Geomorphic Modeling and Routing Improvements for GIS-Based Watershed Assessment in Arid Regions

1. Geomorphic Modeling and Routing Improvements for GIS-Based Watershed Assessment in Arid Regions Darius J. Semmens Ph.D. Candidate, Watershed Management March 5, 2004

2. Acknowledgements • USDA-ARS Southwest Watershed Research Center • David Goodrich, Scott Miller, Carl Unkrich • USGS – Waite Osterkamp • U of AZ • Phil Guertin, Richard Hawkins, Vicente Lopes, Craig Wissler • U.S. EPA, Landscape Ecology Branch • Bill Kepner, Bruce Jones • Betsy Semmens

3. Introduction • Hydrologic and geomorphic systems are defined and linked by the movement of water on the Earth’s surface • Management and planning for land and water resources is facilitated by watershed models • Recent improvements to watershed models have been primarily focused on humid environments • Arid regions characterized by processes operating at different temporal and spatial scales, thus require specialized conceptual models • This research addresses two limitations of arid-region distributed watershed models that hinder their use as assessment and planning tools • Limitation of spatial scale (hydrologic model) • Inability to simulate geomorphic response to landscape change (geomorphic model)

4. Problem Statement – Hydrologic Model • Small-watershed models designed to simulate short-duration ephemeral flows • Performance declines when applied to areas larger than about 100 km2 • Large-watershed models simulate longer-term water balance • Performance declines when applied to areas smaller than about 1,000 km2 • Ephemeral runoff in medium-sized arid-region watersheds is best described by small-watershed models • Modifications are needed to improve the performance of small-watershed models at larger scales

5. Problem Statement – Geomorphic Model • To understand how an individual stream reach responds to external stresses it is necessary to study the channel network as a whole • Geomorphic watershed models are thus necessary to evaluate long-term (years) impacts of landscape change • Event-based watershed models simulate erosion and deposition based on assumption that channel geometry is static during the course of an event • Prevents simulation of cumulative impacts from multiple events • No event-based watershed models that track cumulative adjustment of the channel network in terms of channel width, depth, and slope.

6. Identifying the Scale Gap in Watershed Modeling Range of characteristic space – time scales BMP implementation Ecosystem restoration Urbanization Large WS Models (e.g. SWAT) Small WS Models (e.g. KINEROS2) Intermediate-Scale WS Models (This Research) From: Bloschl and Sivapalan (1995)

7. Study Area • USDA-ARS Walnut Gulch Experimental Watershed • Semi-arid rangeland • Desert scrub (brush) and grassland • ~150 km2 • Rainfall and runoff measured by a network of recording rain gauges, flumes, and weirs

8. Watershed 1 (WG1) 2 3 4 5 6 (WG6) 7 8 9 10 11 (WG11) 15 Area (km2) 148 112 9.42 2.29 22.1 93.6 13.6 14.8 23.9 15.8 7.85 23.7 nested subwatersheds and measuring devices LH104 (0.047 km2) Primary Watershed Areas

9. Hydrologic Model • Hypothesis • A significant source of error at intermediate scales results from the inability to account directly for diffusion of the flood wave as it is routed through the channel network • Approach • Implement Variable Parameter Muskingum-Cunge (VPMC) routing in KINEROS2 (Smith et al., 1995) • Compare with kinematic routing at multiple scales

10. Geomorphic Model • Hypothesis • A continuous-simulation, event-based geomorphic model describing channel width, depth and slope adjustments can predict reasonable geomorphic change in semi-arid watersheds • Approach • Implement channel-geometry adjustments in KINEROS2 based on total stream power minimization • Develop a GIS-based interface to facilitate model parameterization, multiple-event simulations, and results visualization • Evaluate generalized model behavior in absence of observed channel-geometry change • Sensitivity to initial channel geometry • Response to different precipitation records • Response to land-cover change

11. KINEROS2 Geomorphic Model (K2G) • Width and depth adjusted to minimize total stream power at end of each time step • Depth adjustments • Maximum erodible depth • Bank failure • Width adjustments • Compound channels Depth

12. AGWA-G • GIS-based interface for K2G, customized version of AGWA • Watershed delineation and discretization • Land cover and soils parameterization • Coordinates multiple consecutive simulations and tracks cumulative outputs • Results visualization • Differencing results from two simulations – relative assessment

13. Geomorphic Model Testing • Observed, distributed precipitation input • SSURGO Soils • Hydraulic-geometry and observed-geometry channels • Four land-cover scenarios • Compare results for 1964, 1977, & 1978 monsoon season on WG11 Discretization Elevation Soils Rain Gauges 1973 1997 All urban Part urban

14. Simulation Inputs Sediment grain-size distributions Land-cover scenarios Precipitation record characteristics for the 1964, 1977 and 1978 monsoon seasons

15. Results 1964 • Hydraulic-geometry channels • 1997 land cover • Wet (top), intermediate (middle), and dry (bottom) year simulation results • Depth changes mapped on the left, width changes mapped on the right • Erosion during wet year, and deposition during dry year Decreasing Precipitation 1977 1978

