The implications of weather, nutrient prices, and other factors on nutrient concentrations in agricultural watersheds

This paper examines how nutrient prices, weather, and other factors influenced P outputs in agricultural watersheds using a detailed daily dataset of water quality observations over a 40-year period. Because policies have focused differentially on soluble P through federal permitting programs for point sources and sediments through federal subsidies for conservation, we examine sediment, particulate P and soluble P separately. A novel element of this study is the inclusion of farm fertilizer and output (i.e., corn) prices, which affect agricultural sources of P in these watersheds. We do not find that sediment concentrations are influenced by P prices, but sediment has trended downward, and is seasonally lower in all months except February and March in the Maumee. In contrast, we find that soluble P concentrations are heavily influenced by P prices. They trended downward through 1995, but upwards since. While concerns about fall and winter P application have emerged, we do not find evidence that the distribution of soluble P concentrations shifted towards winter over time. Weather accounts for about 50% of the higher soluble P loadings in 1996-2011, but higher P prices in 2005-2011 lowered P concentrations relative to what they would have been. Other factors account for the remaining 50% of the increase in soluble P concentrations in 1996-2011.