A GIS-based groundwater travel time model to evaluate stream nitrate concentration reductions from land use change |
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Authors: | Keith E Schilling Calvin F Wolter |
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Institution: | (1) Iowa Geological Survey, 109 Trowbridge Hall, Iowa City, IA 52242-1319, USA |
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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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Keywords: | Geographic information system (GIS) Groundwater Travel time Land use change Conservation Iowa |
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