The Desert What makes a desert? U.S. deserts Life in the desert What makes a desert? U.S. deserts...

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Transcript of The Desert What makes a desert? U.S. deserts Life in the desert What makes a desert? U.S. deserts...

  • Slide 1
  • The Desert What makes a desert? U.S. deserts Life in the desert What makes a desert? U.S. deserts Life in the desert
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  • What is a desert? All deserts are dry. Not all deserts are hot. Some definitions say deserts are arid (dry) regions that get less than 25 cm (ten inches) of rain a year. The problem with this is that there are places that get less than 10 of rain a year, but arent deserts. A better definition is one that relates the amount of rain to how fast water can evaporate (or transpire from plants) in that place. This is the PET/P ratio (or Potential EvapoTranspiration to Precipitation ratio). Simply, a desert has a PET/P ratio of 3 or higher. All deserts are dry. Not all deserts are hot. Some definitions say deserts are arid (dry) regions that get less than 25 cm (ten inches) of rain a year. The problem with this is that there are places that get less than 10 of rain a year, but arent deserts. A better definition is one that relates the amount of rain to how fast water can evaporate (or transpire from plants) in that place. This is the PET/P ratio (or Potential EvapoTranspiration to Precipitation ratio). Simply, a desert has a PET/P ratio of 3 or higher. This is what most people think a desert looks like. In fact, even only a small part of the Sahara is sand dunes.
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  • PET/P ratios To measure this, the rate of water evaporating from a pan is measured (pan evaporation). Studies with plants show that actual transpiration loss is about 60% of the pan evaporation rate. Studies show that in Tucson, for instance, pan evaporation is 100 a year (so PET = 60), and we get at most 12 of rain a year. From this, PET/P = 60/12 = 5 Since this is higher than 3, Tucson is officially a desert. Yuma has a PET/P ratio of 30. The Sahara Desert has a PET/P ratio of 600! To measure this, the rate of water evaporating from a pan is measured (pan evaporation). Studies with plants show that actual transpiration loss is about 60% of the pan evaporation rate. Studies show that in Tucson, for instance, pan evaporation is 100 a year (so PET = 60), and we get at most 12 of rain a year. From this, PET/P = 60/12 = 5 Since this is higher than 3, Tucson is officially a desert. Yuma has a PET/P ratio of 30. The Sahara Desert has a PET/P ratio of 600!
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  • Another definition A desert is a place where water is severely limiting to life most of the time. Without the word severely this could also apply to semi-arid habitats such as chaparral or grasslands. A desert is a place where water is severely limiting to life most of the time. Without the word severely this could also apply to semi-arid habitats such as chaparral or grasslands.
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  • Why so dry? Several situations can result in deserts: Rainshadow effect Global wind patterns Ocean currents Distance from water Several situations can result in deserts: Rainshadow effect Global wind patterns Ocean currents Distance from water
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  • Rainshadow As warm moist air rises, it cools, and rain precipitates out. This means most of the moisture falls on one side of mountains. As warm moist air rises, it cools, and rain precipitates out. This means most of the moisture falls on one side of mountains.
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  • Wind patterns Sunlight is stronger and delivers more energy near the equator, and much less as you move toward the poles. This causes a zone of warm rising air near the equator, so (like with the rain shadow effect) it rains a lot near the equator. As the air cools and drops (around 20-30 N and 20-30 S) it is very dry. Because of this, there are many deserts near these latitudes. Sunlight is stronger and delivers more energy near the equator, and much less as you move toward the poles. This causes a zone of warm rising air near the equator, so (like with the rain shadow effect) it rains a lot near the equator. As the air cools and drops (around 20-30 N and 20-30 S) it is very dry. Because of this, there are many deserts near these latitudes. Wind pattern animation
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  • Ocean Currents Warm or cold ocean currents also affect local climates. Cold water holds less moisture, and there is less evaporation from cold water. For instance, the Galapagos Islands are on the equator, and we would expect them to be warm and humid. The cold Humboldt current, though, robs them of much moisture and causes them to be a desert. Warm or cold ocean currents also affect local climates. Cold water holds less moisture, and there is less evaporation from cold water. For instance, the Galapagos Islands are on the equator, and we would expect them to be warm and humid. The cold Humboldt current, though, robs them of much moisture and causes them to be a desert.
