Nutrient concentrations for various forms of N, P, C, as well as suspended sediments, are determined from monthly grab samples taken at the Ipswich and Parker dams. These nutrient concentrations are then used in conjunction with USGS discharge data (recorded at gages in the Parker River at Byfield, MA and the Ipswich River at Ipswich, MA) to calculate annual nutrient loading to the Estuary, coming over each dam. Annual yield is also calculated for both dams.
Grab samples are collected monthly at the Parker and Ipswich dams. Water is filtered and/or partitioned for analysis for suspended solids, particulate organic nitrogen (PON), total dissolved nitrogen (TDN), ammonium (NH4), nitrate (NO3), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), particulate phosphorus (PPO4), total dissolved phosphorus (TDP), phosphate (PO4), particulate organic carbon (POC), dissolved organic carbon (DOC). Analyses are conducted according to PIE-LTER protocols (http://pie-lter.ecosystems.mbl.edu/content/analytical-methods).
Discharge data is mined from the USGS website (http://waterdata.usgs.gov/nwis) for the Parker River gage at Byfield, MA and the Ipswich River gage at Ipswich, MA. Since the gages are NOT located directly at the dams where nutrient sampling occurs, the USGS discharge data is multiplied by a correction factor (1.1812 for the Parker data, and 1.1964 for the Ipswich data) to obtain discharge over the respective dams.
Monthly nutrient concentrations are multiplied by total monthly discharge over the dams to estimate nutrient loading on a monthly timescale. The sum of discharge approximately 15 days before and after the nutrient concentrations is used. Monthly loading numbers are added together to obtain annual loading per water year. Water years are determined according to USGS data updates, and generally start in September. The numerical year given to a water year is the new year reached in January during the September-September cycle (i.e. Sept 2003 - Sept 2004 is water year 2004). Yield calculations are derived from the loading calculations by dividing the nutrient loading by watershed area upstream of the dam (at Parker this is 65.2km2, and Ipswich this is 387.4km2). Total N and P loads and yields reflects organic and inorganic forms of nitrogen. Total C loads and yields reflect only organic forms of carbon, ie dissolved inorganic carbon was not measured and is not included.
Sample yield calculation:
1 tonne = 1000kg 1km^2 = 100 hectares
Ipswich Dam, Nitrogen Yield,
1994 126 tonnes x (1000 kg/tonne) = 126,000 kg
387.4 km^2 x (100 hectares/km^2) = 38,740 hectares
126,000 kg / 38,740 hectares = 3.25 kg / hectare / year
Nutrient samples continue to be taken monthly.
Monthly nutrient concentrations are updated annually in WAT-VA-Inputs.
Every 2-3 years, WAT-VA-Load is updated with loading calculated from concentrations in WAT-VA-Inputs.
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