Climate Variability and Streamfiow

At a given point along a river, streamfiow is the product of the total catchment area above the gage and the average rate at which runoff is generated from snow and/or rain in that catchment. Runoff within a basin depends not only on rainfall but on its temporal distribution, vegetation cover (amounts and types), évapotranspiration, soil moisture storage capacity, rate of groundwater outflow, amount of paved area, etc. The seasonality of streamfiow varies widely from river to river and is influenced mostly by the local seasonal cycle of precipitation, by timing of snowmelt (where appropriate), by the travel time of water from the runoff source areas, through surface and subsurface reservoirs and channels, and by large-scale human interventions.

Year-to-year variations in streamfiow timing and magnitude play important roles in the development and management of water resources in most regions. Such interannual variations may be superimposed on longer (decadal to century-scale) fluctuations. Regime shifts from wetter to drier periods (and vice versa) on the decadal scale can be seen, for instance, in the annual streamfiow data for the Colorado Basin in the southwestern United States (Fig. 2). Some but not all of these variations and their characteristics can be attributed to El Nino-Southern Oscillation events, climate forcings on the decadal time scale in the north Pacific, etc. There are many exceptions to large-scale patterns of streamfiow seasonality in a region. Spatial differences in and between main streams and tributaries pose significant problems for flow estimation and planning. The shifts, and the surprises they introduce, provide a variable background against which allocations and water quality requirements are to be developed, agreed upon, and implemented.

Water Year (October-September)

Figure 2 Colorado River streamflow: Annual deviations from the long-term mean (1896-1996) at Lee Ferry and 9-year moving

Water Year (October-September)

Figure 2 Colorado River streamflow: Annual deviations from the long-term mean (1896-1996) at Lee Ferry and 9-year moving

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