akes, rivers, and streams provide many benefits to the general public, but these services are not captured in markets and have proven difficult to quantify (Brauman et al. 2007; Keeler et al. 2012) . This is problematic because information on the value of water resources is needed in many policy and regulatory contexts. For example, the US Environmental Protection Agency is charged with estimating the benefits and costs associated with major rules and regulations designed to safeguard aquatic habitats (Griffiths et al. 2012) . Cost-benefit assessments for water-quality changes are also considered in the design of payment and incentive programs, as well as in spatial planning decisions related to investments in conservation or habitat restoration (Olmstead 2010; Griffiths et al. 2012) . Lack of information about the value of water-quality benefits can complicate justifying major spending on improved water quality. Despite high demand, estimates of the value of clean water are often not available at the relevant scale for proposed interventions, are time-and resource-intensive to obtain, and are difficult to link to empirical measurements of water quality (Iovanna and Griffiths 2006; Keeler et al. 2012) . The most common non-market methods for estimating the value of water-quality improvements typically require time-consuming and costly surveys, either to assess respondents' stated willingness-to-pay for improved water quality (Carson and Mitchell 1993) or to gather information on past recreational behavior (sites visited and distances traveled to sites; see Feather et al. 1995; Phaneuf and Smith 2005; Egan et al. 2009 ). In the first case, a survey is administered to ask respondents how much they would be willing to pay for a given change in water quality (stated preference approach). In the second case, users are asked information about their past behavior, and values are assessed based on how much (in terms of time and lost wages) they have conceded to obtain a higher-quality resource or experience (revealed preference approach). By design, survey data are often site-specific, limited in temporal and spatial scale, and not easily applicable to other decision-making contexts (Freeman et al. 2014) . Estimates of benefits are also not typically expressed in terms of changes in water quality that can be linked to pollutant loads or land-use change, making it difficult to compare the costs and benefits associated with additional restoration or protection measures (Wilson and Carpenter 1999; Keeler et al. 2012) . We investigated how recreational lake users respond to variations in lake water quality, using data from an online social-media source as an alternative to survey data. Specifically, we used geotagged photographs (images associated with spatial-coordinate metadata) uploaded to the photo-sharing website Flickr (www.flickr.com) to estimate the number of visits to different lakes. We counted the number of uploaded photos taken by individual users on unique days that fell within selected boundaries of each lake More than 41 000 water bodies are listed as impaired by the US Environmental Protection Agency under the Clean Water Act. Implementation and enforcement of regulations designed to address these impairments can be costly, raising questions about the value of the public benefits derived from improved surface water quality. Here, we assess the recreational value of changes in water quality using freely available geotagged photographs, taken by members of the public, as a proxy for recreational visits to lakes. We found that improved water clarity is associated with increased numbers of visits to lakes and that lake users were willing to incur greater costs to visit clearer lakes. Lake users were willing to travel 56 minutes farther (equivalent to US$22 in travel costs) for every one-meter increase in water clarity in Minnesota and Iowa lakes, when controlling for other lake attributes. Our approach demonstrates the potential for social-media data to inform social-ecological research, including assessment of the recreational benefits of improvements in water quality. RESEARCH COMMUNICATIONS RESEARCH COMMUNICATIONS
Keeler et al. REF assess the relationship between lake visitation and selected lake attributes for more than 1000 lakes in the Midwestern US states of Minnesota and Iowa.
Recreational demand for clean water: evidence from geotagged photographs by visitors to lakes
Frontiers in Ecology and the Environment
Environmental Science Biology