Predicting the hydrologic implications of land use change in forested catchments

1. 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

2. 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

3. 1) Continental and regional signatures of land cover change

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

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

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

9. Assessment approach – spatially distributed hydrologic modeling

13. 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

14. Investigation of forest canopy effects on snow accumulation and melt Measurement of Canopy Processes via two 25 m2 weighing lysimeters (shown here) and additional lysimeters in an adjacent clear-cut. Direct measurement of snow interception

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

17. Simulated response to forest harvestSub-basins of the Deschutes River, WA

18. Sources of road-derived runoff

19. Surface routes for road runoff

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

21. Simulated streamflow w/ and w/o forest roads Hard Creek Ware Creek Hard Creek Ware Creek

22. 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

23. DHSVM MASS WASTING SURFACE EROSION CHANNEL EROSION Qsed Q OUTPUT Sediment Modeling with the DHSVM Watershed Sediment Module Watershed Sediment Module

24. Portraying Watershed ChangeSediment Model Wildfire in the Icicle Creek basin

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

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

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

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

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

30. Regional Land Use Change Presettlement Land Use Modern Land Use

31. Land Use Changes:Change in Forest Cover

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

33. Evaporation Changes Presettlement Land Use Modern Land Use Evaporation (mm) Differences (mm)

34. Snow Cover Changes Presettlement Land Use Modern Land Use Snow Water Equivalence (mm) Differences (mm)

35. Calibrated Flow Comparison • Discharge was calibrated using: • Modern land use types • Discharge Observations from 1980-1989 • Plots compare discharge for the first 5 simulation years (1951-1955) • Discharge also generated for presettlement land use using the same parameters

36. Extreme Flow Comparison • Annual peak and low flow events for Water Years 1951-1995 • Compares simulated flow with presettlement and modern land use • Both peaks and low flows are greater with modern land use

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

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

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

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