|value||Understanding the atmospheric mechanisms behind precipitation events, and their hydrologic impacts on the land surface is crucial for ecosystem functioning, water supply, fire suppression, and in the case of extreme events, for water quality, flood control and dam safety. Recent studies have shown that more than 60% of heavy cool season precipitation events (Nov-Mar) in the Verde River basin in northeastern Arizona, are related to the occurrence of Atmospheric Rivers (ARs) (Rivera, 2014). ARs are narrow corridors of concentrated water vapor that bring copious amount of rainfall along the U.S. west coast (Dettinger et al., 2011; Neiman et al., 2008; Ralph et al., 2013; Ralph and Dettinger, 2012; Ralph et al., 2006; Rutz et al., 2014) and penetrate inland where they interact with the topography to produce heavy precipitation and flooding (Neiman et al., 2013; Rivera, 2014). Additionally, the arrival of ARs can contribute to the end of persistent droughts in the west coast (Dettinger, 2013). The Salt and Verde basins provide most of the water supply for the greater Phoenix metropolitan area, the 12th largest in the USA, and for two tribal reservations: the White Mountain Apache tribe and the San Carlos Apache tribe which jointly own 59.4% of the land. While current drier-than-normal atmospheric conditions have increased forest fire risk and have put more pressure on the available water for human and ecosystem consumption; near and mid-term (months to decades) precipitation projections are of great concern to natural-resource managers who depend on the basins’ resources for water supply, ecosystem health, and economic activities such as timber, carbon credit projects, wildlife and recreational enterprises. Intense precipitation events can cause flooding which results in turbid water that is untreatable and damages the recharge facilities, this creates difficulties for water managers in the region (Doug Toy, pers. comm.).
The objectives of the proposed research, responding to Science Priorities 1 and 2, are: 1) Analyze the predictability of ARs at intermediate timescales (10 years) using decadal climate model simulations by identifying the large scale-atmospheric circulation patterns associated to ARs. Are ARs modulated by ENSO events? Are ARs likely to become more frequent and intense under warmer climates? 2) Develop and apply hydrological models to simulate streamflows in the basins for water management (e.g., reservoir levels and releases) and for flood control purposes. How is the frequency and intensity of ARs linked to drought periods in the basin?
We plan to use an automated AR detection algorithm (Rivera and Dominguez, in preparation) to identify ARs affecting the Salt-Verde basin in the decadal climate simulations from the CMIP5 experiment. These simulations are initialized with observed sea surface temperatures every 5 years (1960, 1965,…,2005) and run 10 years into the future and could potentially yield a better representation of ocean-atmosphere procesess (Meehl et al., 2009; Taylor et al., 2012). We will evaluate the predictability of ARs based on changes in the location of Pacific jet stream linked to ENSO events which results in wetter than normal winters (Canon et al., 2007; Kim et al., 2006). It is important to keep in mind that the intensity and spatial extent of precipitation linked to ARs is due to the interaction of the atmospheric moisture with the local topography, however, the CMIP5 GCMs are still too coarse to realistically represent these fine-scale processes. We will obtain a detailed representation of the projected precipitation in the basin, by dynamically downscaling individual AR events using the Weather Research and Forecasting (WRF) Regional Climate Model for both the historical and projected future climates.
While understanding the precipitation response to AR events is important, this information is not particularly useful to stakeholders who are primarily concerned with the impact of precipitation events on runoff in the reservoirs. Consequently, most of our efforts will be focused on analyzing the hydrologic response of the Salt and Verde basins to AR events. We will use both observed and simulated historical and future precipitation to drive the Variable Infiltration Capacity (VIC) hydrologic model. The model will provide information on streamflow in the Salt and Verde, as well as soil moisture, evapotranspiration, and other hydrologic variables through the basin. We will simulate event-driven daily streamflows along the main channels placing particular attention to the role of antecedent moisture conditions in the basin on flooding. Since extreme wet events cause flooding that results in untreatable turbid waters, we will analyze how often water quality issues are associated to an AR occurrence. Flooding along the main channels will also be simulated with the Hydrologic Engineering Center River Analysis System model (HEC-RAS) for maximum daily simulated flows. |