About the Sediment & Nutrient Assessment Program
The Susquehanna River Basin Commission (www.srbc.net) implemented a five-year nutrient-monitoring program in October 1984 to establish a database for estimating nutrient and suspended sediment loads in the Susquehanna River Basin. This monitoring effort, conducted as part of the Chesapeake Bay Restoration Program, consisted of monthly base flow sampling and periodic sampling throughout the high flow hydrograph for a minimum of five storms per year.
Initially, 12 sampling sites were established. This sampling network included a series of mainstem and major tributary sites, and a series of sites located on smaller watersheds that had significant areas of specific land use, or representative combinations of land uses. Collection of data at stations on the mainstem and major tributaries was necessary to enable accurate allocation of nutrient and suspended sediment loads to the main river reaches and to major subbasins. Data were collected for the Susquehanna River at Danville and Harrisburg, the West Branch Susquehanna River at Lewisburg, the Juniata River at Newport, the Swatara Creek near Hershey, the West Conewago Creek near Manchester, and the Conestoga River at Conestoga, Pennsylvania, to confirm the Chesapeake Bay Program watershed model load allocations and provide the basis for refining the model outputs for these areas. Each site represented large areas having significant differences and levels of complexity in terms of geological setting and combinations of land uses.
The stations located on relatively small watersheds of varying geologic settings provided a range of land uses or representative combinations of land uses. The sites represent: (1) a total wilderness area--Stony Creek at Water Tank Trail near Dauphin, Pa.; (2) a sparsely populated forest/cropland watershed with no areas of concentrated residential development--Sherman Creek at Shermans Dale, Pa.; (3) a small watershed of intense suburban development throughout its headwaters area--Paxton Creek near Penbrook, Pa.; (4) a complex area that drains extensive cropland with a number of fairly large reservoirs--Codorus Creek near York, Pa.; and (5) a suburban and urban development--Codorus Creek at Pleasureville. These watersheds were considered to be representative of many areas throughout the basin and would provide detailed information for: (1) refining the Bay watershed model; (2) identifying problems related to specific land uses and combinations of land uses; and (3) identifying future management actions in the area.
In 1987, an additional site was added on the Susquehanna River at Marietta, Pa., to provide a better estimate of the nutrient and suspended sediment loads transported by the Susquehanna River prior to entering the hydropower reservoirs in the lower river. In 1989, another sampling site was established on the Susquehanna River at Towanda, Pa., to provide an estimate of nutrient and suspended sediment loads from New York State.
The initial five-year program was concluded at the end of December 1989, and five of the 12 original sites were selected for continued long-term monitoring. The Susquehanna River at Towanda, Danville, and Marietta, and the West Branch Susquehanna River at Lewisburg were selected to provide long-term data to evaluate trends in nutrient and suspended sediment transport from the major subbasin. The fifth site, the Conestoga River at Conestoga, was selected to provide long-term data from a major tributary watershed with intensive agricultural activity and increasing development. Implementation of Best Management Practices is also being actively pursued. In October 1993, a sixth site, the Juniata River at Newport was re-established for long-term monitoring.
In October of 2004, 13 additional sites were added to the monitoring network as part of the Chesapeake Bay Program’s Non-tidal Monitoring Network. Additional sites were added in 2005 and 2012. This effort was conducted by the CBP Non-Tidal Water Quality Workgroup based on the following objectives:
- To measure and assess the actual nutrient and sediment concentration and load reductions in the tributary strategy basins across the watershed.
- To improve calibration and verification of partners’ watershed models.
- To help assess the factors affecting nutrient and sediment distributions and trends.
These new sites were chosen with the following priorities:
- Outlets of major streams draining the tributary strategy basins.
- Areas within the tributary strategy basins that have the highest nutrient delivery to the bay and tidal tributaries.
- Represent the overall range of conditions in the Bay watershed. This would include range of loads from different land cover types (urban, agriculture, and forestland covers), physiographic/geologic setting, and range of watershed sizes.
Loads & Yields
Loads (aka Flux) and yields represent two methods for describing nutrient and suspended sediment amounts within a basin. Loads refer to the actual amount of the constituent being transported in the water column past a given point over a specific duration of time and are expressed in pounds (or kilograms). Yields compare the transported load with the acreage of the watershed and are expressed in lbs/acre. This allows for easy watershed comparisons.
This project reports loads and yields of total and dissolved nitrogen, organic nitrogen, ammonia nitrogen, nitrate + nitrite nitrogen, and phosphorus, total organic carbon, suspended sediment, and inorganic phosphorus. Prior to 2014, load estimation was conducted using the Minimum Variance Unbiased Estimator (MVUE or ESTIMATOR). Beginning in 2014, load estimation was conducted using the USGS developed model, Weighted Regression on Time, Discharge, and Season (WRTDS) due to better accuracy of load estimation. A description of this model can be found at Weighted Regression on Time, Discharge, and Season (WRTDS).
The Susquehanna River Basin Commission estimates nutrient, suspended sediment, and flow trends as part of the Chesapeake Bay Program's annual trend update. A substantial challenge to understanding changes in load, and concentration is whether management outcome effects can be separated from the plethora of factors that influence nutrient and sediment dynamics. The most significant factor affecting concentrations and loads is rainfall and its effect on flow. Trends in flow indicate changes in hydrology that have occurred. Changes in flow, the sources of flow (base flow and overland runoff) and the management of flow, affect the observed concentrations and the estimated loads of nutrients and suspended sediment. Several procedures have been developed to remove the effects of flow so that the resulting trend represents the effects of management action and not variations in precipitation.
Prior to 2014, trend estimation was conducted using the Minimum Variance Unbiased Estimator (MVUE or ESTIMATOR). Beginning in 2014, trend estimation was conducted using the USGS developed model, Weighted Regression on Time, Discharge, and Season (WRTDS). Both models utilize a specific methodology to minimize the effects of flow such that the resultant trend represents changes in water quality that are due to management actions as opposed to year-to-year variations of flow. WRTDS trends were reported in Flow Normalized Flux (Load) and Flow Normalized Concentrations. Advantages of WRTDS include a more robust flow adjustment methodology as well as an ability to identify non-linear trends that likely exist within larger datasets.