1. 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

2.  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)

3. Agriculture in Minnesota Major Crops: Corn Soybeans

4. Agriculture in Minnesota

5. Lesser Known Crops: Grapes

6. There are 8.5 turkeys per Minnesotan...

7. ... which beats to 6.2 pigs per Iowan.

8.  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)

9. Watershed Characteristics  36.4 km 2 watershed  8 km perennial stream (Avg. Q = 0.45 m 3 /sec)  Average annual precipitation of 87 cm  89 % Corn / soybean 8% ungrazed grassland (buffer strips) Source: Iowa Geologic Survey and Iowa DNR, 2002 Yellow = Corn Green = Soybean Temporal and Spatial Variability in Nitrate in Subsurface Drains

10.  146 drains total  93 sub-surface 8 large diameter  60-108 cm 73 medium diameter  27-48 cm 12 small diameter  17-23 cm  53 surface/overland Temporal and Spatial Variability in Nitrate in Subsurface Drains

11. Five synoptic sampling periods July 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

12. Nitrate Concentrations: Summers 2006-80 Temporal and Spatial Variability in Nitrate in Subsurface Drains

13. Temporal and Spatial Variability May 2008 Discharge Specific conductance Nitrate Spatial Variability

14. Temporal and Spatial Variability in Nitrate in Subsurface Drains Variability in Nitrate Concentration by Diameter

15.  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

16. 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)312027031 Nitrate (kg/day N)623828031 Chemical load = instantaneous discharge * chemical concentration Temporal and Spatial Variability in Nitrate in Subsurface Drains

17. Log nitrate load versus log discharge R 2 > 0.94 (for all) Temporal and Spatial Variability in Nitrate in Subsurface Drains

18. Comparison of Synoptics with Fixed Sites Temporal and Spatial Variability in Nitrate in Subsurface Drains Two fixed sampling locations February 2007 – November 2008