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Types of droughtWe can definedrought in three main ways: Meteorological drought:related to rainfall amounts Hydrological drought:determined by water levels inreservoirs Agricultural drought:related totheavailabilityof water for crops

Meteorological DroughtMeteorological drought is generally defined by comparing the rainfall in a particular place and at a particular time with the average rainfall for that place. The definition is, therefore, specific to a particular location. Meteorological drought leads to a depletion of soil moisture and this almost always has an impact on crop production.When we define drought this way, we only consider the reduction in rainfall amounts and don't take into account the effects of the lack of wateron water resevoirs, human needs or on agriculture.

1. Yearly averages of precipitation in Spain.Adapted from "Libro Blanco del Agua" Spain.From this graph it's easy to see the years with very low rainfall in Spain.During the1990's, thewhole country suffered one of the worst droughts in many years. The lowest rainfall was seen in 1995.

Over the last three decades, spring rainfall has decreased in many Mediterranean areas causing severe drought related problems. From 1951 to 2001 rainfall amounts decreased in most areas of Spain, particularly in the Central and Southern regions. Drought affected more people thanany other natural disaster over the same period. The worstyear was in 1995 and oversix million people were affected by the drought in that year alone.

2. Reservoir levels. From CEDEX.

In this picture you can see how the drought that affected Spain during the first half of the nineties alsoaffected thewater level in reservoirs. In 1995 the volume of water held in the reservoirs was only about 10% of their totalcapacity.

Hydrological DroughtHydrological drought is associated with the effect of low rainfall on water levels in rivers,reservoirs,lakes and aquifers. Hydrological droughts usually are noticed some time after meteorological droughts. First precipitation decreases and, some time after that, water levels in rivers and lakes drops.Hydrological drought affects uses which depend on the water levels. Changes in water levels affect ecosystems, hydroelectrical power production and recreational, industrial and urban water use.

Agricultural drought.Agricultural drought occurs when there is not enough water available for a particular crop to grow at a particular time. This drought doesnt depend only in the amount of rainfall, but also on the correct use of that water. Imaginea period of low rainfall where waterisused carelesslyforirrigation and other purposes. Under these circumstances, the effect of the drought becomes more pronounced than it wasbefore.Agricultural drought is typically seen aftermeteorological drought (when rainfall decreases) but before a hydrological drought (when the water level in rivers, lakes and reservoirs decreases).It is important to mention that the effects of droughts are different in irrigated and non-irrigated agriculture. In regions which rely on irrigation, the impacts of short lived agricultural droughts are usually lower than in regions where crops are not irrigated. Irrigated agriculture relies on stocks of water so if it doesn't rain, these crops still get the water they need (until the resevoirs run dry). However, in non-irrigated agriculture crops depend directly on the rain as their water source. If it doesn't rain, the crops don't get the water they need tosurvive.

3. Rainfall and agricultural production. Adapted from Libro blanco del Agua Spain.This graph shows the total rainfallfor different years in Spain and the total agricultural production (irrigated and non irrigated agriculture). The year 1995 saw the lowest rainfall in Spain in the 1990's and this graph shows how agricultural production suffered from the lack of water.

About this page:Author: Marta Moneo andDr. Ana Iglesias- Universidad Politcnica de Madrid,Espaa1. scientific reviewer: Alex de Sherbinin - CIESIN, Columbia University, USA2. scientific reviewer: Lily Parshall - Goddard Institute for Space Studies, Columbia University, USAeducational reviewer: Emilio Sternfeld -Colegio Virgen de Mirasierra - Espaalast published: 2004-05-12

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last updated 23.02.200620:48:57|ESPERE-ENC 2003 - 2011

Floods:Causes and EffectsHomeTsunamisTornadoesHurricanesFloodsFiresVolcanoesActivitiesAbout UsCreditsWater covers 70% of the Earths surface, such as lakes, oceans, seas, rivers and much more.Causes1. Floods are formed when big waves are rolled on normally dry places.2. Rain can also cause floods when it rains more than the land can handle.

