Alan F. Hamlet Dennis P. Lettenmaier JISAO Center for Science in the Earth System Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington January, 2004 Effects of Land Cover, Topography, and Climate on Pacific Northwest Flooding and Flood Forecasting

Hydroclimatology of the Pacific Northwest

Annual PNW Precipitation (mm) Elevation (m) The Dalles

Winter Precipitation Summer Precipitation (mm)

Hydrologic Characteristics of PNW Rivers

Temperature warms, precipitation unaltered: Streamflow timing is altered Annual volume stays about the same Precipitation increases, temperature unaltered: Streamflow timing stays about the same Annual volume is altered Sensitivity of Snowmelt and Transient Rivers to Changes in Temperature and Precipitation

Characteristics of Flooding Events West and East of the Cascades

Coastal and Transient Snow Basins (West of the Cascades) Flooding frequently occurs in Nov-Dec when intense rain storms with temperatures above freezing are most likely. In so-called “Rain on Snow” events that produce severe flooding, the presence of snow is actually not the major driver. Instead, intense and sustained precipitation over enlarged basin areas (due to warm temperatures) with fully saturated soils produce the major component of the runoff in the largest events. In moderate flooding events, snow melt and precipitation tend to be more comparable in their contribution to peak streamflows and antecedent snowpack is more important.

ColdWarm Effective basin area contributing direct runoff to the river channel system increases in warm winter storm events. Skagit River Basin

Snow Melt Dominant Basins (East of the Cascades) Flooding mostly occurs in spring when snow melt peaks. Severe flooding can result from extraordinarily heavy snowpacks over large spatial areas (e.g. WY 1997), rapid snowmelt due to extremely warm or clear weather, or from a combination of sustained snow melt and heavy precipitation (e.g. the Vanport Flood in 1948). Moderate snowmelt floods can have much longer duration in comparison with flooding produced by individual rain storms. Note that huge snowpacks do not necessarily produce severe flooding in spring (e.g. WY 1999).

Effects of Land Cover on Flooding in the Pacific Northwest

Urbanization (increased impervious surfaces and removal of active soil storage during development) Altered streamflows: Increased magnitude and “flashiness” of peak flows More rapid recession and lower base flows in late summer Stream channel erosion and instability Capacity problems in storm water drainage systems Ecological problems due to erosion, scouring, or increased nutrient and sediment loadings

Source: Booth D.B., 2000, Forest Cover, Impervious-Surface Area, and the Mitigation of Urbanization Impacts in King County, WA Typical Effects of Urbanization on a Small Watershed Des Moines Creek

Effects of Logging and Road Networks Loss of forest canopy increases total snow accumulation Increased exposure to wind and solar radiation increases melt rates Road building and culverts alter natural drainage networks creating “pipes” to the stream channel which increase peak flows during moderate flooding events Loss of vegetation can produce larger sediment loads or trigger debris flows Effects of logging and road building are roughly additive.

Source: Storck, P., 2000, Trees, Snow and Flooding: An Investigation of Forest Canopy Effects on Snow Accumulation and Melt at the Plot and Watershed Scales in the Pacific Northwest, Water Resources Series Technical Report No. 161, Dept of CEE, University of Washington Effects of Forest Canopy on Snow Accumulation Loss of canopy increases the snow water equivalent and increases the rate of melt.

Sources: Storck, P., 2000, Trees, Snow and Flooding: An Investigation of Forest Canopy Effects on Snow Accumulation and Melt at the Plot and Watershed Scales in the Pacific Northwest, Water Resources Series Technical Report No. 161, Dept of CEE, University of Washington Bowling, L.C., P. Storck and D.P. Lettenmaier, 2000, Hydrologic effects of logging in Western Washington, United States, Water Resources Research, 36 (11), Modeling studies (Storck 2000) and comparative analysis of observations in paired catchments (Bowling et al. 2000) show that large scale clearcutting results in increased flood peaks on the order of 10% for small basins in the transient snow zone of the Cascades. Effects of Harvest Strategies on Magnitude of Flood Peaks

