Surface Energy and Water Balance for the Arkansas–Red River Basin from the ECMWF Reanalysis

Average surface energy and water budgets, subsurface variables, and atmospheric profiles were computed online with an hourly timescale from the ECMWF reanalysis for five subbasins of the Mississippi River from 1985–93. The results for the Arkansas–Red River basin are discussed on diurnal, 5-day, monthly, seasonal, and interannual timescales, and compared with the observed basin-scale precipitation and streamflow. The model shows the seasonal and interannual variability of precipitation, evaporation, and soil water. The annual range of soil water is typically 100 mm, and the interannual range is somewhat smaller. The model has a significant spinup of about 29% in precipitation from the analysis cycle to a 12–24-h forecast. The spinup of the model “large-scale” precipitation is 39%, double that of the spinup of the model “convective” precipitation of 18%. When compared with 5-day and monthly basin averages of hourly rain gauge observations (corrected for a probable 10% low bias), the precipitation in the reanalysis is low by 20%–25%, while the 12–24-h forecast precipitation is high by about 5%; so the model precipitation estimates the bracket observations. The nudging of soil water in the analysis cycle, based on 0–6-h forecast errors in low-level humidity, plays an important role in the model liquid hydrology. It prevents the downward interannual drift of soil water, associated with a shortfall of precipitation in the analysis cycle, while allowing interannual variations of soil water. However, the nudging appears to be trying to compensate for other errors in the model: such as errors in the diurnal cycle of low-level mixing ratio and in the seasonal cycle of evaporation. Evaporation in the model is probably high in winter, and on an annual basis may have a small high bias in comparison to a basin evaporation estimate derived from observed precipitation and streamflow. An internal inconsistency of 7% in the evaporation term in the model surface energy and subsurface water budgets is also found, dating from an earlier model version. The coupling of soil water in the model to evaporative fraction and the low-level thermodynamics is similar to that observed. The model runoff, which is all deep runoff from the base soil layer, is low by a factor of 2, when compared to observed streamflow on an annual basis. The model diurnal cycle of precipitation has a near-noon maximum, while that observed is late afternoon and evening. This is probably related to the model error in the diurnal cycle of mixing ratio and boundary layer depth. Overall the ECMWF reanalysis gives a valuable description of the surface energy and water balance of the Arkansas–Red River basin on timescales longer than the diurnal.