Thanos Papanicolaou
IIHR - Hydroscience & Engineering, The University of Iowa

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Application Note: Watershed Studies

Watershed dynamics: Many watershed simulations today work in the batch world; an event is simulated based on a static set of field data. If newer data become available, the simulation is simply rerun. For example, hydrodynamic and sediment transport simulations to predict geomorphologic changes within a stream and the impact of these changes to the aquatic life are conducted by considering a constant sediment input value from terrestrial sources such as roads, floodplains, and other natural occurring disturbances (i.e., landslides, fires). As a result perturbations that exist in the system due to the spatial and temporal variability in the terrestrial sediment input are not accounted. Very few applications use real time data even if the capability to do so is available. A great effort has been recently devoted to run simulations faster than real time based on static data sets. However, this is highly inefficient and leads to multiple sediment predictions that are conflicting when major events are predicted. This lack of ability to dynamically inject data into simulations and other applications, as these applications execute, limits the analysis and the predictive capabilities of these applications. The novel capabilities to be sought here are application simulations that can dynamically accept and respond to on-line field data and measurements and/or control such measurements. This synergistic and symbiotic feedback control-loop between simulations and measurements is a novel technical direction that can open domains in the capabilities of simulations within watersheds that can facilitate the “capturing” of episodic catastrophic events.

*Biogeochemical Fingerprinting and Isotopes:* Contaminated sediment is the leading cause of stream impairment in the United States, yet a robust method to identify and quantify watershed sediment sources still has not been developed. Development of biogeochemical fingerprinting methods such as nitrogen (N) and carbon (C) values and C/N ratios enabled radical improvement in sediment and pollutant load predictions. It provided means to evaluate Best Management Practices and other policy decisions involved with water quality management plans under the TMDL process. As landscapes are stripped of valuable, nutrient rich topsoils and streams are clouded with habitat degrading fine sediment, it becomes increasingly important to identify and mitigate erosive surfaces. Particle tracking using carbon (C) and nitrogen (N) isotopic signatures and carbon/nitrogen (C/N) ratios offer a promising technique to identify such problematic sources. The stable isotope C-13 reflects the signature of the parent vegetation at different spatial scales, whereas the stable isotope N-15 reflects the temporal variations in the organic content of soils, primarily due to volatilization and organic decomposition. The ratio C/N ratio is reflective of plants and microorganisms that have inhabited the soil at different spatial scales. A study was conducted to estimate sediment source and erosion rates within a watershed using this isotopic technology coupled with mineralogy fingerprinting techniques, radionuclide transport monitoring, and erosion-transport models. The study was extended to cover field studies of upland erosion processes, such as, solifluction, mass wasting, creep, fluvial erosion, and vegetative induced erosion. Upland and floodplain sediment profiles and riverine suspended sediment were sampled in the upper Palouse River watershed of Northern Idaho. Over 300 samples were obtained from deep intermountain valley (i.e. forest) and rolling crop field (i.e. agriculture) locations. Analysis of the results indicated distinct N isotopic signatures and C/N ratios for forest and agriculture sediment sources. In addition, unique C and N isotopic signature and C/N ratios exist within floodplain and upland surfaces, and within the 10 centimeter profiles of erosion and deposition locations. C and N isotopic signature and C/N ratio were dependent upon land use and soil moisture conditions, and served as a useful technique in quantifying erosive source rates and understanding upland erosion processes.
Studies have shown that certain pedologic and climatic factors related to C and N biogeochemical cycles could possibly alter the isotopic signature of soils. Although variability C-13, N-15 and C/N fingerprints due to these factors may ultimately be an advantage for developing a field-based method that enables short-term erosion predictions, knowledge of the significance of these factors and the degree of variability that they introduce to C-13, N-15 and C/N is imperative. A study was conducted to address sensitivity issues of the proposed fingerprint techniques for different spatial and temporal scales. The study was performed by excavating 294 original soil samples from the Upper Palouse Watershed, Northwestern Idaho, and analyzing the soil samples for C-13, N-15 and C/N using isotope ratio mass spectrometry. Statistical methods were used to test the significance of these factors on C-13, N-15 and C/N under different scales. This unique approach facilitated the development of a composite tool that for the first time incorporated both statistically verified isotopic signatures of soils and a mixing optimization model to identify of the soil provenance. Results indicate that C-13, N-15 and C/N were statistically different among forest vs. agriculture soils and show greater variability within forest soils as compared to agriculture soils. Plot-location and soil-pit within plot induced random variability for both agriculture and forest soils. Slope-location was a significant factor in the agriculture land-use but not the forest. In both land-uses C-13 and N-15 exhibited an increase andexhibited a decrease when excavating deeper in the soil-profile and for the smaller size-class; thus agreeing with previous soil organic matter studies. Discernible data trends were not apparent across multiple seasons.