TR-543 Texas Coastal Nutrient Input Repository - Task 4 Report Statistical Models for Nutrient Loading into Lavaca Bay
Authors: Michael Schramm
We quantified daily, monthly, and annual nutrient loading from freshwater segments in the Lavaca Bay watershed using a statistical modeling approach relating time and discharge to nutrient concentrations. Total combined annual nitrate loads ranged from 12,574 kg in 2011 to 794,510 kg in 2007 with a mean annual load of 205,405 kg per year from 2005 through 2020. Total combined annual total phosphorus loads ranges from 7,839 kg in 2011 to 916,908 kg in 2004 with a mean annual load of 241,681 kg per year from 2000 through 2020. Model validation indicated the approach performed well for predicting nutrient loads at the two most downstream sites in the watershed (Lavaca River near Edna, USGS-08164000 and Lake Texana at Palmetto Bend Dam, USGS-0816425). Additional flow-biased monitoring would improve model performance and validation during important high flow events that are the primary drivers of loading to Lavaca Bay. Model performance suffered at upstream sites to due higher variance in the measured data and insufficient sampling.