1. MODELING THE IMPACT OF IRRIGATION ON NUTRIENT EXPORT FROM AGRICULTURAL FIELDS IN THE SOUTHEASTERN UNITED STATES W. Lee Ellenburg Graduate Research Assistant Earth System Science Center UAHuntsviile 2013 Alabama Water Resources Conference

2. Introduction  Agricultural runoff has the potential to contribute to a litany of water quality problems  Excessive Nutrients  Storm Runoff  Inter-storm Periods

3. Does Irrigation Impact Nutrient Runoff?  Irrigation increases soil moisture and thus runoff and leaching  Irrigated crops are more efficient than rain-fed crops in up-taking nutrients/sequestering biomass.  Droughts are known to substantially increase the nitrogen left in the soil [Nafziger, 2013; USDA NRCS, 2012]

4. In The News Iowa copes with nitrate surge in drinking water  Summer 2012 - Drought plagues the Midwest  Fertilizer in the fields goes unused  Spring, 2013 - Wettest spring in 141 years  Sharp increase in nutrient runoff and leaching

5. Does Irrigation Impact Nutrient Runoff?  Nutrient export and cycling kinetics is closely related to soil moisture via “hydraulic flushing” [Hornberger et al. (1994)]  Soil moisture content, temperature and organic carbon content are the dominant factors in the nutrient cycling process

6.  This project explored the differences of the nitrogen cycle dynamics (i.e. nitrogen movement) in irrigated and rain-fed fields using a physically based distributed hydrologic model coupled with a sophisticated nutrient cycling and plant uptake model (DSSAT). All that… to say this

7. Background  Many models have been developed to simulate agricultural runoff  ARS Models CREAMS, EPIC, SWAT, etc.  Hydrologic-Ecosystem Models PnET-BGC, SPARROW, LASCAM, etc.  More commonly, field studies have been employed

8. Agricultural Model  DSSAT  Decision Support System For Agro-technology Transfer  Experimental inputs: Planting dates Irrigation thresholds Fertilization schedules Climate data Soil Data  Used to simulate:  nitrogen cycle  plant uptake

9. Nutrient Transport  Kinematic wave approximation:  Surface  Subsurface  Nitrogen Concentrations

10. Study Area  Corn (Zea mays L.)  Silty Clay Loam (NRCS)  Bulk Density  1.29-1.2  Porosity (Sat.) . 521-.485  Lower Limit . 172-.332  Drained Upper Limit .346-.483

11. Experimental Set-up  Split Plot Design  4 Replications 2010; 5 Replications 2011; All Irrigated Treatment Number Nitrogen Application Treatment Number Nitrogen Application 1100-0960-40 250-5010250-0 330-7011125-125 4150-01280-170 575-75130-0 650-1001450-0 7200-01525-25 8100-100160-50

12. Irrigation Applications 2010 Description Amount (inches) Yearly Precipitation 71.2 Seasonal Precipitation 27.5 Irrigation 8.4

13. Irrigation Applications 2011 Description Amount (inches) Yearly Precipitation 68.2 Seasonal Precipitation 35.5 Irrigation 5.5

14. Model Calibration - 2010 Observed Mean = 146.5 Simulated Mean = 157.7 P-value =.105 Assuming Unequal Variances:

15. Model Calibration - 2010 Variance in the observed replications: 18195 kg/ha (289.3)

16. Model Validation - 2009 Observed Mean = 8375 kg/ha (133.2) Simulated Mean = 8288 kg/ha (131.8) P-value =.460 Assuming Unequal Variances:

17. Model Validation - 2011 Observed Mean = 4.38 mg/l Simulated Mean = 4.34 mg/l Observed Variance = 7.57 mg/l Simulated Variance = 50.89 mg/l RMSE = 164% In terms of the processes being modeled, the overwhelming source of nitrogen loss from the field is the plant uptake which accounts for about 90% of the losses in this study (compared to de-nitrification and volitization). **

18. Results

19. Surface Runoff

20. Surface Nitrate Export 2010 2011

21. Results and Discussion – Treatment 10 (280-0) Comparing 2011 with 2010 N

22. Surface Nitrate Export

24. Trt 1

25. Trt 11

26. Results and Discussion – Subsurface Treatment 6-2011 (56-112)

27. Lateral Subsurface Nitrate Export 2010 2011 Growing Season

28. Lateral Subsurface Nitrate Export 2010 2011 Fallow Season

29. Total Lateral Nitrate Export

30. Conclusions  Surface Runoff  Early season frontal systems have major impact on surface nitrate runoff. Irrigation benefits by allowing the nitrate to move beyond the surface layer.  Runoff can be mitigated with flexible application schedules.

31. Conclusions  Subsurface Lateral Leaching  Irrigation has minimal effect on lateral leaching of N during the growing season.  Irrigation decreases the residual (fallow/post season) nitrate in the soil column decreasing the lateral and vertical leaching  Irrigation provides the vertical movement and aerobic conditions for nitrogen to be consumed by the plant.