2.1.5 Deficit within the water cycle

What is a deficit within the water cycle

When water outputs exceed inputs causing a decline in water stored in soils rivers lakes vegetation and groundwater

What are the main meteorological causes of water cycle deficit

Seasonal variation prolonged drought and long term climate change

How does seasonal variation cause water deficits

Dry seasons reduce precipitation while higher temperatures increase evapotranspiration lowering soil moisture and river flow

How does climate change increase water cycle deficits

By increasing temperatures drought frequency and evapotranspiration while reducing effective rainfall and recharge

What happens to soil moisture during a water deficit

Soil moisture declines reducing vegetation growth and infiltration capacity

How do rivers respond to prolonged deficits

River discharge decreases channels may dry up and baseflow is reduced

How are groundwater stores affected by deficits

Recharge declines abstraction continues and water tables fall

What is a meteorological drought

A prolonged period of below average rainfall caused by atmospheric conditions

What caused the UK 1976 drought

Persistent high pressure systems and a northward shifted jet stream blocking Atlantic rainfall

When did the UK 1976 drought begin

Autumn 1975 with rainfall significantly below average until summer 1976

How long did the rainfall deficit last before the 1976 drought peaked

Around 16 consecutive months of below average rainfall

Why were antecedent conditions important in the 1976 drought

Dry conditions in 1975 depleted soil moisture and groundwater before 1976 began

How did evapotranspiration affect the 1976 drought

High temperatures increased evapotranspiration worsening the water deficit

What happened to soil moisture in summer 1976

It fell below 20 percent in parts of southern England

How were river flows affected in 1976

Many rivers reached record low flows including the Thames

What was the Thames discharge in July 1976

Around 1.5 cubic metres per second compared to a normal 5.5

How were groundwater levels affected in 1976

Aquifer recharge was minimal and water tables fell several metres below average

What saw impacts did reservoirs face in 1976

Reservoir levels fell below 30 percent in some areas

What human impacts occurred during the 1976 drought

Hosepipe bans standpipes drought orders and appointment of a Minister for Drought

Why did flooding occur after the 1976 drought ended

Hardened dry soils caused infiltration excess overland flow during intense rainfall

What is the London Aquifer

A major chalk and Thanet Sands aquifer supplying groundwater to London

Where does recharge of the London Aquifer mainly occur

Chalk outcrops in the North Downs and Chilterns

When does most recharge of the London Aquifer occur

Winter and early spring when precipitation exceeds evapotranspiration

How do dry summers affect London Aquifer recharge

High evapotranspiration prevents water from reaching the saturated zone

How did the 2010 to 2012 drought affect the London Aquifer

Two dry winters reduced recharge causing groundwater levels to fall 10 to 15 metres below normal

How does urbanisation reduce aquifer recharge

Impermeable surfaces prevent infiltration and increase runoff

How does climate change threaten future London Aquifer recharge

Hotter drier summers and intense rainfall reduce effective infiltration

What is GARDIT

A groundwater management strategy to control London Aquifer levels

Why was GARDIT introduced

To prevent rising groundwater from flooding underground infrastructure

How does GARDIT work

By regulated abstraction monitoring boreholes and maintaining target groundwater levels

How successful has GARDIT been

It has stabilised groundwater levels for over two decades

What challenges threaten GARDIT in the future

Climate change population growth and reduced recharge

Why is the Amazon rainforest important to the water cycle

It recycles moisture through evapotranspiration generating up to half its own rainfall

How has climate change affected the Amazon water cycle

Longer dry seasons higher temperatures and reduced rainfall reliability

What happened to soil moisture in Amazon droughts

Soil moisture fell 20 to 30 percent below average

What feedback loop worsens Amazon droughts

Reduced evapotranspiration lowers rainfall intensifying drought

How have rivers in the Amazon been affected by drought

River discharge has fallen by up to 50 percent in some tributaries

What ecological impacts have Amazon deficits caused

Tree mortality reduced biodiversity and weakened carbon sequestration

What is a tipping point risk in the Amazon

Transition from rainforest to savannah due to sustained deficits

What is the Aral Sea example of

Human induced water cycle deficit due to over abstraction

What caused the Aral Sea deficit

Diversion of the Amu Darya and Syr Darya for irrigation

How much inflow to the Aral Sea was diverted

Over 90 percent by the 1980s

How much has the Aral Sea shrunk

Over 90 percent of its original surface area

What environmental impacts resulted from the Aral Sea deficit

Salinisation toxic dust storms biodiversity collapse and climate extremes

What does the Aral Sea show about water cycle deficits

That human management can rapidly and permanently disrupt water stores

What is over abstraction

Withdrawal of water faster than natural recharge rates

How does over abstraction cause water deficits

It lowers groundwater tables reduces river baseflow and depletes surface stores

Where is over abstraction a major issue globally

South Asia the Aral Sea basin and the Colorado River Basin

Why is sustainable water management important

To prevent long term deficits ecosystem damage and water insecurity