Citation:

Semmens, D J., W. R. Osterkamp, P. Guertin, AND D. C. Goodrich. SIMULATING SUB-DECADAL CHANNEL MORPHOLOGIC CHANGE IN EPHEMERAL STREAM NETWORKS. Presented at American Geophysical Union Conference, San Francisco, CA, December 13-17, 2004.

Impact/Purpose:

The primary objectives of this research are to:

Develop methodologies so that landscape indicator values generated from different sensors on different dates (but in the same areas) are comparable; differences in metric values result from landscape changes and not differences in the sensors;

Quantify relationships between landscape metrics generated from wall-to-wall spatial data and (1) specific parameters related to water resource conditions in different environmental settings across the US, including but not limited to nutrients, sediment, and benthic communities, and (2) multi-species habitat suitability;

Develop and validate multivariate models based on quantification studies;

Develop GIS/model assessment protocols and tools to characterize risk of nutrient and sediment TMDL exceedence;

Complete an initial draft (potentially web based) of a national landscape condition assessment.

This research directly supports long-term goals established in ORDs multiyear plans related to GPRA Goal 2 (Water) and GPRA Goal 4 (Healthy Communities and Ecosystems), although funding for this task comes from Goal 4. Relative to the GRPA Goal 2 multiyear plan, this research is intended to "provide tools to assess and diagnose impairment in aquatic systems and the sources of associated stressors." Relative to the Goal 4 Multiyear Plan this research is intended to (1) provide states and tribes with an ability to assess the condition of waterbodies in a scientifically defensible and representative way, while allowing for aggregation and assessment of trends at multiple scales, (2) assist Federal, State and Local managers in diagnosing the probable cause and forecasting future conditions in a scientifically defensible manner to protect and restore ecosystems, and (3) provide Federal, State and Local managers with a scientifically defensible way to assess current and future ecological conditions, and probable causes of impairments, and a way to evaluate alternative future management scenarios.

Description:

A distributed watershed model was modified to simulate cumulative channel morphologic
change from multiple runoff events in ephemeral stream networks. The model incorporates the general design of the event-based Kinematic Runoff and" Erosion Model (KINEROS), which describes the processes of interception, infiltration, surface runoff, and erosion from small watersheds predominantly characterized by overland flow; A geomorphic sub-model was added to compute geometric adjustment of the channel reaches in response to computed changes in sediment storage by minimizing the total stream power. The event-based watershed model framework permits exceptional temporal resolution of channel response to ephemeral water and sediment fluxes, including impacts resulting from distributed land-use/cover management. A GIS-based interface was developed to facilitate parameterization of the model, track cumulative geomorphic change and errors resulting from multiple distributed rainfall events. In addition the interface allows visualization of the spatial patterns of computed geomorphic adjustment, and permits comparisons of results from different simulations.

Results from multiple-event simulations indicate that morphologic adjustment within channel networks is predominantly controlled by the frequency and magnitude of rainfall events. In ephemeral stream networks transmission losses progressively decrease the transport capa.city of flows as they move downstream. Drier periods, characterized by small runoff events, thus produce aggradation and increasing width-to-depth ratios within the smallest headwater
channels. As rainfall and the number of large events increases, headwater channels begin to incise and sediment is progressively redistributed further downstream. Disturbance within a portion of the watershed can interfere with this response pattern, with significant alterations to channel morphology extending well downstream of the disturbance. Comparisons of simulated patterns of morphologic adjustment with and without localized urbanization demonstrate the anticipatedgeomorphic response of incision within the urbanized area and increasing width-to-depth ratios downstream.

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