16. Results 1964 • Hydraulic-geometry channels • Partially urbanized land cover • Differences from 1997 land cover not obvious • Less erosion within, and more deposition and downstream of urbanized tributary Decreasing Precipitation 1977 1978

17. Results 1964 • Runoff depth (mm) per unit contributing area • Runoff highest as flows coalesce in the headwaters, then decreases in the downstream direction because of channel infiltration • Significant decreases occur further upstream for drier years • Deposition occurs downstream of transition Decreasing Precipitation 1977 1978

18. Results 1964 • Observed-geometry channels • 1997 land cover • Wet (top), intermediate (middle), and dry (bottom) year simulation results • Depth changes mapped on the left, width changes mapped on the right • Reach adjustments more spatially varied with observed channels Decreasing Precipitation 1977 1978

19. Results 1964 • Observed-geometry channels • Partially urbanized land cover • Reach adjustments more spatially varied with observed channels • Can see preferential change on southern tributary Decreasing Precipitation 1977 1978

20. Mass-Balance Error • Mass-balance error for modeled change in sediment storage (red) and equivalent mass of geometric adjustment (blue) for entire channel network • Modeled cumulative magnitude of deposition/erosion (burgundy), and equivalent mass of geometric adjustment (cream) • Geometric adjustments conserve mass reasonably well for hydraulic-geometry simulations, but not for the observed-geometry simulations Mass-balance error (%) Magnitude of deposition/erosion (kg)

21. Relative Assessment • Error in watershed modeling is substantial • Even carefully calibrated models yield poor results when applied to events significantly larger or smaller than those used in the calibration • Geomorphic model is thus most useful for evaluating where in the watershed change is likely to be most significant • Assuming the basic processes are represented accurately, and error is spatially uniform, it can be largely removed through differencing simulation results • Relative assessment can thus identify general patterns of response to landscape change, even if the specific magnitude of that change is not correct

22. Results – Relative Assessment • Hydraulic-geometry channels • Difference in computed depth (left) and width (right) changes between PU and 97 simulation results for wet (top) and intermediate monsoon seasons • Significant differences concentrated on urbanized tributary • Erosion increases within urbanized area more pronounced for wet year • Reduced erosion or increased deposition begins further upstream during drier year • Aggradation downstream characterized by depth decreases and width increases 1964 Decreasing Precipitation 1977 Difference in depth changes Difference in width changes

23. Results – Relative Assessment • Observed-geometry channels • Magnitude of differences is different from the hydraulic geometry simulations • Pattern of adjustment very similar to that for the hydraulic-geometry channels – erosion in urbanized area and deposition downstream • Suggests that channel slope and discharge are the most important parameters governing channel response 1964 Decreasing Precipitation 1977 Difference in depth changes Difference in width changes

24. Hydraulic-Geometry Channels Observed-Geometry Channels Results • Wet monsoon cumulative runoff (mm), infiltration (m3/km), and sediment yield (kg/ha) • Runoff increases from urbanization decrease in downstream direction • Infiltration increases in downstream direction • Sediment yield increases from urbanization increase in downstream direction • Spatial patterns very similar for both hydraulic and observed geometries • Increased deposition downstream where stream power decreases Runoff Runoff Infiltration Infiltration Sed. Yield Sed. Yield

25. Hydraulic-Geometry Channels Observed-Geometry Channels Results • Intermediate monsoon cumulative runoff (mm), infiltration (m3/km), and sediment yield (kg/ha) • Runoff increase from urbanization dissipates more rapidly in downstream direction • Infiltration increase peaks further upstream • Sediment yield increase peaks further upstream • Locus of deposition shifts upstream • Spatial patterns very similar for both hydraulic and observed geometries Runoff Runoff Infiltration Infiltration Sed. Yield Sed. Yield

26. Conclusions – Geomorphic Model Individual Batch Simulations • Network-wide mass conservation is reasonable when hydraulic-geometry channels are used, but needs work for more variable observed-geometry channels • Erosion is most widespread during the wettest year, erosion and deposition mixed during intermediate year, and most widespread deposition for driest year • Specific channel adjustments sensitive to initial channel geometry – more uniform for hydraulic-geometry channels

27. Conclusions – Geomorphic Model Relative Assessment • Results of the scenario-output differencing show the concentration of impacts within and downstream of the urbanized area, and no significant changes in the unaffected areas • Geomorphic impacts of urbanization varied with the number and magnitude of precipitation events, but the general response was erosion in the urbanized area and deposition downstream • Spatial pattern of geomorphic response closely linked to changes in cumulative runoff and channel infiltration • Spatial pattern of geomorphic response relatively insensitive to initial channel geometry, suggesting that a suitable hydraulic-geometry relation may be sufficient for broad-scale application of the model

28. Future Research – Geomorphic Model • Model validation – need to demonstrate that simulated geomorphic adjustments are representative of observed adjustments • Increase upper watershed size limit for K2G • Diffusion-wave routing • Discritization of channel • Link simulated geomorphic change and channel stability, or vulnerability to degradation • Evaluate model behavior over broader range of precipitation records, and over longer periods of time • Evaluate model response to major disturbance, and whether response is persistent or transitive • Link simulated geomorphic change and riparian condition