Abstract
Excessive nitrate-nitrogen (nitrate) loss from agricultural watersheds is an environmental concern. A common conservation practice to improve stream water quality is to retire vulnerable row croplands to grass. In this paper, a groundwater travel time model based on a geographic information system (GIS) analysis of readily available soil and topographic variables was used to evaluate the time needed to observe stream nitrate concentration reductions from conversion of row crop land to native prairie in Walnut Creek watershed, Iowa. Average linear groundwater velocity in 5-m cells was estimated by overlaying GIS layers of soil permeability, land slope (surrogates for hydraulic conductivity and gradient, respectively) and porosity. Cells were summed backwards from the stream network to watershed divide to develop a travel time distribution map. Results suggested that groundwater from half of the land planted in prairie has reached the stream network during the 10 years of ongoing water quality monitoring. The mean travel time for the watershed was estimated to be 10.1 years, consistent with results from a simple analytical model. The proportion of land in the watershed and subbasins with prairie groundwater reaching the stream (10–22%) was similar to the measured reduction of stream nitrate (11–36%). Results provide encouragement that additional nitrate reductions in Walnut Creek are probable in the future as reduced nitrate groundwater from distal locations discharges to the stream network in the coming years. The high spatial resolution of the model (5-m cells) and its simplicity may make it potentially applicable for land managers interested in communicating lag time issues to the public, particularly related to nitrate concentration reductions over time.
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Acknowledgments
The Walnut Creek Nonpoint Source Monitoring Project was supported in part by Region VII of the US Environmental Protection Agency through a 319-Nonpoint Source Program grant to the Iowa Department of Natural Resources. Additional support was provided by US Department of Agriculture Cooperative State Research, Education, and Extension Service grant 2004-51130-03120. The Neal Smith National Wildlife Refuge contributed greatly to the success of the project.
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Schilling, K.E., Wolter, C.F. A GIS-based groundwater travel time model to evaluate stream nitrate concentration reductions from land use change. Environ Geol 53, 433–443 (2007). https://doi.org/10.1007/s00254-007-0659-0
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DOI: https://doi.org/10.1007/s00254-007-0659-0