EQ2 Flashcards
the UN’s definition of drought
‘ an extended period – a season, a year or several years- of deficient rainfall relative to the statistical multi-year average for a region
meteorological drought
Shortfalls in precipitation due to short-term variability, or, longer term trends, which increase the duration of the dry period. Lack of rainfall is often combined with high temperatures, high winds, strong sunshine, and low humidity. This increases evaporation.
agricultural drought
Can be accelerated by farming practices such as overgrazing. Rainfall deficiency leads to a deficiency in soil moisture and soil water availability which then has a knock on effect on plant growth and reduces biomass. The result is poor crop yields, and decline in pasture quality.
hydrological drought
Reduced stream flow and groundwater levels which decrease because of reduced inputs of precipitation and continued high rates of evaporation. It also in reduced storage in any lakes or reservoirs. There are major threats to wetlands and other wildlife habitats. Also linked to decreasing water supplies for urban areas which results in water-use restrictions.
socio-economic drought
occurs when the water demand for social and economic purposes exceeds water availability
this could be the result of a weather-related shortfall in water supply or overuse of the available water supplies
it differs from the other types of drought because its occurrence depends on temporal and spacial variations in supply and demand
The Inter Tropical Convergence Zone, or ITCZ
is a belt of low pressure which circles the Earth generally near the equator where the trade winds of the Northern and Southern Hemispheres come together.
Trade winds
A wind blowing steadily towards the equator from the north-east in the northern hemisphere or the south-east in the southern hemisphere, especially at sea. Two belts of trade winds encircle the earth, blowing from the tropical high-pressure belts to the low-pressure zone at the equator.
famine drought
Widespread failure of agricultural systems, food shortages develop into famines that have severe social, economic and environmental impacts. As populations grow and become wealthier, their demand for water also increases. Also natural variability in the climate can cause a temporary decline in supply and stores are not replenished.
why are urbanised areas at risk of flooding
Urbanised areas have increased amounts of impermeable surfaces and so become more at risk of surface water flooding. Water is unable to infiltrate soil so it flows overland.
why are small basins at risk of flooding
Small basins especially in semi arid or arid areas are subject to flash flooding due to short lag times (minutes or hours). Infiltration of semi permeable sloped surfaces is limited allowing surface overland flow to develop rapidly.
why are low lying parts of flood plains and river estuaries at risk of flooding
Low-lying parts of flood plains and river estuaries are not just prone to river flooding but also to groundwater flooding due to the ground becoming saturated after heavy rainfall.
deforestation
reduces interception and evapotranspiration, resulting in greater volumes and rates of surface runoff, which ensures that precipitation reaches river channels faster, creating more flashy hydrographs. Furthermore, deforestation and intensive crop-growing expose soil to greater rates of erosion, increasing river sediment load and deposition within channels: this reduces the capacity of the river to carry water and increases the likelihood of flooding. Eg. deforestation in Nepal and Tibet is known to be increasing the frequency and magnitude of floods in Bangladesh from the Ganges and Brahmaputra rivers.
urbanisation
Urbanisation causes a number of changes that also increase flood risk. Eg, the expansion of impermeable surfaces such as roads, roofs and patios increase the rate of the surface runoff into rivers via the urban drainage system. River lag times are shortened by urban drainage systems, which aim to transfer water efficiently into watercourses so that streets do not flood or have standing water. In addition, bridges and culverts - underground channels designed to divert river water under infrastructure such as roads - often reduce river capacity.
floodplain drainage
Floodplain drainage is common in developed countries to provide land for agriculture and to expand urban areas. The drainage process reduces the natural storage capacity of the floodplain, especially where natural wetlands are lost. The land may shrink as it dries out, getting lower and thus even more susceptible to flooding.
flood mismanagment
Flood mismanagement - alterations at one point in a drainage basin can cause negative consequences further downstream. In most cases, flood management by hard engineering is designed to reduce the frequency of floods. Hard engineering includes the building of embankments (artificial levees) to increase channel capacity, but it may simply operate by transferring the discharge to unprotected areas or choke (narrow) points downstream.
