HYDROLOGICAL PROCESSES Hydrol. Process. 24, 3864–3877 (2010) Published online 29 July 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.7818 Climate change and future flows of Rocky Mountain rivers: converging forecasts from empirical trend projection and down-scaled global circulation modelling Anita Shepherd, 1,2 Karen M. Gill 3 and Stewart B. Rood 3 * 1 Irrigation Branch, Alberta Agriculture and Food, Lethbridge, Alberta T1J 4V6, Canada 2 North Wyke Research Station, Devon, EX20 2SB, UK 3 Environmental Science Program, University of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada Abstract: To predict future river flows, empirical trend projection (ETP) analyses and extends historic trends, while hydroclimatic modelling (HCM) incorporates regional downscaling from global circulation model (GCM) outputs. We applied both approaches to the extensively allocated Oldman River Basin that drains the North American Rocky Mountains and provides an international focus for water sharing. For ETP, we analysed monthly discharges from 1912 to 2008 with non-parametric regression, and extrapolated changes to 2055. For modelling, we refined the physical models MTCLIM and SNOPAC to provide water inputs into RIVRQ (river discharge), a model that assesses the streamflow regime as involving dynamic peaks superimposed on stable baseflow. After parameterization with 1960–1989 data, we assessed climate forecasts from six GCMs: CGCM1-A, HadCM3, NCAR-CCM3, ECHAM4 and 5 and GCM2. Modelling reasonably reconstructed monthly hydrographs (R 2 about 0Ð7), and averaging over three decades closely reconstructed the monthly pattern (R 2 D 0Ð94). When applied to the GCM forecasts, the model predicted that summer flows would decline considerably, while winter and early spring flows would increase, producing a slight decline in the annual discharge (3%, 2005–2055). The ETP predicted similarly decreased summer flows but slight change in winter flows and greater annual flow reduction (9%). The partial convergence of the seasonal flow projections increases confidence in a composite analysis and we thus predict further declines in summer (about 15%) and annual flows (about 5%). This composite projection indicates a more modest change than had been anticipated based on earlier GCM analyses or trend projections that considered only three or four decades. For other river basins, we recommend the utilization of ETP based on the longest available streamflow records, and HCM with multiple GCMs. The degree of correspondence from these two independent approaches would provide a basis for assessing the confidence in projections for future river flows and surface water supplies. Copyright  2010 John Wiley & Sons, Ltd. KEY WORDS climate change; historic hydrology; hydroclimatic modelling; Oldman River Received 3 July 2009; Accepted 14 June 2010 INTRODUCTION There is growing concern about environmental conse- quences from climate change and it is recognized that a critical impact in the Rocky Mountain region of western North America could be changes in the magnitude and seasonality of river flows, which provide primary surface water supplies (Barnett et al., 2005; Schindler and Don- ahue, 2006). To assess future river flows, there are two relatively independent analytical approaches that have been applied (Stewart et al., 2004, 2005). One approach, empirical trend projection (ETP), assesses actual his- toric streamflow data and applies statistical analyses to detect and quantify historic patterns or trends (Burn, 1994; Zhang et al., 2001; Stewart et al., 2005; Rood et al., 2008). The statistical analyses often include non- parametric rank-order tests to detect trends and subse- quent linear or non-parametric regressions to characterize historic patterns. A strength of ETP is that it is based on * Correspondence to: Stewart B. Rood, Environmental Science Program, University of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada. E-mail: rood@uleth.ca actual, historic data and the statistical analysis is rela- tively straightforward, requiring few assumptions about underlying processes. The future forecasting with ETP is based on the assumption that the near future will extend the recent past, with respect to the direction and mag- nitude of change. It is recognized that this assumption may not be valid, since changes such as those due to the accumulation of carbon dioxide and other greenhouse gases may accelerate climate change. It is also likely that some climate parameters may display aspects of non-linear dynamics including thresholds and feedbacks. Subsequently, the near future will probably extend the direction of climate change but the response magnitude may increase. A complementary approach involves hydroclimatic modelling (HCM), which involves river flow forecasting after regional downscaling of climate projections from global circulation (or ‘climate’) models (GCMs) (Shep- herd and McGinn, 2003; Lapp et al., 2005). The GCMs represent physically-based computer models that consider energy inputs and flows and atmospheric processes, and these forecast future climate conditions with different Copyright  2010 John Wiley & Sons, Ltd.