Estudiante

WATER RESOURCES RESEARCH, VOL. 38, NO. 6, 1083, 10.1029/2001WR000619, 2002

Annual hydroclimatology of the United States
A. Sankarasubramanian
International Research Institute for Climate Prediction, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA

Richard M. Vogel
WaterSHED Center, Department of Civil and Environmental Engineering, Tufts University,Medford, Massachusetts, USA Received 13 April 2001; revised 21 January 2002; accepted 21 January 2002; published 25 June 2002.

[1] An overview of the annual hydroclimatology of the United States is provided. Time series of monthly streamflow, temperature, and precipitation are developed for 1337 watersheds in the United States. This unique data set is then used to evaluate several approaches forestimating the long-term water balance and the interannual variability of streamflow. Traditional relationships which predict either actual evapotranspiration or the interannual variability of streamflow from an aridity index f ¼ PE=P are shown to perform poorly for basins with low soil moisture storage capacity. A water balance model is used to formulate new relationships for predicting actualevapotranspiration and the interannual variability of streamflow. These relationships depend on both the aridity index f ¼ PE=P and a new soil moisture storage index. A physically based approach for estimating the soil moisture storage index is introduced which requires monthly time series of precipitation, potential evapotranspiration, and an estimate of maximum soil moisture holding capacity. The netresults are improved expressions for the long-term water balance and the interannual variability of streamflow which do not require either INDEX TERMS: 1833 Hydrology: Hydroclimatology; 1836 calibration or streamflow data.
Hydrology: Hydrologic budget (1655); 1818 Hydrology: Evapotranspiration; 1860 Hydrology: Runoff and streamflow; 1878 Hydrology: Water/energy interactions; KEYWORDS:hydroclimatology, water balance, aridity, soil moisture, humidity, evapotranspiration

1. Introduction
[2] Hydroclimatologic models describe the interactions between land surface and atmospheric processes at different spatial and temporal scales. The simplest hydroclimatologic model which is valid across all spatial and temporal scales is the lumped form of the continuity equation applied to a watershed:dS ¼PÀEÀQÀG; dt ð1Þ

[1974] hypothesized a generalized functional relationship between the evapotranspiration ratio and the aridity index:
  E PE : ¼y P Å ð2Þ

where P, E, and Q are the average depth of precipitation, actual evapotranspiration, and runoff respectively, G is the net amount of groundwater that leaves aquifer storage, and dS represents the change in storage in the basin overthe time interval dt. Equation (1) is commonly used as the basis for describing the annual hydroclimatology of a region [Schreiber, 1904; Ol’dekop, 1911; Budyko, 1974; Milly, 1994a, 1994b; Wolock and McCabe, 1999; Zhang et al., 2001]. [3] A common approach for summarizing the long-term hydroclimatology of a region is to plot the ratio of mean annual evapotranspiration to precipitation E/P versusthe ratio of mean annual potential evapotranspiration to precipitation PE=P. The indices E/P and PE=P are termed the evapotranspiration and aridity ratios, respectively. Budyko
Copyright 2002 by the American Geophysical Union. 0043-1397/02/2001WR000619$09.00

Figure 1 summarizes relationships of the form given in (2) introduced by Schreiber [1904], Ol’dekop [1911], Budyko [1974], and Pike [1964].Asymptotes A and B in Figure 1 correspond to the upper limits on the evapotranspiration ratio or the lower limits on runoff. For regions with unlimited moisture supply the actual evapotranspiration approaches potential evapotranspiration, which is illustrated by line B in Figure 1. Similarly, when potential evapotranspiration exceeds precipitation, then actual evapotranspiration approaches...