Predicting the hydrologic implications of land use change in forested catchments Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington Chapman Conference on Ecosystem Interactions with Land Use Change June 14, 2003 Santa Fe, New Mexico

Outline of this talk Background – the signature of land use change Example 1 – Logging and flooding in the Pacific Northwest Example 2 –Hydrologic effects of vegetation change in the upper Midwest Some outstanding issues in prediction of hydrologic effects of land cover change

1) Continental and regional signatures of land cover change

Source: National Institute of Public Health and the Environment (RIVM, Netherlands) and the Center for Sustainability and the Global Environment (SAGE, University of Wisconsin). Estimated 1850 and 1990 global land cover

Early Conifer Middle Conifer Late Conifer Early Deciduous Middle Deciduous Late Deciduous Brush Agriculture Water Historical (1900) Current (1990) Columbia River basin estimated 1900 and 1990 vegetation cover (from ICBEMP)

2) Example 1: Logging and flooding in the Pacific Northwest

Assessment approach – spatially distributed hydrologic modeling

Mechanisms for hydrologic change Rain-on-snow runoff generation Channel manipulation via forest roads Water table (hence saturated area changes) via altered evaporative demand Combinations of above

Investigation of forest canopy effects on snow accumulation and melt Measurement of Canopy Processes via two 25 m 2 weighing lysimeters (shown here) and additional lysimeters in an adjacent clear-cut. Direct measurement of snow interception

SWE difference for February 1996 ROS event; harvest - no harvest More snow at beginning of event Less snow at end of event

Simulated response to forest harvest Sub-basins of the Deschutes River, WA

Sources of road-derived runoff

Surface routes for road runoff

Effect of forest roads on water table Drier with roads Wetter with roads

Simulated streamflow w/ and w/o forest roads Hard Creek Ware CreekHard Creek Ware Creek

Bottom line: Both vegetation removal and roads contribute to increased peak runoff Effects more or less superimpose For the Deschutes basin, each effect represents about a 10% increase in the ~10 yr flood Relative magnitude of vegetation effect decreases with return period, road effect increases

Sediment Modeling with the DHSVM Watershed Sediment Module DHSVM MASS WASTING SURFACE EROSION CHANNEL EROSION Watershed Sediment Module OUTPUT Q Q sed

Portraying Watershed Change Sediment Model Wildfire in the Icicle Creek basin

Mass Wasting Module MASS WASTING Multiple realizations of total failure locations Multiple time series of sediment supply Soil depth DEM Soil type Vegetation type P(F) Soil cohesion Root cohesion Veg. Surcharge Friction angle

Surface Erosion Module Multiple time series of sediment supply Overland flow SURFACE EROSION Roads and streams Soil Precipitation Vegetation DEM Distribution of sediment delivery to channels (roads and streams)

Channel Erosion Module CHANNEL EROSION Channel flow Mean and standard deviation of sediment load for selected channel reaches Distribution of sediment delivery to channels (roads and streams)

Probability of slope failure before and after Fourth of July Fire Icicle Creek Vegetation Pre-fire Post-fire Approximate extent of August 2001 fire

3) Example 2 –Hydrologic effects of vegetation change in the upper Midwest

Regional Land Use Change Presettlement Land UseModern Land Use

Land Use Changes: Change in Forest Cover

Variable Infiltration Capacity (VIC) Macroscale Hydrologic Model Full Energy Balance Full Water Balance Mosaic Vegetation Cover Variable Infiltration Curve Generates Runoff Arno Baseflow Curve

Evaporation Changes Differences (mm) Evaporation (mm) Presettlement Land UseModern Land Use

Snow Cover Changes Differences (mm) Snow Water Equivalence (mm) Presettlement Land UseModern Land Use

Calibrated Flow Comparison Discharge was calibrated using: –Modern land use types –Discharge Observations from Plots compare discharge for the first 5 simulation years ( ) Discharge also generated for presettlement land use using the same parameters

Extreme Flow Comparison Annual peak and low flow events for Water Years Compares simulated flow with presettlement and modern land use Both peaks and low flows are greater with modern land use

Cumulative Flow Comparison Cumulative discharge from 1951 to 1995 Decreased evaporation from smaller forested areas yields more runoff with modern land use

4) Some outstanding issues in prediction of hydrologic effects of land cover change

1. The calibration problem 1965 Flood Hydrograph (Flood of Record) 1969 Flood Hydrograph Mississippi River at Anoka, MN

2. The model complexity problem Effect of wildfire on simulated root cohesion