and ENSO events are similar to the precipitation patterns discussed above
(Piechota et al. 1997).
Factors Controlling Runoff
Regardless of the source of precipitation, runoff in streams begins when
depression storage in the watershed (e.g., ponds, small depressions) is full and
the rate of rainfall exceeds the rate of infiltration (Dunne and Leopold 1978).
Many factors control the ultimate timing and magnitude of runoff into streams,
but only some of these factors are important for understanding differences in
runoff characteristics between arid and humid climates: spatial and temporal
variability of rainfall, interception, evaporation and transpiration, channel trans-
mission losses, and time to the onset of runoff after rainfall begins (Pilgrim et al.
1988, Niemczynowicz 1990, Nouh 1990). In addition to these general compari-
sons between humid and arid regions, the highly varied topography and soils in
the Southwest (Fig. 8) lead to locally variable precipitation (and streamflow)
Arid regions are well known for highly variable spatial and temporal rainfall
(Graf 1988, Pilgrim et al. 1988, Reid and Frostick 1997). Intense rains can fall in
one watershed or portion of a watershed while an adjacent watershed or portion
remains dry. This extreme variability, especially characteristic of summer
thunderstorms, can occur in all seasons because of the variable topography in the
Southwest (Fig. 9). Attempts to predict runoff from a given rainfall event in arid
climates are complicated by the fact that even in small watersheds the very spotty
rain makes it difficult to know precisely how much of the watershed is receiving
precipitation, especially given the low density of rain gauges in desert regions
(Pilgrim et al. 1988). Interannual variability in precipitation is high in arid
regions, with a high ratio of record peak to average annual peak discharge (Graf
1988). Along many very arid region river systems, as in southeastern California,
no flow is experienced during some years. Runoff patterns are also variable on
the decadal and century scale and appear related to ENSO events (Ely 1997).
Recognition of a link between ENSO and annual and longer cycles in precipita-
tion holds promise for improving long-term drought and flood forecasting
(Schonher and Nicholson 1989).
Interception represents that portion of the rainfall that is held on the surface
of leaves and branches of trees and other vegetation. Most of this moisture is
later returned to the atmosphere through evaporation, so it does not contribute to
runoff in streams. Interception is likely most important with low-intensity rainfall
events (Pilgrim et al. 1988), which, in the Southwest, occur during winter frontal
storms. Interception is unimportant (and therefore a greater percentage of rainfall