3.1.1.2 The Water Cycle

Inputs

Precipitation, conventional/relief/frontal rainfall

Processes of moving water

Evaporation, condensation and cloud formation (driven by atmospheric circulation cells)

Causes of precipitation

Condensed water droplets become too heavy to remain suspended

Conventional rainfall

Warm air rises from solar radiation and then condenses into rain

Relief rainfall

Warm air forced up by a barrier which condenses into rain

Frontal rainfall

Warm air rises over cool air and falls as rain

Outputs

Evapotranspiration, Transpiration, Streamflow

Transpiration

Plants respire water absorbed in roots which evaporates

Streamflow

Water in the drainage basin leaving in tributaries

Flows (examples)

Infiltration, Percolation, Throughflow, Surface runoff, Groundwater flow, Streamflow, Stemflow

Factors impacting infiltration

Tree roots and precipitation rate

Factors impacting percolation

Number of fractures and permeability of rocks

Factors impacting throughflow

Clay soils have high capacity and small pores so a slow flow rate. Sandy soils have low capacity and large pores so a fast flow rate.

Stores

Soil water, Groundwater, River channel, Interception, Surface storage

Water table

Upper surface where pores are saturated

Water balance

Formula which expresses the process of water storage and transfers

Water balance equation

Precipitation = Total runoff + Evapotranspiration +- Storage

Factors affecting the water balance (Don’t stop saving urban areas)

Deforestation, Storm events, Seasonal changes, Urbanisation, Agriculture

How deforestation affects the water balance

Less interception by trees

Transpiration decrease

Soil no longer held together by roots so water storage decreases

How does storm events affect the water balance

Saturation of soil

Less effective recharging of water stores

How do seasonal changes affect the water balance

Spring has more vegetation

Summer means the ground is harder

Autumn has less vegetation and more rainfall

Winter means the ground is impermeable

How does agriculture affect the water balance

Pastoral farms trample soil and reduce infiltration

Irrigation causes groundwater depletion

Ploughing increases infiltration due to looser soils

Urbanisation

Impermeable surfaces reduce infiltration

SUDS reduce number of impermeable surfaces

Soil water budget

Annual balance of inputs/outputs which show effects on water storage and availability

What is the soil water budget dependent on excluding seasonal variation

Soil type, depth, permeability and bedrock

How does autumn affect the soil water budget

Greater precipitation than evapotranspiration means soil moisture and surplus increases

How does winter affect the soil water budget

No evapotranspiration so soil water stores refill

How does spring affect the soil water budget

Evapotranspiration increases due to photosynthesis but still a water surplus

How does summer affect the soil water budget

Utilisation of soil water as no precipitation can lead to a deficit

Major global stores

Atmosphere (0.001%), Lithosphere, Cryosphere, Oceans, Biosphere

Factors affecting global distrubution

Land/Sea, Geology, Atmospheric circulation (ITCZ create variations in precipitation), Climate change/cycles (Ice ages, 1/3 of world ice covered)

Inter - Tropical Convergence Zone (ITCZ)

Moves north/south seasonally which dicattes where intense low pressure rainfall occurs (shifts storage magnitude temporally and spatially)

Aquifers (Lithosphere)

Underground stores of water (30% of freshwater) for up to 10,000 years

Glaciers (Cryosphere)

69% of freshwater for 20-100 years

Flood hydrograph

A graph showing the discharge of a river after a flood event

Lag time

Difference between peak rainfall and discharge

Flashy hydrography

Short lag time and high peak discharge

Factors causing a flashy hydrography

Small basin, saturated soil, steep slopes, impermeable rock, urbanisation, heavy rainfall

Subdued hydrograph

Long lag time and low peak discharge

Factors causing a subdued hydrography

Large basins, permeable rocks, forested land, gentle slopes, dry soil, light rainfall