Understanding what determines municipal (i.e., urban and sub-urban) water demand is key to developing resilient drinking water systems. Important drivers are the quantity of consumers (population), types of consumers (e.g., single- or multi-unit residential, commercial, governmental), consumer income, water pricing, water system efficiency, and weather and climate, among others [1, 2]. Making long-term (i.e., multi-decadal) projections of water demand is challenging because many of these factors will vary over time in uncertain ways. For example, anthropogenically forced climate change is, and will be for the foreseeable future, occurring at a rate that its impact on future water demand can be important at time horizons of a few decades, which aligns with the lifetime of major water system infrastructure [3–5]. However, the rate of future climate change is highly uncertain, particularly at regional scales [6, 7].
Quantifying the potential effect of climate change on future water demand has often been based on empirical analyses of historical sensitivity of demand to changes in meteorological variables, typically air temperature and precipitation, integrated over time intervals of a day to months. Mostly, studies revealed a positive relationship of demand with temperature and/or an inverse relationship to precipitation [8–13]. Temperature affected demand more strongly in some systems [13–15] while precipitation was the larger factor in others [10, 16, 17]. The positive relationship of demand to temperature and negative relationship to precipitation can be partially attributed to the need, whether actual or perceived, for increased watering of lawns, gardens, and parks when soil moisture is low due to high evapotranspiration rates driven by high temperature and due to a paucity of rain [1, 18]. Water consumption related to other outdoor activities and water used for cooling can also increase when temperatures are higher [19].
Given the above relationships, projected increases in temperature across the globe [7] imply an increase in future municipal water demand - all else being fixed - such that the effect of rising temperature on demand is only a question of magnitude, not direction. In contrast, precipitation projections for summer, the season with typically the highest water demand, range from increases to decreases across the globe [7], therefore both the sign and magnitude of the effect of precipitation changes will vary regionally.
Since as early as Cohen [20], numerous studies have estimated the effects of climate change on future municipal water demand. Many analyses focused on large cities and metropolitan areas, such as Portland, Oregon [4, 21, 22], Seattle and Eastern Puget Sound Washington [23], Chicago and northeastern Illinois [24], Birmingham, UK [25], Phoenix, Arizona [22], Bangkok, Thailand [26], Sydney and the Blue Mountains Region, Australia [14], and Naples, Italy [5]. Other analyses were national [27–30] or even global [31] in extent. Although these latter large-scale studies help with developing regional or national policy, they lack local-scale information needed by water providers and the communities they serve. Some studies estimated the effects of climate change on multiple water sectors including the municipal sector but only reported on total water demand, therefore the impact to municipal water alone is not available to the reader [27–29, 32], or the contribution of climate change was not given separate from other factors [33].
Although the above does not provide an exhaustive list of studies of the effects of climate change on future water demand, it is notable that none discussed the role that the interaction of tourism and climate change may have on municipal water demand. In fact, only one study even mentioned tourism [26] and then only to state that peak demand was generally higher during peak tourism season. One multi-city study of the United States and Canada [30] even intentionally excluded cities with seasonally varying populations (presumably largely due to tourism) because of their potentially confounding effect. Yet, the relative impact of tourism on water consumption is expected to increase globally; Gössling and Peeters [34], for example, estimated that both direct and indirect water consumption from tourism would increase by 50% or even 90% in a more extreme scenario, from 2010 to 2050, far outpacing global population growth.
Tourism not only increases the number of water consumers, but tourist use differs from residential use. Tourists often can be characterized by more lavish water use related to more or longer showering and bathing, more use of water-intensive leisure and sport facilities such as swimming pools, hot tubs, spas, saunas, and golf courses, more laundering from the frequent changing of bed and bath linens, and more restaurant dining [35–37]. Given the different water demand of tourists compared to residents, and the transient nature of tourists, the sensitivity of water demand to weather and climate may be different for tourists [38, 39]. Consequently, tourist and resident water demand may respond differently to future climate change.
Despite the recognized role that climate change will have on water demand, climate change impacts have been incorporated into relatively few water management plans in Oregon, United States, outside of the well-resourced Portland metropolitan area. For example, our review of the most recent water master plans and water management and conservation plans of water providers on the middle coast of Oregon (“Mid-Coast”), a region with an important contribution of tourism to the economy, found none that accounted for future climate on water demand. We have little reason to believe the situation is different elsewhere along the Oregon coast and even for small water providers in most places across the globe.
Our objective was to address two unanswered questions. The first is how water demand in relatively small, tourist-based communities in the Mid-Coast respond to climate variability. Although Toth et al. [38] and Mazzoni et al. [39] addressed this question for a region of northern Italy, the extent to which their results are generalizable is not clear. The second question is how climate change may affect water demand in these communities over the next several decades.
To meet our objective, we first estimated sensitivities of historical municipal monthly water demand to air temperature and precipitation for Mid-Coast communities that varied with respect to the relative importance of tourism to their overall economies. Secondly, we used these sensitivities to project the effect of climate change on municipal water demand to the year 2070. We also compared the effect of climate change to the effect of population change alone to consider the relative roles that climate and population may have on water demand over the next several decades.