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Chapter 3: Hydrological Processes in Riparian Areas
Precipitation
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Precipitation in Arizona, and in the southwest, exhibits some of the greatest variability
within the United States. - Precipitation varies temporally at several scales: 1) daily, in response to summer thunderstorms, 2) seasonally, 3) annually with drought and flood cycles, and 4) large scale, in response to atmospheric circulation patterns such as El Niño and La Niña.
- Precipitation also varies spatially across the landscape and with elevation.
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Generally, precipitation is the result of four types of storms: convective storms, tropical storms, uplift near mountains, and frontal storms.
- Across a significant portion of Arizona, annual rainfall is bimodally distributed and provides both winter and summer rain.
- Winter precipitation results from frontal storms characterized by long duration, low intensity, and large areal coverage.
- In the summer, rainfall is generally the result of short duration, intense thunderstorms.
Figure 3.4. Precipitation can be highly variable both in time and space (left). Intense thunderstorms are typical in the summer, with precipitation accompanied by lightning (right).
- a significant portion of precipitation in Arizona can occur as snow in higher elevations (> 7000 ft).
Figure 3.5. Snowmelt can also be a significant portion of the runoff that reaches the streams in Arizona.
- The characteristics of precipitation have implications for those hydrologic processes that follow, including surface runoff, infiltration, evaporation and transpiration, and subsurface flow.
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