Modeling Future Residential Outdoor Water Demand in the U.S. West
This article is adapted from a report on a project funded by the Babbitt Center for Land and Water Policy and the Water Research Foundation. Several municipal staff members in Aurora, Colorado, generously provided data and support for the project, including Lyle Whitney, Zach Vernon, and Timothy York at Aurora Water and Karen Hancock, principal planner with the City of Aurora.
The Front Range of Colorado, an urban corridor east of the Rocky Mountains that includes Denver, Boulder, and other densely populated areas, increasingly faces a perilous combination: rapid population growth and a capricious water supply. With a semi-arid climate and a history of lengthy drought periods, the region is experiencing an increasing imbalance between water supply and demand, a situation aggravated by climate change. Water managers are faced with the challenge of securing an adequate water supply for future growth and new development.
Our research at the University of Colorado applies an innovative methodology to water demand forecasting that provides key insights into the relationships between development decisions, water needs, and growth. This in-depth analysis of residential outdoor water use in Aurora, Colorado, uses a novel housing typology to inform 50 unique future development scenarios, providing a water blueprint for future projected growth, a methodology that can be applied to other communities. The study forecasts future water demand under various growth, climate change, and design scenarios, and demonstrates the inherent value and necessity of connecting water management and land use planning.
Projecting Water Demand in Aurora
The study area is the City of Aurora, Colorado, a growing municipality located in the Denver metro area. Like most communities in the region, Aurora is facing intense population growth and development pressures, with close to 2 percent annual projected growth to 2070, which will almost double the current population (Data & Demographics: Population, 2020).
While Aurora has prioritized securing water rights to ensure the community has the needed water supply to support that growth, these are predominantly surface water supplies. They are vulnerable to drought and climate change and can be usurped by those with higher-priority rights to Colorado River water. These circumstances leave the community’s water security vulnerable, providing ample incentive for developing a better understanding of a variety of possible trajectories of future water demand, which can also inform and prioritize land use policy changes.
Based on the best available data for population growth, land use, and water use, this study generated growth scenarios out to the year 2050. The study methodology is based on the creation of a unique housing typology, which reflects the current housing stock in Aurora. The housing types were defined by yard size, home value, and current outdoor water use. This data was then analyzed to determine the key housing characteristics that predicted significantly different water use, based on built environment characteristics. The two most significant variables were house value and pervious lot areas—in other words, larger houses with larger yards predictably used more water. This is consistent with other findings in the literature pointing to income levels and yard size as key drivers for water demand (Arbues and Villanua 2006, Dalhuisen et al. 2003, Locke et al. 2018, Rogers 2002). The analysis allowed for the classification of all residential units in the data set into eight distinct housing types.
The scenario development was initiated by establishing a baseline growth scenario which projects full build out of Aurora’s approved master planned (MP) developments. The housing typology was matched to the expected future units in the MPs, based on an analysis of their yard sizes and home values. Using the water use data for each house type, the outdoor water demand was calculated for the projected MP developments. This calculation was used as the baseline scenario for comparison with 50 subsequently developed scenarios.
The subsequent scenarios combine six factors: predicted development extent, pervious area, home values, climate change severity, landscape design, and level of irrigation efficiency. A portion of the scenarios includes projection of growth beyond the extent of MPs, simulating unplanned development of what is currently open space prairie lands. Because the distribution of housing types in these unplanned future development areas is unknown, the researchers used Aurora’s Unified Development Ordinance guidelines to develop scenarios with different combinations of housing types.
Landscape design, irrigation efficiency, and climate variables were added to the scenario model as multipliers that either increase or decrease overall outdoor water use per unit area. The 50 scenarios were organized into ten groups of five scenarios, which had the same sequence of landscape design, climate change extent, and irrigation efficiency values. This allowed for a clear understanding of the impact of the multipliers and the development extent.
The resulting scenarios address a wide set of possible future circumstances that were designed to reflect a range of realistic possible outcomes. By providing insight into how impacts may change based on factors like climate and population growth, this methodology can inform management and policy decisions related to lot size and landscaping standards. For example, the results show that having a development ordinance that limits lot sizes for single family residential development and requires at least 50 percent low-water (xeric) landscaping would yield water savings up to 35 percent.
Conclusions and Scalability
Given the multiple constraints on sustaining future water supply in the Colorado River Basin, creating a portfolio of reasonable outcomes can be critical for managing sustainable growth. The key factors within the bounds of land use planning include growth boundaries, lot sizes and housing types, landscape design regulations (e.g., xeric requirements or turf limits), and irrigation efficiency standards.
The results from this model show that new development that requires 50 percent xeric landscaping combined with efficient irrigation significantly reduces outdoor water demand, even when using the highest climate change impact multiplier and the greatest extent of development scenarios.
Notably, this model uses only half-xeric yards, versus modeling the entire outdoor pervious space with xeric design. This is because the current UDO calls for low-water-use front yards, while back yards have no regulatory parameters on landscape design. This suggests that creating a 100 percent xeric landscape yard would likely prove twice as effective. The half-xeric yard was chosen due to the guidelines of Aurora’s UDO ordinance, which requires small lot homes to have xeric designed front yards. Expanding that ordinance to 100 percent xeric landscaping for future growth would significantly contribute to meeting water supply goals, while still accommodating growth projections.
The scenario modeling in this study provides a unique look at the coupled effects of multiple factors that influence outdoor water demand. The results show that best practices for low water use landscape design and irrigation efficiency can lower water footprints for high population growth in a warming climate, as predicted for Aurora. These outcomes reinforce the importance of integrating land use planning and water management.
The conservative projections for climate change used in this study are likely estimates of future realities, and our results reveal that community planning that includes water smart land use zoning, building code improvements, and landscape requirements yields positive effects for future water resilience.
Beyond the direct usefulness of this study to Aurora, much of its value lies in demonstrating that the methodology is feasible, yields reasonable results, and is scalable to larger regions.
Many cities in the Colorado River Basin and throughout the West are facing similar growth pressures to those seen along Colorado’s Front Range. Our findings support the recommendation for jurisdictions to carefully consider where and how development occurs, before extending future populations away from the urban core through annexations of undeveloped open spaces, prairies, and greenfields. Moreover, this water demand forecasting model demonstrates the promising opportunities available for the arid West to use water-smart land use planning to create a more resilient future.
Gretel Follingstad is a PhD candidate in Geography, Planning & Design, with a research and pedological focus on Climate Resilience Planning, in the Department of Urban and Regional Planning at the University of Colorado, Denver
Austin Troy is a professor in the Department of Urban and Regional Planning at the University of Colorado, Denver.
Lead image: A waterwise yard in Aurora, Colorado. Credit: City of Aurora.
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