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  • Distance from water is also important. The Gobi Desert in Asia is dry because it is a very long way from most bodies of water. Together, these factors explain the distribution of most major deserts. Distance from water is also important. The Gobi Desert in Asia is dry because it is a very long way from most bodies of water. Together, these factors explain the distribution of most major deserts. Gobi Desert
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  • U.S. Deserts Four deserts: Great Basin Desert Mojave Desert Sonoran Desert Chihuahuan Desert Note: By some measures, the Sonoran and Chihuahuan deserts do overlap in Southeastern Arizona, around Bisbee. Four deserts: Great Basin Desert Mojave Desert Sonoran Desert Chihuahuan Desert Note: By some measures, the Sonoran and Chihuahuan deserts do overlap in Southeastern Arizona, around Bisbee.
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  • Great Basin Desert Nevada, Utah, Oregon, Idaho High elevation and farthest north, the Great basin is in the rain shadow from mountains along Pacific coast. Cold desert - average temperature 50F. Severe hard freezes in winter. Not all deserts are hot! Most of the precipitation is snow. Typical (indicator) plants: big sagebrush (Artemisia tridentata). Many small-leaf shrubs, few trees, few succulents. Nevada, Utah, Oregon, Idaho High elevation and farthest north, the Great basin is in the rain shadow from mountains along Pacific coast. Cold desert - average temperature 50F. Severe hard freezes in winter. Not all deserts are hot! Most of the precipitation is snow. Typical (indicator) plants: big sagebrush (Artemisia tridentata). Many small-leaf shrubs, few trees, few succulents.
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  • Great Basin Desert
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  • Mojave Desert Southern California and Western Arizona. In the rain shadow from mountains along Pacific coast. Hard freezes in winter but not as severe as in Great Basin. Most precipitation is in winter. Indicator plants: Joshua tree (Yucca brevifolia), low shrubs, a few succulents and trees. Southern California and Western Arizona. In the rain shadow from mountains along Pacific coast. Hard freezes in winter but not as severe as in Great Basin. Most precipitation is in winter. Indicator plants: Joshua tree (Yucca brevifolia), low shrubs, a few succulents and trees.
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  • Mojave Desert Michael Frank 2006 Joshua Tree National Monument
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  • Sonoran Desert Our desert Arizona, Baja and Sonora in Mexico In Pacific rain shadow and at 20-30 latitude. Two rainy seasons, winter and summer, with about half the rainfall during each. Freezes happen but are rare and usually dont last long. Indicator species - Saguaro (Carnegiea gigantea), creosote (Larrea tridentata), mesquite (Prosopsis spp). Our desert Arizona, Baja and Sonora in Mexico In Pacific rain shadow and at 20-30 latitude. Two rainy seasons, winter and summer, with about half the rainfall during each. Freezes happen but are rare and usually dont last long. Indicator species - Saguaro (Carnegiea gigantea), creosote (Larrea tridentata), mesquite (Prosopsis spp). Michael Frank 2005
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  • Southeastern Arizona, Southern New Mexico, North/Central Mexico High elevation, not protected from arctic air masses so hard winter freezes. Summer rain. Low shrubs (creosote, sage), leaf succulents (Yucca spp), small cacti, few trees. Southeastern Arizona, Southern New Mexico, North/Central Mexico High elevation, not protected from arctic air masses so hard winter freezes. Summer rain. Low shrubs (creosote, sage), leaf succulents (Yucca spp), small cacti, few trees. Chihuahuan Desert
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  • Living in a desert For both plants and animals, the lack of water is the biggest factor in determining what plants and animals live there. Most have adaptations that let them take advantage of what water there is, and survive when there is no water. These can take two forms: Drought avoidance Drought tolerance For both plants and animals, the lack of water is the biggest factor in determining what plants and animals live there. Most have adaptations that let them take advantage of what water there is, and survive when there is no water. These can take two forms: Drought avoidance Drought tolerance
  • Slide 19
  • Drought avoidance - annuals Annuals (those that just live one season) escape harsh conditions by not existing. They only sprout from seed when conditions are right. Air and soil temperature, and available water have to be right. Then they grow and flower very quickly. If rain continues, they may continue to grow and flower until things dry out. Since we have two rainy seasons here, we often get two growing seasons for annuals. Strategy for annuals: grow fast, set seeds while the conditions are right. Seeds may go several years before the