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

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

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

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

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

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 25-30 cm.

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

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)

Cross-Comparison Location

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

Variation in Precipitation July 04 02 Rainfall (mm)

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

Energy Balance Closure

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

Seasonal Variation in Rn - Corn

Seasonal Variation in Rn - Soybean

Seasonal Variation in G - Corn

Seasonal Variation in G- Soybean

Variation in LE and CO2

Variation in LE - Soybean

Variation in Energy Balance Components Blue- Corn Red- Soybean

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

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