mental energy within the various reservoirs is required. This paper explores our current ability to define this partitioning and draws conclusions about the resulting implications on the hydrology of the western United States.

To illustrate the complexity of the hydrologic random walk, it is useful to consider the concept of teleconnections (Namias, 1981), which is a statistical approach relating the magnitudes of weather or hydrologic events that occur at great distances from each other. For teleconnections to be more than a statistical oddity, they must be the result of seasonally preferred paths through the hydrologic random walk. The relation of weather patterns around the globe to an aperiodic anomalous warming of the eastern Pacific Ocean, the El Niño Southern Oscillation (ENSO), is a teleconnection that has been much explored recently. For example, as shown in Figure 8.1, Ropelewski and Halpert (1987) demonstrated a positive correlation between ENSO events and precipitation magnitudes over much of the Colorado River basin. This correlation implies that an energy exchange between the atmosphere and the Pacific Ocean can alter the probabilities of precipitation in the western United States during ENSO events.

A more explicit depiction of a preferred path of a similar or even greater spatial scale is found in the work of Koster as reported by Eagleson (1986). Figure 8.2 shows the regions where water that is evaporated in the month of March from a grid cell of 10 degrees longitude by 8 degrees latitude located in southeastern Asia is first redeposited on earth. Most of the land area under the mandate of the Bureau of Reclamation receives moisture from this cell, as does most of eastern Asia and the northern Pacific. Thus, evidence indicates a very complex system of reservoirs of moisture and energy in the oceans, in the atmosphere, and on the land that interacts with itself to define the existing climate and hydrology in the western United States.

What do we know about the response of this complex system to an increased greenhouse effect? Probably, we know best the physics of the transport of mass and energy in most reservoirs of the system. However, we know the physics only at spatial scales that are not fully compatible with the data bases and computing facilities that are available today. Climatologists and oceanographers have attempted to bridge this incompatibility by constructing mathematical general circulation models (GCMs) of the earth's atmosphere and oceans. GCMs are, at best, compromises between the sophistication of the description of the physics and the temporal and spatial scales at which transport is computed; thus,