3. Floods can happen after a hurricane, such as after hurricane Katrina it flooded the neighborhoods in New Orleans.Effects1. Floods can make people drown and die.2.Floods can destroy a lot of Types of FloodingFlooding can be divided into different categories according to their duration:1. Slow-Onset Floods Slow-Onset Floods usually last for a relatively longer period, it may last for one or more weeks, or even months. As this kind of flood last for a long period, it can lead to lose of stock, damage to agricultural products, roads and rail links.2. Rapid-Onset Floods Rapid-Onset Floods last for a relatively shorter period, they usually last for one or two days only. Although this kind of flood lasts for a shorter period, it can cause more damages and pose a greater risk to life and property as people usually have less time to take preventative action during rapid-onset floods.3. Flash Floods Flash Floods may occur within minutes or a few hours after heavy rainfall, tropical storm, failure of dams or levees or releases of ice jams. And it causes the greatest damages to society.Flooding can also be divided into different categories according to their location:1. Coastal Floods Coastal Floods usually occur along coastal areas. When there are hurricanes and tropical storms which will produce heavy rains, or giant tidal waves created by volcanoes or earthquakes, ocean water may be driven onto the coastal areas and cause coastal floods.2. Arroyos Floods A arroyo is river which is normally dry. When there are storms approaching these areas, fast-moving river will normally form along the gully and cause damages.3. River Floods This is the most common type of flooding. When the actual amount of river flow is larger than the amount that the channel can hold, river will overflow its banks and flood the areas alongside the river. And this may cause by reasons like snow melt or heavy spring rain.4. Urban Floods In most of the urban area, roads are usually paved. With heavy rain, the large amount of rain water cannot be absorbed into the ground and leads to urban floods.peoples homes.3. It can flood a lot of places that we really need like homes, stores and much more.What is a Flood?Anywhere it rains, it can flood. A flood is a general and temporary condition where two or more acres of normally dry land or two or more properties are inundated by water or mudflow. Many conditions can result in a flood: hurricanes, broken levees, outdated or clogged drainage systems and rapid accumulation of rainfall. In addition, floods may occur when levees or spillways are intentionally opened to alleviate the impact of a flood event.Just because you haven't experienced a flood in the past, doesn't mean you won't in the future. Flood risk isn't just based on history, it's also based on a number of factors: rainfall, river-flow and tidal-surge data, topography, flood-control measures, and changes due to building and development.Flood-hazard mapshave been created to show different degrees of risk for your community, which help determine the cost of flood insurance. The lower the degree of risk, the lower the flood insurance premium.

Flooding and Flood Risk

What Causes Flooding Flash Floods Flood After Fire Heavy Rains Ice Jams La Nia Levees Mudflows New Development Snowmelt Spring Thaw Tropical Storms and Hurricanes West Coast Rainy Season Defining Flood Risks Understanding Flood Maps Undergoing a Map Change Flood Map Update Schedule Floodsmart Video Library Flood Risk Scenarios The Cost of Flooding The Levee Simulator

La Nia is characterized by unusually cold ocean temperatures in the Equatorial Pacific (unlike its brother El Nio, which is characterized by unusually warm ocean temperatures in the Equatorial Pacifc). Each La Nia event is unique and can vary in strength, impact and duration. The current La Nia is quite strong and forecasters cannot rule out the possibility that it might last throughout the year.http://www.floodsmart.gov/floodsmart/images/what_causes_flooding/levee_images/levees_hdr.jpg

Levees are designed to protect against a certain level of flooding. However, levees can and and do decay over time, making maintenance a serious challenge. Levees can also be overtopped, or even fail during large floods. Because of the escalating flood risks in areas with levees, FEMA strongly recommends flood insurance for all homeowners in these areas. Homeowners should also develop a disaster plan. To download FEMA's Family Disaster Plan Guide,click here.New DevelopmentConstruction and development can change the natural drainage and create brand new flood risks. That's because new buildings, parking lots, and roads mean less land to absorb excess precipitation from heavy rains, hurricanes, and tropical storms.Long cold spells can cause the surface of rivers to freeze, leading to ice jams. When a rise in the water level or a thaw breaks the ice into large chunks, these chunks can become jammed at man-made and natural obstructions, rehttp://www.floodsmart.gov/floodsmart/images/what_causes_flooding/springthaw_hdr.jpgsulting in severe flooding.

