Spatial and Temporal Variation of Energy and Carbon Fluxes in Central Iowa

Energy balance and CO2 exchange of agricultural crops has been investigated through limited field studies because of the expense of the monitoring equipment and availability of fields to place equipment. Quantifying the spatial and temporal variation in the energy balance and CO2 dynamics over crop canopies will improve regional-scale estimates of water and C fluxes. A study was conducted in central Iowa during 2002 as part of the Soil Moisture Experiment to evaluate soil moisture energy exchange across an intensive corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production area near Ames, IA (lat. 41.98380985, long. 293.75497316). Surface energy balance and CO2 flux stations were placed in 12 corn and soybean fields across different soils and landscapes in and around the Walnut Creek watershed. Variability among fields was induced by three factors. Within a day, variations among fields were due to the presence of cumulus cloud formation in the afternoon. Short-term differences across days among the fields were due to variation in the spatial pattern of rainfall events causing differential drying. Throughout the season, differences among fields were due to soil water availability, which affected crop growth and ground cover. Differences in early season ground cover were correlated with energy balance (r 5 0.80). During the growing season, latent heat and CO2 uptake were closely related (r 5 0.85). Characterization of the spatial distribution of energy balance and CO2 uptake in an intensive cropping region provides guidance on the confidence that can be placed in interpreting single-site measurements. EXCHANGES of energy and CO2 from the soil and canopy surface in the Corn Belt are influenced by changes in land cover and cropping practices. Houghton (1999) estimated 124 Pg C (1 petagram 5 10 g) have been added to the atmosphere as a result of land use changes between 1850 and 1990, of which 13% has been from conversion of midlatitude grasslands to cultivated cropland. One of the most intense areas for this conversion is the Upper Midwest corn–soybean region of the USA, comprised of .60 million ha representing 60% of cultivated cropland in the USA. There are important implications for surface energy balance and CO2 uptake associated with major land use conversions. In the Upper Midwest region, native prairie grasslands have been replaced with corn and soybean crops. This conversion is important because of the configuration of the rows planted on the soil surface and the reduction in the length of the year that the crop provides cover on the soil surface. Latent heat flux (LE) exchange processes between the surface and boundary layer of the atmosphere for a native prairie were altered when prairie was transformed from a continuously covered grass surface to a corn and soybean production system in which the soil surface annually transitions from bare soil to full canopy cover to crop residue and finally to bare soil again. The exposed soil surface during the non-growing portion of a year as well as during the emerging crop phase represents an important energy exchange period (Ham et al., 1991) that responds to varying soil surface conditions. Surface heterogeneity induced by early row crop development (exposed soil with changing crop canopy) contributes to the challenge of quantifying and understanding energy exchange processes from bare soil and emerging crops (Luxmoore et al., 1973; Hatfield, 1989; Ham et al., 1991). Byre et al. (2000) reported that hydrologic budgets were significantly altered for Wisconsin prairies after conversion to a corn–soybean agroecosystem. Although cumulative evapotranspiration (ET) was comparable between the two crops there was a different temporal distribution of ET between corn and prairie fields (Byre et al., 2000). Climate and landscape were found to be two critical factors affecting the water balance in Australia (Farmer et al., 2003). These findings are similar to observations by Kustas and Albertson (2003), who used a large eddy simulation model to quantify spatial variations of sensible and latent heat fluxes. They found that spatial variation was present across landscapes and that the mechanisms for these differences needed to be understood to help improve regional-scale model predictions. Small and Kure (2003) suggested that coupling between soil moisture and the radiation budget would create significant differences across regions. Walker et al. (2001) observed that the spatial variation of soil moisture was critical to understanding the water balance of a watershed in eastern Australia. Lyons and Halldin (2004) observed that surface heterogeneity was a major factor in the spatial variation of sensible and latent heat exchanges in southern Sweden. Estimating the surface energy balance components and understanding the complex processes of energy and mass exchange from land surfaces and the boundary layer of the atmosphere is critical to many applications in meteorology and hydrology. Measurement of mass, energy, and CO2 fluxes between the terrestrial surface and the atmosphere was the major focus of this effort. Assuming advection is negligible, the surface energy fluxes are related by Q* 1 G 1 H 1 LE 5 0 [1] J.L. Hatfield and J.H. Prueger, National Soil Tilth Lab,, 2150 Pammel Drive, Ames, IA 50011; and W.P. Kustas, Hyrdology and Remote Sensing Lab., Beltsville, MD 20705. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. Received 25 Apr. 2005. *Corresponding author ([email protected]). Published in Agron. J. 99:285–296 (2007). Special Submissions doi:10.2134/agronj2005.0116S a American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CO2 flux, carbon dioxide uptake; DOY, day of the year; EC, eddy covariance; LAI, leaf area index; SMEX02, Soil Moisture Experiment 2002. R e p ro d u c e d fr o m A g ro n o m y J o u rn a l. P u b lis h e d b y A m e ri c a n S o c ie ty o f A g ro n o m y . A ll c o p y ri g h ts re s e rv e d .