The hydrological cycle influences water budgets and river systems at a local scale

Water Budgets

Balance between precipitation, evaporation and run-off can be useful at global, regional and local scales

Available Soil Water

Amount of water that can be stored in the soil and is available for growing crops

• At certain times of the year there may be a shortage of water but at other times a surplus
• At a local scale water budgets show the annual balance between inputs (precipitation) and outputs (EVT) Water balance equation:

$$P = Q + E ± S$$

• P ⇒ Precipitation, Q ⇒ Discharge
• E ⇒ Evapotranspiration, S ⇒ Changes in storage River regimes

Water Availability

River regime

Annual variation in discharge or flow of a river at a particular point ir gauging station, usually measured in cumecs

River system

To the entire drainage basin, including all the tributaries connected to it

The Character of a Regime of the Resulting Stream or River is Influenced by Several Variable Factors:

• The size of the river and where discharge measurements are taken along its course
• The amount, seasonality and intensity of the precipitation
• The temperatures, with possible meltwater and high rates of evaporation in the summer
• The geology and soils particularly their permeability and porosity; groundwater noted in permeable rocks is gradually released into the river as base flow
• The type of vegetation cover: wetlands can hold water and release it slowly into the river
• Human activities aimed at regulating a river's discharge (dam building)

River Levels in Northern Areas Often Higher Due to Snow Melt

Winters warmer than -8C are making snow disappear - and threatening Europe's water supplies

Storm Hydrographs

↳ Shows the variation of discharge within a short period of time (usually a singular storm)

• Rate of flow against time past a specific point in river
• Before storm starts, main supply of water to river/stream = groundwater/base flow
• As storm develops, water goes to stream via various methods (infiltration, throughflow, overland flow)

Main Features

• Once rainfall input begins, discharge starts to rise - shown on rising limb

Rising Limb

Part of a storm hydrograph in which the discharge starts to rise

• Peak discharge eventually reached as water takes time to move through system to the gauging station of the basin

Peak Discharge

Time when river reaches its highest flow

• Time interval between peak rainfall and peak discharge = lag time
• Once storm input has ceased the amount of water in the river starts to ↓ (shown by recessional limb)

Recessional Limb

Part of the storm hydrograph in which the discharge starts to ↓

• Eventually the discharge returns to base flow

Base Flow

The normal day-to-day discharge of the river

• Peak rainfall → Part of a storm hydrograph where the water in the river decreases

Different storm hydrographs

• Shape of a storm hydrograph of the same river may vary from one rain event to the next
  • Variation closely linked to the nature of the rainfall event
• Shape of hydrograph also varies from one river to another
• Flashy hydrographs ⇒ Hydrographs with very steep limbs, especially rising limbs, a high peak discharge and a short lag time

Impact of Urbanisation on Hydrological Processes

Most sig factor that leads to ↑ flood risk

• Building activity leads to clearing of vegetation - exposes soil and ↑ overland Piles of disturbed and dumped soil ↑ erodibility. Eventually bare soil is replaced by a concrete and tarmac (both impermeable)

• High density of buildings means that rain falls onto roofs and is then swiftly dispatched into drains by gutters

• Drains and sewers built → ↓ distance storm water must travel before reaching a channel

• Urban rivers → Tend to be channelised with embankments to guard against flooding

• Bridges can restrain the free discharge of floodwaters and act as local dams for upstream floods

  • In extreme weather events urban areas are highly vulnerable

Factors that Affect the Type of Storm Hydrograph

Factor	Flashy river	Flat river
Characteristics of hydrograph	● Short lag time ● High peak ● Steep rising limb	● Long lag time ● Low peak ● Gently sloping rising limb
Climate	● Intense storm which exceeds the infiltration capacity of the soil ● Rapid snow melt as temperatures suddenly rise above zero ● Low evaporation rates due to low temperatures	● Steady rainfall which is below infiltration capacity of soil ● Slow snow melt as temperatures gradually rise above zero ● High evaporation rates due to high temperatures
Rock type	Impermeable rocks Eg. Granite	Permeable rocks Eg. Limestone
Soils	Low infiltration (eg. clay based soil)	High infiltration rates (eg. sandy soil)
Relief	High, steep slopes which promote surface runoff	Low, gentle slopes that allow infiltration and percolation
Basin size	Small	Large
Drainage density	High drainage density • more streams and rivers per unit area	Low drainage density • less streams and rivers per unit area
Vegetation	Bare • low levels of interception	Dense • higher levels of interception
Antecedent conditions	Basin already wet from a previous rain, high water table	Basin dry, low water table, unsaturated soil so high infiltration
Human activity	● Urbanisation producing impermeable concrete and tarmac surfaces ● Deforestation reduces interception ● Arable land	● Low population density with few impermeable surfaces ● Reforestation increases interception ● Pastoral and forested land