Causes & types of drought
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Causes & types of drought
Types of drought
We can define drought in three main ways:
1. Meteorological drought: related to rainfall amounts
2. Hydrological drought: determined by water levels in reservoirs
3. Agricultural drought: related to the availability of water for crops
Meteorological 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 water on 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 the 1990's, the whole 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 than any other natural disaster over the same period. The worst year was in 1995 and over six million people were affected by the drought in that year alone.
Hydrological 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.
2. Reservoir levels. From CEDEX.
In this picture you can see how the drought that affected Spain during the first half of the nineties also affected the water level in reservoirs. In 1995 the volume of water held in the reservoirs was only about 10% of their total capacity.
Agricultural drought occurs when there is not enough water available for a particular crop to grow at a particular time. This drought doesn’t depend only in the amount of rainfall, but also on the correct use of that water. Imagine a period of low rainfall where water is used carelessly for irrigation and other purposes. Under these circumstances, the effect of the drought becomes more pronounced than it was before.
Agricultural drought is typically seen after meteorological 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 to survive.
3. Rainfall and agricultural production. Adapted from “Libro blanco del Agua” Spain.
This graph shows the total rainfall for 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 and Dr. Ana Iglesias - Universidad Politécnica de Madrid, España
1. scientific reviewer: Alex de Sherbinin - CIESIN, Columbia University, USA
2. scientific reviewer: Lily Parshall - Goddard Institute for Space Studies, Columbia University, USA
educational reviewer: Emilio Sternfeld - Colegio Virgen de Mirasierra, España
last published: 2004-05-12