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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
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)
Agriculture in Minnesota
Major Crops:
Corn
Soybeans
Agriculture in Minnesota
Agriculture in Minnesota
Lesser Known Crops: Grapes
There are 8.5 turkeys per Minnesotan ...
... which beats to 6.2 pigs per Iowan.
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)
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
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
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
Nitrate Concentrations: Summers 2006-80
Temporal and Spatial Variability in Nitrate in Subsurface Drains
Temporal and Spatial Variability
May 2008 Discharge
Specific conductance
Nitrate
Spatial Variability
Temporal and Spatial Variability in Nitrate in Subsurface Drains
Variability in Nitrate Concentration by Diameter
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
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
Log nitrate load versus logdischarge
R2 > 0.94 (for all)
Temporal and Spatial Variability in Nitrate in Subsurface Drains
Comparison of Synoptics with Fixed Sites
Temporal and Spatial Variability in Nitrate in Subsurface Drains
Two fixed sampling locationsFebruary 2007 – November 2008
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