The primary objective of the current effort is to develop hydrologic and nutrient budgets for UKL for the 19- year period record (WY 1992-2010). The updated analysis utilizes substantial hydrologic and water quality data collected by various agencies in the lake, lake outflow, major tributaries, sub-watersheds, and agricultural areas adjacent to the lake, portions of which were restored to wetlands over the WY 1998-2010 period. Relatively minor refinements to WY 19921998 budgets are made to reflect adjustments to the historical hydrologic data, additional water quality data from pumped agricultural areas and restored wetlands, and an improved method for computing daily nutrient loads based upon daily flow and biweekly water quality measurements at the major monitoring sites.
One secondary objective is to evaluate time-series trends in flow, nutrient load, and concentrations in each inflow and in the overall lake nutrient budget terms (total inflow, outflow, storage, retention) to determine whether changes occurred over the WY 1992-2010 time period in response to management efforts or other factors.
Another secondary objective is to differentiate between background and anthropogenic nutrient sources based upon spatial variations in unit area export (load / drainage area) and flow-weightedmean concentration (FWM = load / flow volume) across watershed monitoring sites.
Specific tasks for the WY 1992-2010 analysis include:
- Compile all relevant data (nutrient concentration, flow, water level, precipitation, evaporation) for tributaries, pumped agricultural areas, lake, and lake outflow to develop monthly time series for flow, load, and FWM concentration at watershed sites and components of the overall lake nutrient balances (inflow, outflow, storage, net retention).
- Update the previous hydrologic and nutrient budgets for UKL using the above generated time-series data and summarize at monthly, seasonal, yearly, and multi-year time intervals.
- Provide tabular and graphical analyses of inter-annual and seasonal variations in TP and total nitrogen (TN) loading for inflow and outflow stations, and for storage and net retention of nutrients in the lake.
- Summarize seasonal and annual variations in concentrations of nutrient species (TP, SRP, TN, NH4-N and NO23-N) at major tributary monitoring sites.
- Perform trend analyses using both nonparametric (seasonal Kendall test) and parametric (linear regression) statistical techniques to evaluate time-series trajectories in the flow volumes, total nutrient loads, and FWM concentrations associated with each individual source and component of the lake mass balances over the 19-year period of record.
- Evaluate spatial variations in flow, total nutrient loads and FWM concentrations throughout the watershed. These variations reflect the relative contributions of natural background and anthropogenic sources. To the extent possible with the existing data, parse out sub-watershed loading and unit area export for various reaches of the Wood River, Sevenmile Creek, and Sprague River systems.
Detailed results of the mass balance computations are contained in Appendix E. The main goal of this effort is not to comprehensively analyze and interpret the budgets and trends, but rather to generate the information needed to support further analyses, interpretation, and modeling. Additional efforts would be needed to place results in the context of the specific management measures implemented during this period, assess the magnitude and effect of climatic trends on inflows and nutrient loads, and evaluate responses of the lake internal nutrient dynamics to variations in external nutrient loads and other controlling factors.
- Posted on:
- July 26, 2012
- 3 minute read, 535 words
- Water Resources Engineering
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