Overview: What is wrong with simple statistical regressions of hydrologic response on impervious area? Toward a more complete understanding of normal flows. Distributed Hydrological Modeling Example from applications of Distributed Hydrological Modeling at UW Changes in impervious area. Changes in forest cover. Global Climate Change.
Percent Impervious Area 0 100% 0 1 Runoff Coefficient Typical Representation of Effect of Impervious Area on Runoff Coefficient
What we really want to know is: What is the change from normal? Previous graph is 100% correct for dry initial conditions. What if it has just rained nonstop for five days... Well that never happens around here?
Percent Impervious Area 0 1 0 1 Runoff Coefficient Representation of Effect of Impervious Area on Runoff Coefficient for extremely wet initial conditions
Therefore, normal response depends: Static Variables: Land Cover Impervious Area, etc Dynamic Variables: Soil Moisture Precipitation Intensity Storm Duration, etc. Numerous Studies have shown decreased effects of land use Change as antecedent conditions become wetter. Our task is to build a predictive model of what is normal… And that can’t be done without considering interaction of meteorology with land cover changes
Terrain - 150 m. aggregated from 10 m. resolution DEM Land Cover - 19 classes aggregated from over 200 GAP classes Soils - 3 layers aggregated from 13 layers (31 different classes); variable soil depth from 1-3 meters Stream Network - based on 0.25 km 2 source area Land surface characterization required by DHSVM
Calibration Location (Snoqualmie) Testing: Cedar Calibration to two USGS sites Split sample validation at over 60 sites Parameters transfer extremely well to other watersheds without recalibration
Application of DHSVM to lower Cedar River Watershed to assess impacts of changes in impervious area on basin hydrology Fraction Impervious Area (1998) 100 % 75 % 50 % 25 % 0 % Taylor Creek (14 km 2 ) 5% imperv. Madsen Creek (5.4 km 2 ) 20% imperv.
DHSVM Calibration to determine baseline parameters. Taylor Creek (5% impervious area) Feb 1991 Mar 1991 Apr 1991 May 1991 CFS Test of Impervious Area Representation (no re-calibration) Madsen Creek (20% impervious area) 4/1/91 4/4/91 4/7/91 4/10/91 CFS 0 20 40 60 80 100 120
cfs 4/5/1991 4/15/1991 4/20/1991 Old Growth Forest: 58 cfs peak, 2.3 inches total runoff 100 % increase in peak 1991 Land Cover (20 % imperv.): 115 cfs peak, 3.2 inches total runoff Observed (1991): 120 cfs peak, 3.6 inches total runoff
Predicted Change in Mean Monthly Temperature due to Increased Carbon Dioxide Levels (Mean of 4 GCM’s) Temperature Increase (degrees C) 2020’s (w.r.t mid 20th century climate) 2040’s (w.r.t mid 20th century climate) 2020’s Mean Winter Increase = 1.6 C 2040’s Mean Winter Increase = 2.4 C
Methodology for Assessing Impacts of Climate Change on Watershed hydrology Observed Meteorology At Stations in and near Target Watershed Synthetic “Observed” Record: Talt=Tobs + Delta T Palt = Pobs*(Delta P) Change In Temperature Change in Precipitation DHSVM SWE Reservoir Inflow
Snow Water Equivalent (mm) Cedar River Watershed: Retrospective Analysis of Average Snow Water Equivalent Under Current and Altered Climates Current 2025 2045 Drought Current Low Year Becomes... 2025/2045 Best Case
Effect of Climate Change on Mean Monthly Inflow (1988 to 1996) to Cedar Reservoir Month Monthly Inflow (meters) Higher Winter Flows: Increased Precipitation Higher Freezing Level -> More Rain 126,000 acre-ft 90,000 acre-ft 78,000 acre-ft Snowmelt Current 2025 2045
Basins for which streamflow was simulated for each vegetation scenario. GAP, 1991 is based on a 1991 LandSat image. Band Harvest has a total clear-cut area identical to GAP, 1991 but concentrated in the transient snow zone (700- 900 m). The control simulation is the historic vegetation coverage (based on GAP with all clear-cuts regrown). GAP, 1991 Band harvestHistoric Vegetation
Hourly Precipitation (mm) Low elevation(<300 m) snow (mm SWE) Effect of forest canopy removal, Snoqualmie River at Snoqualmie Falls, February 1996 event