1. Spatial Variation in Corn and Soybean Fields in Central Iowa
Energy Balance and Carbon Dioxide
Exchanges

2. Authors
J.L. Hatfield
J.H. Prueger
W.P. Kustas

3. Background
Experiment conducted in central Iowa during 2002 as part of the Soil Moisture experiment (SMEX02)
Opportunity to combine a Soil Moisture Atmospheric Coupling Experiment (SMACEX) to quantify the spatial variation in energy balance and carbon dioxide exchanges across a watershed

4. Objectives
Quantify the differences among energy balance and CO2 exchanges across multiple sites in a corn-soybean area
Quantify the differences among sites caused by soil and crop growth differences

5. Site Walnut Creek Watershed
5200 ha corn/soybean production area in central Iowa

7. Soils within Fields Eight sites were in Clarion soil
Two sites in Nicollet soil
Two sites in Webster soil
Differences among these soils is due to soil water holding capacity in the upper cm.

8. Instrumentation 12 EC 3-D sonic anemometer (CSAT3)
10 Open path CO2/H2O IRGA (LI7500)
2 Open Path H2O (KH20)
Data Collection
20 Hz, 20 day times series
30 Minute data, remainder of season

9. Ancillary Instrumentation
Net radiation (7-Kipp & Zonen, 7-REBS)
Soil heat flux plates with soil T (REBS)
Vitel θv sensors
Temperature and humidity (Vaisala)
Radiometric temperatures (soil & canopy + soil, Apogee)

10. Cross-Comparison Location

11. Exploratory Equivalence Analysis (are the EC’s equivalent)
Variable
June 06
Sept. 17 CV
CO2
Yes
~ <09% H
~ <11% LE
~ <10%

12. Variation in Precipitation
July Rainfall (mm)

13. Variation in Precipitation
Total Rainfall (6/1/02-8/19/02) (cm)

14. Energy Balance Closure

15. Variation in Net Radiation
Little difference among net radiation across the sites

16. Seasonal Variation in Rn - Corn

17. Seasonal Variation in Rn - Soybean

18. Seasonal Variation in G - Corn

19. Seasonal Variation in G- Soybean

20. Variation in LE and CO2

21. Variation in LE - Soybean

22. Variation in Energy Balance Components
Blue- Corn
Red- Soybean

23. Observations
Variation in energy balance components and CO2 among sites was larger than the comparison differences
Differences among sites were caused the crop growth due to soil types and soil water availability
Variation remained consistent throughout the study period

24. Implications
Multiple sites provide measure of the differences across soils within a watershed settings
Careful selection of sites would be necessary to obtain data typical of regional scale estimates