Estimates of Watershed-Scale Nitrate Fate In Agricultural Settings: An Example of National Water Quality Studies First Envisioned In 1985

In the fall of 1985, Jacob Rubin and his colleagues presented their vision for the future U.S. Geological Survey National Water Quality Assessment Program (NAWQA). Their strategy involved implementing an extensive monitoring program, supplemented by later studies focused on enhancing geographic assessment and process understanding. An outcome of this vision, the Agricultural Chemicals Team (ACT), initiated studies in 2001 to develop an understanding of the sources, transport, and fate of nutrients and pesticides in agricultural settings. A major goal of ACT has been to estimate the watershed-scale nitrate mass budget in multiple agricultural watersheds: Morgan Creek, MD; Leary Weber Ditch, IN; Maple Creek, NE; and Granger Drain, WA.

For each watershed, “excess nitrogen” was determined by subtracting estimates of crop uptake and volatilization from estimates of nitrogen input from atmospheric deposition, plant fixation, and fertilizer and manure applications. Surface-water discharge and concentration data were used to estimate watershed nitrogen export. End-member-mixing analysis was used to estimate the amount of nitrogen removed by streambed/in-stream reactions. Denitrification in the vadose/ground-water zones was assumed to equal excess nitrogen minus surface-water export minus streambed/in-stream removal. Nitrogen inputs ranged from a low of 124 kg N/ha (WA) to a high of 163 kg N/ha (NE). Excess nitrogen ranged from about 10% (IN) to 30% (MD) of nitrogen input. The fraction of excess exported in surface water was almost 100% in the heavily tile-drained IN watershed, and ranged from 30-50% in the other watersheds. For these watersheds, streambed/stream removal of excess nitrogen ranged from 12% (WA) to 48% (MD) and was dependent on ground-water inflow and streambed characteristics. Vadose/ground-water denitrification of excess nitrogen ranged from 12% (MD) to 44% (NE) and related to ground-water redox conditions and residence time. NAWQA/ACT results show that watershed-scale nitrogen fate is controlled by the interplay of hydrologic and geochemical conditions.