Here we describe the regional-scale hydrological cycle of Illinois, including both the land and atmospheric branches, using a data set on most of the hydrological variables, i.e., precipitation, streamflow, soil water content, snow depth, groundwater level, and atmospheric flux of water vapor. Since direct observations of evaporation are not available, two different approaches, soil water balance and atmospheric water balance, were applied to estimate the regional evaporation over Illinois from 1983 to 1994. The availability of a comprehensive hydrological data set covering the large area of Illinois facilitated a comparison between these two approaches for estimation of evaporation. To our knowledge, this is the first time such a comparison has been made. The climatologies of the monthly evaporation estimates from the two approaches agree reasonably well and within a 10% error; however, substantial differences exist between the two estimates of evaporation for individual months. The seasonal variability of the evaporation estimates based on soil water balance is largely balanced by the seasonal pattern of subsurface storage, whereas the seasonal variability of evaporation estimates from the atmospheric water balance is almost entirely balanced by the seasonal pattern of lateral fluxes of water vapor. This contrast reflects a fundamental difference in the hydrology of the land and atmospheric branches of the regional water cycle. In light of the fact that independent data sets were used in the two approaches, our results are encouraging: The atmospheric water balance approach has the potential for the accurate estimation of the climatology of regional evaporation, at least for humid regions at a scale similar to that of Illinois (∼105 km2). However, sensitivity analysis suggests that the accuracy of atmospheric water balance computations is rather poor for the scale smaller than 105 km2. For the calculation of evaporation using the soil water balance approach in regions where the groundwater table is rather shallow, the incorporation of the change in groundwater storage is indispensable since groundwater aquifers provide a significant portion of water storage at the monthly timescale.