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The State of New Hampshire has initiated a coordinated effort to proactively prepare for the effects of climate change on the natural and human resources of New Hampshire. An important aspect of this effort is to develop a vulnerability assessment of hydrologic response to climate change. The U.S. Geological Survey, in cooperation with the New Hampshire Department of Health and Human Services, is developing tools to predict how projected changes in temperature and precipitation will affect change in the hydrology of watersheds in the State. This study is a test case to assemble the information and create the tools to assess the hydrologic vulnerabilities in four specific watersheds. The study uses output from general circulation models to drive hydrologic simulations of streamflow, groundwater base flow (hereafter referred to as base flow), and snowfall in four representative watersheds in New Hampshire during the 21st century, including the watersheds of the Ashuelot, Oyster, Pemigewasset, and Souhegan Rivers. Simulations show that on average, relative to current conditions, streamflow is likely to increase and base flow is likely to decrease, although this change is highly variable by geographic location and season. Streamflow variability will likely increase, with more high streamflows and more low streamflows. The largest increases in streamflow are in the winter, with small decreases in summer. Change in base flow varies across the State with the largest change in the northern Pemigewasset River watershed. Changes in snowfall are consistently decreasing for all watersheds on average, with the largest change also in the Pemigewasset. However, monthly snowfall totals during any given winter could be higher in the future than expected under current conditions. Increasing frequency of floods (the largest seven floods expected to occur in 20 years) could be more significant than the size of the floods, except in the northern high altitude watersheds. In other words, the projections indicate a pattern of multiple floods that might not breach the riverbanks, yet the increased frequency could put additional strain on the existing river banks, infrastructure, and nearby human settlements. There is also likely to be an increase in high flows during the winter and spring months, which could result in more uncertainty in planning for the design, operation, and maintenance of infrastructure, including roads and utilities. Similarly, it is expected that, on average, there will be less base flow available and a wider range of seasonal fluctuation in base flow than experienced historically. These issues could necessitate more attention to planning and management of the resource. Based on past experience, the most important effects of climate change could be less certain planning options and a greater need for planning that accounts for the effects of larger streamflows than are currently available. The effects of hydrologic change on human health and well-being could be most readily apparent with respect to changes in streamflow and the subsequent increase in the frequency of minor flooding and the frequency of summer and fall low streamflows. These changes could require the development of plans to adapt, protect, and upgrade infrastructure, such as bridges, culverts, roads, and other structures. The precipitation runoff modeling shows that rivers and watersheds in New Hampshire will likely change in response to climate change, and that this response varies with season and latitude. Although four representative areas were simulated in this study, additional models could be used to predict the response over the entire State. Introduction and Background There is a coordinated effort in New Hampshire to proactively prepare for the effects of climate change with a practical action plan (New Hampshire Climate Change Policy Task Force, 2009). A key component of the action plan is a vulnerability assessment of hydrologic responses to climate change. The U.S. Geological Survey (USGS) has been identified as a partner along with the New Hampshire Department of Health and Human Services and the University of New Hampshire in the development of tools and maps to 1U.S. Geological Survey. 2New Hampshire Department of Health and Human Services. 