water managers in two distinct regions in the southeastern United States responded to the predicted La Niña in their seasonal operations.

The South Florida Water Management District controls water releases for Lake Okeechobee and watersheds in the southern third of Florida's peninsula. Based on precipitation from past La Niña and non-La Niña years, they estimated the variability of the lake levels under a variety of rainfall scenarios. Model projections based on only La Niña years indicated that in the wettest of La Niña years the lake levels could increase somewhat but that lake levels might fall drastically under rainfall conditions such as those in the winter of 2001-2002. If the lowest levels were to occur, then some canals could dry up for a significant length of time. This would cause severe impacts on ecosystems as well as canal and water users in the area. Based on projections of rainfall and appropriate operating decision-making, water managers successfully controlled their water resources in spite of expanding drought conditions.

The Savannah River basin and its set of large reservoirs are managed by the U. S. Army Corps of Engineers to meet the needs of a variety of water users. These stakeholders include power utilities, recreation and tourism, wildlife conservation, and the Savannah River harbor at the mouth of the river. Knowledge of the impending La Niña allowed water managers to use a set of key flow levels that are triggers for severe impacts to expand storage of water upstream in the reservoirs at the top of the watershed. This operational decision was made in anticipation of lower water levels in 2011 based on statistical flow values from preceding La Niña episodes. The U.S. Corps of Engineers is also using the possibility of a second La Niña year to plan for the navigation season in spring 2012.