Recent research showed that active sensors such as Crop Circle can be used to estimate in-season N requirements for corn. The objective of this research was to identify sources of variability in the observed and Crop Circle-estimated optimum N rates. Field experiments were conducted at two locations for a total of five sites during the 2007 growing season using a randomized complete block design with increasing N rates applied at V6-V8 (NV6) as the treatment factor. Field sites were selected from different landscape positions representing variable soil moisture regimes so as to generate a range of optimum N rates at V6. Corn canopy reflectance was measured using Crop Circle prior to N application at V6. Soil and plant biomass samples were obtained at planting, V6, R1 and physiological maturity. A significant grain yield response to NV6 was observed at three of the five sites. The remote sensing model accurately estimated the optimum NV6 rates at three of the five sites. Grain yield did not respond to NV6 applications at two sites, where the remote sensing model over-estimated the optimum NV6 rates. The soil NO3-N and total N uptake data measured during the growing season indicated that the interactive effect of changes in soil moisture and N availability after the NV6 application can influence the accuracy of estimated optimum NV6 rates. A better understanding of the soil moisture redistribution to the depth of root zone in relation to landscape position could help in understanding the influence of water stress on the N utilization of corn and thereby improve estimates of in-season N requirements.