Climate Impacts on Arrow Reservoir by Se-Yeun Lee and Alan Hamlet 1. Introduction Pacific Northwest temperature has increased for 20th century and is predicted to further increase in the 21st century in response to greenhouse forcing. The temperature increase and resultant streamflow timing shifts are likely to decrease the reliability of reservoir refill in Columbia River Basin (Payne et al. 2004; Lee et al., 2009). Lee et al. (2009) developed optimized flood control curves to migrate the hydrologic impacts of global warming and showed that storage deficits decreased without increasing flood risk when the optimized flood control curves were used. As an extension of the Lee et al. (2009) study, we examine potential climate change impacts on the Arrow Lakes from simulated results. 2. Methods The climate change scenario assuming an annual average 2° C warming combined with historical precipitation values was simulated using the Variable Infiltration Capacity hydrologic simulation model (Liang et al. 1994) implemented at 1/8th degree latitude/longitude resolution. The monthly time step Columbia reservoir simulation (ColSim) model, described in more detail by Hamlet and Lettenmaier (1999) is used to examine the impacts of the warming scenario alone, and with adaptive changes in flood control operations based on optimization developed by Lee et al. (2009). Note that we report the effects to storage levels as average “storage deficits”, which are equivalent to the full volume of the reservoir minus the volume of water simulated by the ColSim model in each month. If storage deficit increases on average, this means that the reservoir either successfully fills on that date less frequently, or that the lake levels are lower when the reservoir does not refill (or both). 3. Results Figure 1 shows simulated average end of month storage deficits of Arrow dam from July to September when reservoir refill is important. When the current flood control curves are used for both 20th century climate and the climate change scenario, storage deficits increase for the climate change scenario in comparison to the 20th century climate. When flood control curves are changed from “current flood control curves” to “optimized flood control curves” for the climate change scenario, the storage deficits decrease for all months. For July, storage deficits for the climate change are restored the similar level of those for 20th century, when optimized flood control curves are used. Average Storage Deficit (KAF) 3,000 20th Cent_Cur FC CC_Scen_CurFC CC_Scen_OptFC 2,000 1,000 0 Jul Aug Sep Figure 1. Simulated average end of month storage deficit of Arrow Reservoir for simulated 20th century climate using current flood control curves (20th_Cent_CurFC) and the climate change scenario climate using current flood control curves (CC_Scen_CurFC) and HEC-PRM derived flood control curves (CC_Scen_HecFC). 4. Conclusions In the absence of precipitation changes, streamflow timing shifts related to warmer temperatures are likely to increase storage deficits in the Arrow Lakes. Adaptation measures that revise current flood control operations can potentially reduce these impacts to those consistent with historical values in July. Storage deficits in August and September, however, cannot be fully mitigated by flood control adaptation. This suggests that some impacts in late summer are to be expected even if flood rule curves are revised. Potential impacts to precipitation (and changes in precipitation from decade to decade) may affect these results, and are not considered in this preliminary study. 5. References Hamlet, A. F., and Lettenmaier, D. P. _1999_. “Effects of climate change on hydrology and water resources in the Columbia River Basin.” J.Am. Water Resour. Assoc., 35(6), 1597–1623. Lee, S-Y., Hamlet, A. F., Fitzgerald, C. J., and Burges, S. J. (2009). “Optimized Flood Control in the Columbia River Basin for a Global Warming Scenario.” Journal of Water Resources Planning and Management, DOI 10.1061/(ASCE)0733-9496(2009)135:6(440), 135(6) 440-450. Liang, X., Lettenmaier, D.P., Wood, E.F., and Burges, S.J. (1994). “A simple hydrologically based model of land surface water and energy fluxes for general circulation models.” Journal of Geophysical Research, 99(D7), 14415-14428. Payne, J. T., Wood, A. W., Hamlet, A. F., Palmer, R. N., and Lettenmaier,D. P. (2004). “Mitigating the effects of climate change on the water resources of the Columbia River Basin.” Clim. Change, 62 (1–3), 233–256.