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