Hydrologic flowpath and other natural and anthropogenic factors controlling nitrogen movement from the landscape to streams

Abstract

The growth of food, fuel, feed, and fiber crops on agricultural land requires additions of nitrogen fertilizer to bolster crop yields. The portion of applied nitrogen that is not utilized by the crops is highly susceptible to transport to nearby streams. Elevated nitrogen concentrations in streams can lead to water quality concerns such as ecosystem degradation or drinking water contamination. Identification of geospatial, environmental, and watershed characteristics (variables) that are correlated with nitrogen concentration in streams will provide a greater understanding of the influence that certain variables have on nitrogen transport to streams. The route through which water travels from the landscape to streams (flowpath) is one of these variables. Movement of many forms of nitrogen is linked to the movement of water, therefore, understanding the fluxes of water to streams will help to expand the understanding of nitrogen transport and the affect that landscape management changes will have on the concentrations of nitrogen in streams. In some areas, groundwater is an important flowpath for delivering water and nitrogen to streams. Hydrograph separation can be used to estimate the amount of total streamflow that is attributable to slowflow sources (flowpaths through which water moves slowly) such as groundwater, and fastflow sources (flowpaths through which water moves quickly) such as overland flow. Because flowpaths have an impact on water and nitrogen transport to streams, testing and improving hydrograph separation techniques is needed. Two independent methods of hydrograph separation, the graphical-based BFI program and chemical tracer-based end-member mixing analysis (EMMA), were used to estimate slowflow contributions to the same streams. The estimates of slowflow from the two separate methods of hydrograph separation were not identical, highlighting the differences in how each method works and the difficulty of accurately estimating slowflow. A modified method of EMMA, referred to as a ratio-based EMMA, was created and tested using synthetic and real stream data. The ratio-based EMMA represents a new method of hydrograph separation, as it produced reasonably accurate slowflow estimates when tested against synthetic and real stream data. The importance of flowpath was then tested in six highly modified streams in agricultural watersheds that had extensive data sets and well understood hydrology. The importance of flowpath on stream nitrogen concentrations in these streams aligned with expectations based on what is currently known. Finally, this study was expanded to a large number of small streams where statistical methods were used to gain broader understanding of the controls on water and nitrogen movement to streams, and to allow for extrapolation of this information to unstudied streams. In addition to flowpath, a small number of geospatial, environmental, and watershed variables were shown to be important for estimating total nitrogen loads and concentrations in a large number of small streams. These variables were used in the development of several multiple linear regression models, many of which performed well when applied to a set of validation streams, having reasonable high R2 values and low normalized root mean squared errors. Determination of the important flowpath(s) as well as other important variables which increase nitrogen movement to streams will allow watershed managers to more accurately implement beneficial land management practices in an effort to reduce nitrogen movement to streams. This knowledge will become increasingly important in an effort to maintain or reduce the amount of nitrogen in streams while increasing crop production to support the rising global population.