HYDROCLIMATOLOGY OF ILLINOIS - A COMPARISON OF MONTHLY EVAPORATION ESTIMATES BASED ON ATMOSPHERIC WATER-BALANCE AND SOIL-WATER BALANCE

Here we describe the regional-scale hydrological cycle of Illinois, in cluding both the land and atmospheric branches, using a data set on mo st of the hydrological variables, i.e., precipitation, streamflow, soi l water content, snow depth, groundwater level, and atmospheric flux o f water vapor. Since direct observations of evaporation are not availa ble, mio different approaches, soil water balance and atmospheric wate r balance, were applied to estimate the regional evaporation over Illi nois from 1983 to 1994, The availability of a comprehensive hydrologic al data set covering the large area of Illinois facilitated a comparis on 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 appro aches agree reasonably well and within a 10% error; however, substanti al differences exist between the two estimates of evaporation for indi vidual months. The seasonal variability of the evaporation estimates b ased on soil water balance is largely balanced by the seasonal pattern of subsurface storage, whereas the seasonal variability of evaporatio n estimates from the atmospheric water balance is almost entirely bala nced by the seasonal pattern of lateral fluxes of water vapor. This co ntrast 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 r esults 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 (similar to 10(5) km(2)). However, sensitivity analysis sugge sts that the accuracy of atmospheric water balance computations is rat her poor for the scale smaller than 10(5) km(2). For the calculation o f evaporation using the soil water balance approach in regions where t he groundwater table is rather shallow, the incorporation of the chang e in groundwater storage is indispensable since groundwater aquifers p rovide a significant portion of water storage at the monthly timescale .