There is already clear evidence that changes in the hydrologic cycle are occurring in response to climate change (see, e.g., Trenberth et al., 2007; USGCRP, 2009a). Improved regional projections of changes in precipitation, soil moisture, runoff, and groundwater availability on seasonal to multidecadal time scales are needed to inform water management and planning decisions, especially decisions related to long-term infrastructure investments. Likewise, projections of changes in the frequency and intensity of severe storms, storm paths, floods, and droughts are critical both for water management planning and for many adaption decisions. Developing improved understanding and projections of hydrological and water resource changes will require new multiscale modeling approaches, such as nesting cloud-resolving climate models into regional weather models and then coupling these models to land surface models that are capable of simulating the hydrologic cycle, vegetation, multiple soil layers, groundwater, and stream flow. Improved data collection, data analysis, and linkages with water managers are also critical. See Chapter 8 for additional details.
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