During the spring, frozen land prevents melting snow or rainfall from seeping into the ground. Each cubic foot of compacted snow contains gallons of water and once the snow melts, it can result in the overflow of streams, rivers and lakes. Add spring storms to that and the result is often serious, spring floodinghttp://www.floodsmart.gov/floodsmart/images/what_causes_flooding/ice_jams_header.jpg

ong cold spells can cause the surface of rivers to freeze, leading to ice jams. When a rise in the water level or a thaw breaks the ice into large chunks, these chunks can become jammed at man-made and natural obstructions, resulting in severe flooding.Analysis of the impacts of climate change suggests that agro-ecological systems are the most vulnerable sectors. Agriculture in low latitude developing countries is expected to be especially vulnerable because climates of many of these countries are already too hot. Further warming is consequently expected to reduce crop productivity adversely. These effects are exacerbated by the fact that agriculture and agro-ecological systems are especially prominent in the economies of African countries and the systems tend to be less capital and technology intensive. Predictions of impacts across regions consequently suggest large changes in the agricultural systems of low latitude (mostly, developing) countries.http://www.floodsmart.gov/floodsmart/images/what_causes_flooding/mud_flows_hdr.jpg

Mudflows are rivers of liquid and flowing mud on the surface of normally dry land, often caused by a combination of brush loss and subsequent heavy rains. Mudflows can develop when water saturates the ground, such as from rapid snowmelt or heavy or long periods of rainfall, causing a thick liquid downhill flow of earth.

Cloud seeding is the process of spreading either dry ice, or more commonly, silver iodide aerosols, into the upper part of clouds to try to stimulate the precipitation process and form rain.

Since most rainfall starts through the growth of ice crystals from super-cooled cloud droplets (droplets colder than the freezing point, 32 deg. F or 0 deg. C) in the upper parts of clouds, the silver iodide particles are meant to encourage the growth of new ice particles.

The history of cloud seeding has experienced uncertain results because it can never be known whether a cloud that rains after seeding might have rained anyway. This is because seeding is performed on clouds that look like they have some potential for producing rain.

Interesting facts:

REDUCING THE INTENSITY OF HURRICANES THROUGH SEEDING? NOPE.In the early 1960's, the National Hurricane Center began a series of experiments in seeding of hurricanes to reduce their intensity. Since the most destructive hurricane winds are produced from hurricanes with the smallest diameter eyes, STORMFURY seeding flights tried to stimulate the growth of a new eyewall of convection outside the inner, more destructive eyewall. Unfortunately, the project failed. It has since been determined that there are already an abundance of ice crystals in hurricane rain systems, so the production of new ice particles through cloud seeding probably has little if any effect.

1. vHydrological Drought

Many watersheds experience depleted amounts of available water. Lack of water in river systems and reservoirs can impact hydroelectric power companies, farmers, wildlife, and communities.2. Meteorological Drought

A lack of precipitation is the most common definition of drought and is usually the type of drought referred to in news reports and the media. Most locations around the world have their own meteorological definition of drought based on the climate normals in the area. A normally rainy area that gets less rain than usual can be considered in a drought.3. Agricultural Drought

