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Sensitivity of Boulder Colorado’s Water Supply to Climate Change

Principal Investigator and Institutional Representative

Joel B. Smith, Stratus Consulting Inc.

Co-Principal Investigators

David Atkins, Stratus Consulting Inc.; Carol Ellinghouse, City of Boulder; Lee Rozaklis, Hydrosphere Resource Consultants

Project Period

May 1, 2005 to April 30, 2006

Few local governments in areas that are vulnerable to climate change have conducted an in-depth analysis of how they could be affected and how they might have to change operations or infrastructure. Indeed, it is fair to say that few local governments have conducted an in-depth assessment of how they could be affected by long-term climate variability.

Most of the City of Boulder, Colorado’s water is from melting of snowpack in the Rocky Mountains. Unlike most Front Range communities, which use a majority of their water for irrigation, Boulder uses two-thirds of its water for indoor and industrial uses and one-third for irrigation. Its smaller share of water used for irrigation gives the city less flexibility in responding to droughts than its neighbors.

The City’s water reliability standards impose increased stringency of service reductions depending on the severity of droughts. The City recently analyzed how its water supply system could be affected by a repeat of events during the last 300 years of climate history. The 300-year record consisted mainly of a reconstruction based on tree ring data. The study also considered a 25% increase in interannual variability in the 300-year reconstructed climate record to test sensitivity to an increase in variability and a 15% decrease in average supply to test sensitivity to a long-term change in supply. The study found that Boulder could cope with a repeat of the 300-year reconstructed climate record. A 15% reduction in runoff would adversely affect the city’s water supplies, while a 25% increase in variability would have minimal effect (because of the use of interannual storage).

This project will study the potential effects of climate change in combination with a repeat of long-term climate variability in much more depth. It will analyze how snowpack accumulation and runoff patterns can change because of climate change that could reasonably be expected to happen by 2030. It will then analyze how much of a reduction in runoff would result in missing water supply reliability targets and, based on analysis of output from many climate change models, the rough likelihood of such an event. Finally, it will analyze two relatively extreme wet and dry climate change scenarios to see how the water supply system could be affected.

The study will consist of four tasks:

1. Analyze effects of temperature on snowpack and demand. Stratus Consulting will estimate changes in snowpack and seasonality of runoff from incremental increases in temperature (e.g., +1°C, +2°C, +3°C). They will produce algorithms or look-up tables that can be used by the City of Boulder for future analyses. Based on analysis of the MAGICC/SCENGEN model developed by Dr. Tom Wigley of the National Center for Atmospheric Research, a 1.5°C warming has the highest probability for Colorado by 2030. Hydrosphere will use the snowpack and runoff results as inputs to the City of Boulder's Boulder Creek Watershed Model to examine the effects of a 1.5°C warming on supply, demand, and reliability. The analysis will be done by combining the changes in runoff with the 300 year record of streamflow. That record contains extensive droughts. This approach effectively combines long-term variability with climate change; i.e., assuming the droughts would happen again but under warmer conditions.

2. Determine coping thresholds. The previous analysis of a 15% decrease in long-term runoff found that such a reduction would result in an unacceptable increase in drought conditions. To determine where the threshold for acceptable reductions exists, Hydrosphere will examine 5% and 10% reductions in runoff imposed on the 300 year record. Staff from the City of Boulder will analyze the results to determine what long-term reduction in supply would exceed the city's coping capacity. That level will be considered the threshold coping capacity for the City of Boulder.

3. Examine likelihood of exceeding the coping capacity. Stratus Consulting will work with Dr. Wigley to use the results of MAGICC/SCENGEN to examine the approximate likelihood of exceeding the coping capacity by 2030 and 2070. They will determine what increases in temperature and changes in precipitation could result in runoff reductions that would exceed the coping capacity for Boulder. They will determine the proportion of GCMs that would result in exceeding the threshold. This is not a probability analysis (for one reason, GCM output cannot be interpreted as random independent outcomes), but gives some insight into rough likelihoods of reductions in supply severe enough to exceed Boulder's coping capacity.

4. Examine driest and wettest GCMs. The researchers will use the output of the driest and wettest GCMs to estimate changes in supply and demand. Stratus Consulting will estimate change in runoff and demand. Hydrosphere will use the results to examine implications for Boulder's water management. The results will be interpreted by city staff.

The team will produce a report for NOAA, and if appropriate, will prepare a manuscript to submit to a peer-reviewed journal or other suitable publication. Finally, a one-day workshop will be organized to present the results of the analysis to other communities in the Front Range.