The Atmospheric Land EXchange Inverse (ALEXI) model is used to derive land surface energy budgets over large geographical regions. ALEXI, driven by radiometric temperature changes obtained from GOES satellite infrared measurements and fraction vegetation cover estimates from AVHRR, operates in realtime at horizontal resolutions of 5-10 km. The ALEXI model has been well-validated over a broad range of atmospheric and environmental conditions. The goal of this presentation will be to demonstrate that ALEXI is also a valuable model for estimating the available water in the surface layer (0-5 cm) and root zone (5 cm to 2 m).

The ALEXI algorithm partitions system latent heat estimates into fluxes arising from the soil and canopy components within a GOES pixel scene. These component fluxes are compared to estimates of potential evapotranspiration (ET), using fraction cover to partition net radiation between the soil and canopy. The actual-to-potential ratios for canopy and soil ET are then mapped to available water fractions associated with the root zone and soil surface layer, respectively. On cloudy days, when remotely sensed thermal data are not available and ALEXI cannot be executed, the procedure is inverted; the available water fraction distributions (decremented by daily ET) are used to estimate actual fluxes from potential latent heating. This technique is different from the cumulative soil water budget strategies commonly used in regional-scale models in that soil water estimates are updated whenever remote sensing data are available, making them much less sensitive to cumulative errors in individual water budget components.

ALEXI-estimated soil moisture at 10 km resolution will be compared with National Center for Environmental Prediction (NCEP) Eta-coordinate model output across a large section of the Mississippi River drainage basin. A similar comparison will be conducted for model predictions of latent heat flux. In doing so, we will evaluate the utility of ALEXI model as a means for validating initial surface conditions used in standard regional scale atmospheric models.