RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 8

Figure 1-6 Deep groundwater is discharged by geothermal vents (Yellowstone). (From [7].)
especially in arid regions such as the Southwest, where the extremes of “wet”
and “dry” years can result in a system that experiences a crisis under both cycles.
It is the extremes of the cycle that create the greatest stress in every community,
and the duration of individual droughts or flood-creating rainfall periods that
measure how well or poorly we have built our communities.
Most large river basins in the United States have experienced significant human
alteration or structural intervention over the past two centuries. It is interesting to
consider the net effect of human activities on the regional watershed, although we
have no baseline (pre-disturbance) flow data to compare with current conditions.
However, we can compare the net runoff generated in these large systems with the
rainfall experienced within the watershed (Table 1-3). Also shown is a reference
city, usually situated at the downstream reach of the river basin. In the Santa
Anna basin draining to Los Angeles, the inflow from three diversion canals
affects these statistics significantly.