2 Simulating Hydrologic Response to Climate Change Scenarios in Four Selected Watersheds of New Hampshire help assess the hydrologic effects from shortand long-term climate change. In this partnership, the USGS developed tools (1) to predict how climate change will affect streamflow, base flow, and snowfall and (2) to identify locations that are vulnerable to the effects of climate change in areas across the State. This report documents a test case to create the information and tools that will allow specific users to understand and assess the hydrologic vulnerabilities in four specific watersheds. Vulnerability can be thought of as a measure of the degree to which a system is exposed to, susceptible to, and able to cope with and adapt to adverse effects (highlight 1). Hydrologic vulnerability as a result of climate change is defined for the purposes of this report as a measure of the degree to which a deleterious effect on the hydrologic system results from climate change. Deleterious effects include (1) depletion in reservoir storage, (2) depletion in groundwater storage, (3) decrease in snowpack, (4) decreases in base flow (the component of streamflow made entirely of base flow), (5) low streamflow, and (6) increases in flooding. Vulnerability can also incorporate an assessment of degraded conditions that result from quantitative changes in hydrology (O’Connor and others, 1999). These climatic effects could be exacerbated or alleviated by increased development through changes in land cover and water use. Each region will have differing vulnerabilities to climate change, and assessment of these vulnerabilities is the first step in considering potential adaptive strategies. Hydrologic vulnerability from future climate scenarios has increasingly become a major point of research efforts in New Hampshire and the Northeastern United States (Neff and others, 2000). Mack (2009) assessed a future climate scenario for the seacoast region by simulating potential variations in groundwater recharge and water use. For this region of the State, Mack (2009) found that the potential effect of future climate change on groundwater by 2025 is likely to be more substantial on groundwater recharge than changes in water demand. Using a watershed runoff model to simulate groundwater recharge and snow in the Connecticut River Basin (western New Hampshire), Bjerklie and others (2010b) found that, during the more than five-decade period from 1960 to 2007, snowfall has generally decreased and base flow has generally increased. These changes have not been distributed uniformly in time or space and have marginally decreased or increased in some places in response to local conditions. Similarly, for the 50-year study period from 1961 to 2010, Dudley and Hodgkins (2013) found increasing groundwater levels and streamflow in northern New England. Hamilton and others (2010) reported climate trends in New Hampshire showing air temperature and precipitation increasing throughout the 20th century, with the increases accelerating since 1970. Hamilton and others (2010) also found indications of decreasing snowfall; however, they acknowledge that the trends in snowfall are more complicated with inconsistent trends. Jennifer Jacobs (University of New Hampshire, Durham, oral commun., March 2014) found evidence of consistently decreasing snowfall in New Hampshire. Wake and others (2014) indicate that the climate model consensus shows increasing air temperature and precipitation for New Hampshire in the future, at least into 2100. Bjerklie and others (2010b) project that, in response to this climate change, groundwater recharge (and subsequent base flow) will continue to increase and snowfall will decrease, but not everywhere, depending on the general circulation model (GCM, often referred to as a global climate model) emissions scenario. Similar projections are reported by the U.S. Geological Survey (2014) based on the Intergovernmental Panel on Climate Change (IPCC; 2014) and Melillo and others (2014). Streamflow records for 400 sites in the conterminous United States, measured during a five-decade period from 1941 to 1999, indicate an increase in annual minimum daily and median daily streamflow beginning around 1970 (Lins and Slack, 1999; McCabe and Wolock, 2002), although there is a mixed pattern of increases and decreases in annual maximum daily streamflow across the United States. Annual streamflow in the eastern United States appears to increase as a step change and coincides with an increase in precipitation. Past trends and GCM projections indicate that precipitation will continue to increase in the conterminous United States (Karl and Knight, 1998; Groisman and others, 2005). These trends have also been noted in more localized studies in parts of New England (Miller and others, 2002) and the State of New York (Burns and others, 2007). The measured step increase from about 1970 also coincides with changes in the timing of snowmelt peaks in New England (Hodgkins and others, 2003). Highlight 1 Hydrologic Hazards and Vulnerability Hazard.—A hazard is a deleterious effect, source of potential danger, or condition that can cause harm, deprivation, and stress. The Federal Emergency Management Agency describes hazard identification as a process of “defining and describing a hazard, including its physical characteristics, magnitude and severity, probability and frequency, causative factors, and locations/areas affected” (Schwab and others, 1998). Vulnerability.—How exposed and susceptible the hydrologic system is to deleterious effects from climate change, and how well the human and natural system can cope with and adapt to the deleter

Simulating hydrologic response to climate change scenarios in four selected watersheds of New Hampshire