University of Maryland
On a 7-yr average, the Eta model 12-36 h forecast precipitation averaged over the Mississippi basin differs from the observed precipitation by 2%, while the estimate of evaporation computed as a residual of the water balance equation differs by 5% from the evaporation estimate resulting of a dataset of land surface fluxes prepared with the macroscale hydrologic Variable Infiltration Model (VIC). Notably, the long term average of moisture flux convergence is 0.49 mm day-1 over the Mississippi basin, while streamflow observations at Vicksburg average 0.50 mm day-1. This agreement is a strict test of the quality of the hydrologic cycle estimates, therefore these are promising results for estimates of the water cycle from the Regional Reanalysis currently being produced.
Sub-basins of the Mississippi have diverse land surface-atmosphere interactions as inferred from monthly averages. In the western half of the Mississippi basin, feedbacks can be described as follows: increased soil moisture is associated with a slight increase of net radiation at the surface; latent heat also increases with soil moisture while sensible heat decreases, resulting in an almost linear increase of the evaporative fraction. The overall results support the concept of a positive feedback in which increased soil moisture affects surface fluxes in such a manner that increased precipitation results. However, toward the east (e.g., the Ohio basin), there are no well defined land surface-atmosphere interactions, suggesting that other effects, like the advection of moisture, may be more relevant for precipitation processes.