temporal and spatial variability in nitrate in subsurface drains in a midwestern agricultural...
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Temporal and Spatial Variability in Nitrate in Subsurface Drains in a
Midwestern Agricultural Watershed
Paul Capel, USGS, National Water-Quality Assessment Program
Jonathon Thornburg, University of Minnesota, Water Resources
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Quantify the variability in discharge and nitrate in subsurface drainage within a single small watershed
Spatial Temporal Source
(diameter)
Temporal and Spatial Variability in Nitrate in Subsurface Drains
USGS National Water-Quality Assessment Program (NAWQA)
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Agriculture in Minnesota
Major Crops:
Corn
Soybeans
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Agriculture in Minnesota
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Agriculture in Minnesota
Lesser Known Crops: Grapes
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There are 8.5 turkeys per Minnesotan ...
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... which beats to 6.2 pigs per Iowan.
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Upper portion in the South Fork Iowa
River watershed
Landscape formed by recent glaciations Des Moines Lobe ~10,000 -13,000 BCE
Prairie potholes, ground moraines, and till plains
Temporal and Spatial Variability in Nitrate in Subsurface Drains
Estimated density of subsurface drainage (darker color denotes
greater drainage)
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Watershed Characteristics
36.4 km2 watershed
8 km perennial stream (Avg. Q = 0.45 m3/sec)
Average annual precipitation of 87 cm
89 % Corn / soybean 8% ungrazed grassland (buffer strips)
Source: Iowa Geologic Survey and Iowa DNR, 2002
Yellow = CornGreen = Soybean
Temporal and Spatial Variability in Nitrate in Subsurface Drains
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146 drains total
93 sub-surface 8 large diameter
60-108 cm73 medium diameter
27-48 cm12 small diameter
17-23 cm
53 surface/overland
Temporal and Spatial Variability in Nitrate in Subsurface Drains
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Temporal and Spatial Variability in Nitrate in Subsurface Drains
Five synoptic sampling periodsJuly 2006 – 43 sub-surface samples Mar 2008 – 10 sub-surface samples July 2007 – 34 sub-surface samples May 2008 – 82 sub-surface samples
Aug 2008 – 43 sub-surface samples
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Nitrate Concentrations: Summers 2006-80
Temporal and Spatial Variability in Nitrate in Subsurface Drains
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Temporal and Spatial Variability
May 2008 Discharge
Specific conductance
Nitrate
Spatial Variability
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Temporal and Spatial Variability in Nitrate in Subsurface Drains
Variability in Nitrate Concentration by Diameter
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Least amount of variability in large diameter drains
70-87% of water in the stream from subsurface drains
is from the 8 large diameter drains 70-87% of water in the stream 61-85% of nitrate load in the stream
Smaller drains become more important during periods of high flow
Variability in Concentration by Diameter
Temporal and Spatial Variability in Nitrate in Subsurface Drains
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Variability in Loads as a Function of Time May 2008 had 85% more flow than other periods
May 2008 had 5 to 9 times higher load of nitrate
July 2006
July 2007
May 2008
August 2008
Discharge (L/sec) 31 20 270 31Nitrate (kg/day N) 62 38 280 31
Chemical load = instantaneous discharge * chemical concentration
Temporal and Spatial Variability in Nitrate in Subsurface Drains
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Log nitrate load versus logdischarge
R2 > 0.94 (for all)
Temporal and Spatial Variability in Nitrate in Subsurface Drains
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Comparison of Synoptics with Fixed Sites
Temporal and Spatial Variability in Nitrate in Subsurface Drains
Two fixed sampling locationsFebruary 2007 – November 2008