Monitoring Data for Willow Creek Lake, Nebraska, 2012–14—Nutrient Loading

Metadata Updated: October 10, 2019

A variety of nutrient sources transported by multiple flow paths contribute to the nutrient load entering Willow Creek Lake. In addition to Willow Creek, there are several smaller tributaries that contribute streamflow to the lake. As Spalding and others (1992) noted, groundwater interactions with the lake are likely another important contributor to nutrient loading. Nutrient load was computed as the product of volumetric flow rate and nutrient concentration and was applied to both streamflow and groundwater. A water balance approach was used to estimate the volumetric flow rates (hereinafter referred to as flow) for streams and atmospheric sources, with groundwater flow as the residual of that balance. Nutrient concentrations were determined from sampling data. Nutrient loading into and out of the lake was quantified using multiple techniques, all of which linked volumetric water flux along a flow path with nutrient concentrations. Surface-water inflows were estimated using the program LOADEST (Runkel and others, 2004). Using the streamflow record and sampling results, LOADEST builds a regression for nutrient loads based upon variables such as flow and time of year. Streamflow and nutrient concentrations were both log-transformed to ensure the statistical assumptions of the regression were met, including residual normality (Helsel and Hirsch, 2002). Regression equations were evaluated based upon statistical goodness of fit and avoided the use of quadratic relations to streamflow to prevent excessive extrapolation errors in the load estimates. The LOADEST model is described in Rus and others (2018). Groundwater loading was estimated by assigning a static nutrient concentration to the groundwater flow component estimated from the water balance. Groundwater transport was assumed to be limited to the soluble forms of nitrogen (nitrate plus nitrite) and phosphorus (phosphate). Nutrient concentrations assigned to positive groundwater flow (into the lake) were taken as the unweighted mean of samples from wells in the shallow aquifer. Nutrient concentrations assigned to the negative groundwater flow (out of the lake) were estimated from concentrations measured in the lake. For phosphate, the level was assumed to be zero out of simplicity, as very few lake samples were above the detection limit for phosphate. Nitrate plus nitrite, however, was more complex, with a clear seasonal trend in the lake showing highest concentrations in early spring and lowest concentrations in late summer. Consequently, nitrate plus nitrite in the lake was estimated as a seasonal function of the day of the year that was derived from sample data. Loads calculated at the North Tributary (NT) site had higher uncertainty than the other stream sites, presumably as a result of greater flashiness at the site relative to the other two. The load uncertainty at the NT site was particularly high with the sediment-associated nutrients of total Kjeldahl nitrogen, total phosphorus, and phosphate as phosphorus, with the average standard error being nearly equal to the average daily load. The greatest amounts of uncertainty were associated with the early spring periods, implying that the first flush of sediments from the ephemeral stream were dynamic and had a detrimental effect on the model performance.

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Dates

Metadata Date June 14, 2018
Metadata Created Date July 9, 2019
Metadata Updated Date October 10, 2019
Reference Date(s) January 1, 2018 (publication)
Frequency Of Update notPlanned
Frequency Of Update notPlanned

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Harvested from DOI CKAN Harvest Source

Additional Metadata

Resource Type Dataset
Metadata Date June 14, 2018
Metadata Created Date July 9, 2019
Metadata Updated Date October 10, 2019
Reference Date(s) January 1, 2018 (publication)
Responsible Party U.S. Geological Survey (Point of Contact)
Contact Email
Access Constraints Use Constraints: none, Access Constraints: none
Bbox East Long -97.499999999997
Bbox North Lat 42.4
Bbox South Lat 42.0
Bbox West Long -98.200000000002
Coupled Resource
Frequency Of Update notPlanned
Guid
Harvest Object Id 7fac27ee-6d1f-4813-bfe5-ab30bfc5ed73
Harvest Source Id 34ce571b-cb98-4e0b-979f-30f9ecc452c5
Harvest Source Title DOI CKAN Harvest Source
Licence Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data on any other system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty.
Metadata Language
Metadata Type geospatial
Progress completed
Spatial Data Service Type
Spatial Reference System
Spatial Harvester true
Temporal Extent Begin 2012-03-01
Temporal Extent End 2014-09-30

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