Analysis of the mean annual cycle of the observed water budget based on rad
iosonde data for the 20 year period 1973 - 1992 reveals a negative bias in
the atmospheric estimate of mean moisture flux convergence over the central
United States when compared to mean river discharge for the same period. S
everal possible sources of this bias are examined through the analysis of r
adiosonde, wind profiler, and river discharge data as well as water budget
estimates from NCEP's mesoscale Eta Model. The analysis suggests that 20 ye
ars of radiosonde observations are sufficient to get stable mean water vapo
r flux convergence estimates. The analysis also shows that the bias between
averaged vapor flux convergence and river discharge likely does not depend
on changes in the locations of upper-air observations or on changes in the
radiosondes observational technique. Components of the atmospheric water b
udget diurnal cycle are estimated from hourly wind profiler data and from 3
-hourly Eta Model output. The analyses show that monthly mean atmospheric w
ater budget quantities calculated from twice a day sampling, on average, si
gnificantly underestimate magnitudes of these quantities compared to estima
tes based on hourly or 3-hourly analyses. The magnitude of the underestimat
ion is more than 60% of the difference between mean observed terrestrial an
d atmospheric components of the hydrological cycle over the central United
States, in agreement with earlier studies suggesting that temporal sampling
may account for part of this bias. The effect of spatial sampling was eval
uated using differences between atmospheric water budget terms from the Eta
Model at its full 48 km resolution compared to Eta Model water budget esti
mates sampled at the radiosonde spatial resolution. These analyses show tha
t the magnitude of the effect of spatial resolution is about 40% of the mag
nitude of the mean bias between the average moisture flux convergence and r
iver discharge, The combined effect of temporal and spatial undersampling b
y radiosondes over the central United States is estimated to be 0.44 mm d(-
1). This value is within 8% of the estimated mean difference between river
discharge and moisture flux convergence for the central United States.