The Water Cycle

What is the water cycle?

Shows how water moves around our planet in a cycle between the air, land and sea

The Drainage Basin Hydrological Cycle diagram

And again but a diff one

Drainage basin

The area of land drained by a river and its tributaries

Throughfall (leaf drip)

Water that drips off leaves during a rainstorm

Interception

Where precipitation is prevented from reaching the soil by leaves and branches

Runoff

All the water that flows out of a drainage basin

Infiltration

The movement of water from the surface downwards into the soil

Groundwater flow

The movement of groundwater. This is the slowest movement of water. Sometimes called baseflow.

Overland flow (surface runoff)

The movement of water over saturated or impermeable land

Throughflow

Water that moves downslope through soil

Groundwater store

Water that collects underground in pore spaces in rock

Stemflow

Water that runs down the stems and trunks of plants and trees to the ground

Percolation

The downward movement of water from soil to the rock below

Evaporation

• Where water changes from a liquid to a gas

• Heat from the sun drives the process

• Most evaporation is from the seas and oceans

Evapotranspiration

• The combination of evaporation and transpiration (water lost from plants)

• Water can be lost from the soil even during dry periods as plants continue to draw water from deep below the surface with long roots

Condensation and cloud formation

• Where water vapour is changed back into liquid water

• Caused by a drop in temperature (dew point)

• Clouds form as a result

• When condensation happens near the ground, fog is formed

Precipitation

• Rain

• For precipitation to happen, droplets need to condense on even smaller dust, salt or smoke particles

What is the Bergeron-Findeisen process?

Where droplets of water condense around ice crystals and fall as either snow or rain

Sublimation/desublimation

• Sublimation = when ice turns into water vapour (without becoming water in between stages)

• Desublimation is the opposite - where water vapour becomes snowflakes or frost

• Happens more at high altitude in conditions with low humidity and dry winds

Snowmelt

• Contributes a lot to the global movement of water

• Melting snow is responsible for feeding much of the water into some of the worlds major rivers, e.g. snowmelt from the Himalayas feeding the river Ganges

• In colder climates, it can play a direct part in water availability

Cryospheric processes

• Glaciers change according to the seasons

• In winter, snow accumulates and the glacier advances

• In summer, snow melts (ablates) and the glacier retreats

• The balance is called the glacial budget

What is the water balance?

The balance between inputs and outputs of water in a river

Water budget graph

What is meant by a rivers regime?

The annual variation in a rivers discharge

What does the water budget graph show?

A rivers regime of the period of a year

What happens when precipitation exceeds evaporation?

Water surplus will occur or recharge, depending the time

What happens when evapotranspiration exceeds precipitation?

Water deficit will occur

What is the water balance equation?

precipitation = streamflow + evapotranspiration ± changes in storage

What is a hydrograph?

A graph that shows how the water flow in a drainage basin (particularly river runoff) responds to a period of rain

What is river discharge?

The volume of water which flows through it in a given time. It is usually measured in cubic metres per second (cumecs).

How do you measure the river discharge?

• Calculate the velocity of the water passing by

• Calculate the cross-sectional area of the river

• Discharge = velocity x cross-sectional area

Draw and label a flood hydrograph

Define lag time

The time between peak rainfall and peak discharge

Factors that affect the shape of hydrographs: Precipitation

The most important factors controlling infiltration are the amount and characteristics (intensity and duration) of precipitation that falls as rain or snow

Factors that affect the shape of hydrographs: Soil characteristics

Some soils, such as clays, absorb less water at a slower rate than sandy soils. Soils absorbing less water result in more overland flow into streams.

Factors that affect the shape of hydrographs: Soil saturation

As with a wet sponge, soil already saturated from previous rainfall (antecedent rainfall) cannot absorb much more. Thus more rainfall will become overland flow.

Factors that affect the shape of hydrographs: Land cover

Some land covers have a great impact on infiltration and overland flow. Vegetation can slow the movement of surface flow, allowing more time for it to seep into the ground. Impervious surfaces, such as parking areas, roads and urban developments act as a “fast lane” for rainfall - taking water into storm drains that drain directly into streams. Agriculture and the tillage of land also change the infiltration patterns of a landscape.

Factors that affect the shape of hydrographs: Slope of the land

Water falling on steeply sloped land runs off more quickly and infiltrates less than water falling on flat land

Factors that affect the shape of hydrographs: Evapotranspiration

Some infiltration stays near the land surface, which is where plants put down their roots. Plants need this shallow groundwater to grow and, by the process of evapotranspiration, water is moved back into the atmosphere.

Factors that affect the shape of hydrographs: Antecedent rainfall

Rain falling on a ground surface that is already satured will produce a steep rising limb and a shorter lag time

Factors that affect the shape of hydrographs: Snowmelt

Large amounts of water are released, greatly and rapidly increasing discharge, especially if the ground surface is still frozen since this reduces infiltration

Factors that affect the shape of hydrographs: Vegetation

In summer, deciduous tress have more leaves and so interception is higher, discharge is lower and lag times are longer

Factors that affect the shape of hydrographs: Basin shape

Water takes less time to reach the river in a circular basin than an elongated one

Factors that affect the shape of hydrographs: Slope

In steep-sided drainage basins, water gets to the river more quickly than in an area of gentle slopes

Factors that affect the shape of hydrographs: Geology

Permeable rocks allow percolation to occur, which slows down the rate of transfer of water to the river. Impermeable rocks allow less percolation and have greater amounts of overland flow - hence greater discharge and shorter lag times.

Factors that affect the shape of hydrographs: Urban growth

Impermeable roofs and roads are shaped in such a way as to get rid of water quickly. When combined with a dense network of drains, this means that water gets to the river very quickly, reducing lag time and increase discharge.