8/29/20098 SAHRA - Watershed Visualization

Transcription

1 Module 3 Narration Southwestern Water Cycle 001.wav 26 sec 002.wav 10 sec 003 wav 17 sec Water on Earth is constantly on the move. Water continually circulates between the surface of Earth and the atmosphere in what is called the hydrologic cycle. Within this cycle, water can exist either as a liquid, as a solid (in the form of ice), or as a gas (in the form of water vapor). Water on the surface of Earth is constantly changing between these three phases of matter. The sun provides the energy necessary to transform liquid water in oceans, lakes, and rivers into water vapor. This process is called evaporation. Water vapor is a transparent gas so we cannot see it. It moves wherever air moves. As it cools, water vapor condenses into water droplets to form clouds. The water droplets absorb and reflect sunlight and therefore clouds may appear white or gray. Video sequence Verde Prescott grassland 004.wav 13 sec Once the droplets of condensed water vapor are too heavy to remain in the atmosphere, they fall to Earth as precipitation. Rain, snow, sleet, fog and dew are all forms of precipitation. 1

2 005.wav 12 sec 006.wav 13 sec After the precipitation reaches the surface of Earth, it does one of five things. It evaporates, transpires, runs off, sinks into the soil or becomes part of a plant or animal. Water on plants, in puddles or in the soil may undergo evaporation. The process of water leaving the cells of plants is called transpiration. These two processes are often lumped together and called evapotranspiration. 007.wav 8 sec The process of water sinking into the soil is called infiltration. Water that infiltrates to greater depths adds to groundwater storage in underground aquifers. 2

3 008.wav 10 sec Water that moves downhill on the land s surface and concentrates in streams and rivers is called runoff. In many parts of the world, this runoff eventually flows into the oceans. 009.wav 16 sec The Hydrologic Cycle operates in arid regions like the Southwest as well, but with a little twist. What components of the basic water cycle are missing? pause That s right oceans, lakes and snow are much less important to this arid region water cycle. 010.wav 6 sec However, water vapor which does evaporate from far away oceans is transported to the southwest by prevailing wind currents. 3

4 011.wav 6 sec The moist air condenses as it cools and clouds form. Almost all precipitation falls as rain and quickly evaporates, sometimes before even reaching the ground! 012.wav 5 sec 013.wav 8 sec Only rainfall from severe or persistent storms produces runoff, sometimes in the form of a flash flood. This water quickly infiltrates or soaks into the soils that line the normally dry washes. Arid regions may have large groundwater aquifers that were filled tens of thousands of years ago when the climate was much wetter than today. not used 4

5 014.wav 24 sec So how does this modified hydrologic cycle operate in the southwest? In the Verde watershed, and over much of the Southwest, there are typically two distinct seasonal weather patterns that produce rain or snow. These patterns are caused by the combination of changes in seasonal temperature and changes in moist air flow into this region of the country. Need map of source regions 015.wav 27 sec During the winter, cool, moist air, moving as widespread fronts, slides into the Southwest from the northern Pacific Ocean. The precipitation that occurs tends to be gentle and steady over fairly large portions of the region, but with greater total precipitation in the highlands. The gradual melting of the winter snow cover in the spring supplies the main rivers of the southwest, including the Verde River, with an important source of water. 016.wav 21 sec In the summer, the source of the moist air that moves into the Southwestern U.S. changes from the northern Pacific to the more tropical Sea of Cortez and Gulf of Mexico. This shift to monsoon flow brings surges of wet, warm air that rise in the hot desert climate or are pushed upward by mountains. As the air cools it produces towering clouds. 5

6 018.wav 22 sec In the intense heat of the desert summer, these rising bodies of moist air tend to produce locally heavy but short-lived rainstorm events that are scattered across the region. We call these types of rain events convective storms because they come about from the convection of heat from the surface of the land and are carried higher in the atmosphere by a mass of air. 019.wav 15 sec These convective storms are often thunderstorms which can be quite violent with frequent lightning and high winds. In contrast to the widespread winter storms over the Verde, only a small area of the Verde Basin may receive precipitation from any single summer storm event. 020.wav 8 sec The shape of the land surface, or topography, also affects how and where precipitation tends to fall, in both the winter and summer. 6

7 021.wav 8 sec When a mass of air moves across the landscape and comes to a mountain (or anywhere the ground rises), it is pushed up over the land by the force of the wind. As the air is forced to rise, it cools and condenses to create clouds. This process creates what is called orographic or mountain precipitation. 022.wav 18 sec This video sequence shows the power and complexity of this effect as orographic clouds form over the Santa Catalina Mountains near Tucson, Arizona. During this four hour period, the storm clouds grow and shrink in response to the effects of heating, topography and other meteorological factors. Video sequence 023.wav 21 sec Orographic precipitation is strongly influenced by the Southwest s varied topography, whether it is snowstorms in the winter or thunderstorms in the summer. Because of the orographic process, precipitation tends to fall most heavily on the higher areas. However after forming over the mountains, orographic storms can also drop their rain on nearby lower areas. 2-d map image of area 7

8 024.wav 12 sec Over the mountains of Arizona the orographic effect is significant. The higher you go, the more precipitation you will receive throughout the year. 025.wav 13 sec For example, a rain gauge on the valley floor might receive only 12 inches of precipitation, while a rain gauge at the top of a nearby mountain might receive twice this amount, about 24 inches of precipitation. 026.wav 9 sec We can illustrate this effect by assigning different colors to the various precipitation amounts going from orange in the valley to green at the mountain tops. 8

9 027.wav 5 sec This orographic pattern of precipitation is even more dramatic at the state scale. 028.wav 12 sec By making maps of the monthly average precipitation and then adding these maps together, we see a pattern start to emerge that shows a strong connection between larger precipitation amounts and areas of higher elevation. 029.wav 13 sec In this image of average annual precipitation, red represents lower precipitation amounts (about 4 inches) and the greens and blues represent higher precipitation amounts (about 40 inches). 9

10 030.wav 16 sec Remember that this animation of average precipitation is a simplified representation of what is actually occurring. In reality, precipitation falls from many, many individual convective and frontal precipitation systems as they move across the state. Audio files located at: Stc-admin\WSViz \Storyboards&Narrations\audio_recordings\mod3 10