Bowling, L.C. and Lettenmaier, D.P., 1997, Evaluation of the Effects of Forest Roads on Streamflow in Hard and Ware Creeks, Washington, Water Resources Series Technical Report No. 155, Dept of CEE, University of Washington Bowling and Lettenmaier (1997) estimated that the 10-yr flood peak increased ~10% in two small transient snow basins due to road networks alone. Roads and logging together were estimated to increase the 10-yr flood peak on the order of 20% in the same two small transient snow basins. Effects of Roads Networks on Peak Flows

Effects of Climate Variability on Flooding in the Pacific Northwest

A history of the PDO warm cool warm A history of ENSO Pacific Decadal OscillationEl Niño Southern Oscillation

Effects of the PDO and ENSO on Columbia River Summer Streamflows Cool Warm PDO

Naturalized Summer Streamflow at The Dalles

Selection Criteria: Unregulated Streams Daily Flow Records Records Years Long Pacific Northwest Streamflow Records Selected for Flood Analysis

Data Processing Methods Determine mean annual flood for each basin Set threshold and reset value Determine number of peaks above threshold for each climate category Estimate probability of event above threshold for each basin and climate category

Effects of Climate Change on the Pacific Northwest

Four Delta Method Climate Change Scenarios for the PNW ~ C ~ C Somewhat wetter winters and perhaps somewhat dryer summers

ColSim Reservoir Model VIC Hydrology Model Changes in Mean Temperature and Precipitation or Bias Corrected Output from GCMs

Current Climate 2020s2040s Snow Water Equivalent (mm) VIC Simulations of April 1 Average Snow Water Equivalent for Composite Scenarios (average of four GCM scenarios) The main impact: less snow

Regulated Flow Historic Naturalized Flow Estimated Range of Naturalized Flow With 2040’s Warming Naturalized Flow for Historic and Global Warming Scenarios Compared to Effects of Regulation at 1990 Level Development

Effects to the Cedar River (Seattle Water Supply) for “Middle-of-the-Road” Scenarios

Observed Climate Change: Trends in Temperature, Precipitation, Snowpack, and Streamflow

Area-weighted Regional Avg=1.5 F/century

Annual Precipitation Trends From HCN stations

Relative Trends in April 1 Snow Water Equivalent Relative Trend %/yr Elevation (m)

Trends in Annual Streamflow at The Dalles from are strongly downward.

Some Conclusions Regarding Planning, Project Design Specifications, and Flood Forecasting

“Past Performance is not a Good Measure of Future Performance.” Estimates of flood probability distributions and design specifications (e.g. the “100 year” or “1% likelihood” flood) are a complex function of land surface characteristics, interannual and decadal scale climate variability, long-term climate variations (such as global warming), and water management policies, all of which are non-stationary in time. For convenience, estimates of flood design specifications have traditionally been based on fixed periods of the historic record. In the case of expensive or long-lived structures or for planning processes that should be robust to climate variability and climate change, the use of the historic record for flood estimation is problematic both because of relatively small sample size and changing conditions over time. Note that in the case non-stationary conditions over time, longer streamflow records do not necessarily improve estimates of flood frequencies.

Problems with Forecasting Applications Based on Statistical Relationships Many operational streamflow forecasting applications are currently based on statistical relationships between weather or climate forecasts, snowpack measurements, and streamflow. When land cover of the basin or climate conditions change, the skill of these forecasts can be impaired. Such problems cannot be resolved in the short term because there is no training data available for the altered conditions. These problems have serious implications both for short term flood forecasting applications and forecasts used for water management at seasonal time scales.

Current or Projected Land Surface Conditions Current or Projected Meteorological Data Hydrologic Model Updated or Projected Streamflow Time Series Design Criteria Forecasts Planning Scenarios Use of dynamic models can improve estimates of hydrologic design specifications and short-term and seasonal streamflow forecasts.