Phosphorus Flows and Balances for the Lake Mendota and Yahara River Watersheds: 1992-2017

This study presents an analysis of watershed-level phosphorus (P) mass balances from 1992 to 2017 for the Lake Mendota Watershed (LMW) and Yahara River Watershed (YRW) in south-central Wisconsin. P accumulation rates declined substantially in both watersheds over the time period (from 0.83 million kg-P/year to 0.13 million kg-P/year – an 85% decline – in the LMW and from 1.2 million kg-P/year to 0.15 million kg-P/year – an 88% decline – in the YRW). While declines in agricultural fertilizer P imports were the largest drivers of accumulation rates declining, increases in exports of livestock and manure digester products were also important in the LMW and increases in net crop export and exports of livestock products were also important in the YRW.