EQ2: What factors influence the hydrological system over the short & long term? GapFill

The definition of drought varies from place to place.  The broadest definition is a period with less precipitation than the average, lasting from a few  weekshoursdaysseconds to several years or even decades.  Droughts can have severe socio-economic and environmental impacts, can occur slowly over many years and are influenced by  plantssnowfallsoilclimate change.

There are two basic classifications:

• Meteorological drought – reduced  groundwater storesprecipitationfloodinginterception (e.g. rainfall and snowfall).
• Hydrological drought – reduced  hosepipe usesea levelriver flowflooding and groundwater.

In the UK, we sometimes experience meteorological droughts in the  springwintersummerearly spring.  They may last for several weeks and are associated with hot, dry, sunny weather.  They are caused by blocking high-pressure weather systems called summer  depressionscycloneswetlandsanticyclones.  The hot conditions mean that evaporation potential is high.

ENSO cycles occur in the  ArcticSouthernAtlanticPacific Ocean – but their effects are wide-reaching, changing weather conditions around the world, such as floods and droughts, and extremes in temperature (we call these effects teleconnections).  They are caused by changes in the  CO2 absorptiondepthcolourtemperature of the sea surface, caused by the direction or strength of the trade winds.

After periods of low rainfall,  floodmeterologicallong-termhydrological drought becomes an issue.  This is when streamflow decreases and groundwater supplies are diminished, lowering the water table – so base flow is also reduced.  These droughts affect the environment, such as habitat loss in  grasslandtundrathe UKwetlands, while forest fire risk also increases.

 ENSOAnticycloneDroughtFlooding is a surplus of water within the hydrological cycle, meaning that the land’s surface is submerged.  Sometimes, due to heavy rainfall, rivers burst their banks onto their floodplains.  Humans can increase flood risk through  anticyclonesfloodplain restorationurbanisationafforestation and land-use change, and natural factors also increase the risk.

In the UK, we experience storms – they are periods of wet and windy weather that we call  anticyclonesdepressionstrade windsflood weather or cyclones.  They often occur in the autumn and at the start of the winter.

Very intense precipitation can cause sudden floods called  depressionrainantecedentflash flooding – this often occurs in arid environments where the soil is very dry, and in steep catchments where the water is channelled through a stream.

The ability of the soil to absorb water is affected by the intensity of the rainfall and by prior weather conditions.  Intense rainfall cannot infiltrate the soil fast enough (infiltration excess  fluvialoverlandovergroundunderground flow).  If it has rained a lot recently, such as a succession of storms when the jet stream gets stuck over the UK, there is little air space left in the soil for new water (saturated overland flow).