When soil moisture becomes a problem, the agricultural industry is in trouble with drought. Shortages in precipitation, changes in evapo-transpiration, and reduced ground water levels can create stress and problems for crops.Preventing, Predicting, and Preparing for DroughtsSeveral resources are available to help scientists and forecasters to predict droughts. Drought Index ChartsUse this brief article to determine how to use 8 different drought indices. The US Drought PortalSee how drought impacts your community. The National Drought Mitigation CenterGreat details on the difficulties and successes of predicting droughts are available at the NDMC. 10 Steps to Drought PreparationA great article for communities, businesses, and families. US Seasonal Drought OutlooksThe National Weather Service provides predictions of the chances of drought across the United States.Definition of DroughtDefining a drought is difficult because of the wordnormal. In many areas, normal conditions generally mean conditions that do not deviate from long-term averages. However, these averages themselves can change over time.How Do Jet Streams Form?Warm air masses in the south meet cool air masses from the north and create temperature and air pressure gradients. Essentially, you can compare a "gradient" to a hill on a ski trip. The steeper the hill, the faster you will reach the bottom due to a large difference in thegradeof that hill. In wind speed, the pressure difference between a high and low pressure zone can be very large, thereby creating high winds. Pressure and temperature differences in the jet stream can be large as a global warm front from the south and a cold front from the north meet.What Does the Jet Stream Do?Shallower, (see coastal upwelling above). You have probably seen the other answers about jet streams on this site. The jet streams do have a strong influence on weather patterns and thus short term climate (seasonal to annual basis). They do this by steering weather systems, whether high or low pressure systems, and by acting as effective blocks to the movement of upper level moisture and energy from north to south or vice versa. How they do this is quite complex and you can see, by watching the weather for a few months, that the jet streams can strengthen and weaken, change orientation, and change elevation and depth, sometimes very rapidly, sometimes more slowly.

At times a jet stream will remain nearly stationary for weeks, resulting in nearly unchanging surface weather during the period. You can see how this might affect seasonal climate, by preventing Gulf air from penetrating into the plains and midwest states or the southeastern states, causing drought (a frequent occurrence), or by funneling moisture and energy into those same areas to produce violent storms, flooding, etc., as has happened the past two days in the southeast.

When jet streams move eastward over the U.S. in a more regular pattern, all areas tend to benefit from more consistent rainfall, resulting in improved crop yields in most parts of the country. This happened this year, except for part of the southeast.

The affect of the jet streams on longer term (annual or longer) climate is sometimes more difficult to assess, but is clearly evident during an El Nino, which itself affects the positioning of the jet stream over long periods of time. Severe drought in some areas (southwest U.S.) and flooding in others (western South America) occur during these periods, changing the climate from what is normal over multiple seasons.

David R. CookAtmospheric Research SectionEnvironmental Research DivisionArgonne National Laboratory

Dear Martin-

Generally speaking, jet streams are defined as "rivers of high-speed air in the atmosphere." Jet streams form along the boundaries of global air masses where there is a significant difference in atmospheric temperature. The jet streams may be several hundred miles across and 1-2 miles deep at an altitude of 8-12 miles. They generally move west to east, and are strongest in the winter with core wind speeds as high as 250 mph. Changes in the jet stream indicate changes in the motion of the atmosphere and weather.The El Nio Effect

El Nio is a severeatmosphericandoceanicdisturbance in thePacific Oceanthat occurs every seven to fourteen years. It is called El Nio, meaning the Child, because it usually appears near the Christmas season. Warm surface waters flow from the central Pacific towards the eastern Pacific, suppressing the cold,nutrient-rich upwelling of theHumboldt Currentoff thecoastofSouth America. This disturbance leads to a fall in the number ofplankton, wreaking havoc upon the entire ocean food chain and devastating the fishingindustry.The influence of thesecurrentsalso leads to a complete reversal of thetrade winds, bringing torrential rain,flooding, and mudslides to the usually dry coastal areas ofPeruandEcuador. Another result is the collapse of themonsoonsinAsia, which can bring severedroughttoIndonesiaand northernAustralia. The phenomenon also causes severeweatherdisturbances in other parts of the world, such as droughts in areas ofAfricaand centralNorth America.