3.1.1.2 The Water - *Drainage basins as open systems* + *The water balance*

What is the Water Balance

* Balance between the inputs and outputs of a drainage basin.* Expressed as: P = Q + E (+/- change in storage) P = precipitation Q = run-off E = evapotranspiration

Flows

*Flows of water between different stores* • Surface runoff :Movement of water over the Earth's surface. It is divided into sub-categories: o *Channel flow* - water flowing in a river channel. o *Overland flow* - water flowing across the land • *Infiltration*: Water-- Soil • Percolation: vertical movement through rock underground. This movement is much slower than infiltration. • Throughflow: horizontal flow of water through the soil responding to gravity or water-pressure variations. • *Stemflow*: water intercepted by vegetation drip from leaves or flow down stems and trunks to reach the ground. *Overland flow*- Rain falling faster than infiltration* 1) * Interception* 2) *Surface soil water* *Groundwater flow*- *Water flowing slowly below water table through permeable rock* *Channel flow*- *Water flowing in river or stream*

Drainage basin

*Open system*- Both energy and matter CAN enter and leave *Area surrounding the river where the rain falling on the land flows into the river- River catchment* *This Boundary of a drainage basin is the watershed- ppt falling beyond watershed enters different- drainage basin*

Outline the 'Water balance' and explain seasonal variations (6)

1) *Definition of water balance* *Refers to the balance between inputs and outputs in a water cycle system* Equation *Expressed as: P = Q + E (+/- change in storage) P = precipitation 2) Explanation of seasonal variation * When PPT exceeds Evapotranspiration- water surplus produced- Occurs during the *Winter- UK* iF Evapo exceeds= Higher tEMPS- Water deficit- Summer months (May- Aug) UK 3) *Impacts of Seasonal variation* Lead to different soil conditions Saturated soils in winter + early spring- cause overland flows + potential flooding if streamflow is high OR Drought conditions in the summer Seasonality influences *vegetation cover*= Impacts on the amount of water held in storage as well as water stored as *Snow*

Storage

•1) *Interception by vegetation* Less vegetation- Less interception-More evaporation Dense vegetation- More interception- less evaporation •2) *Absorption by vegetation*- Water take up by plants • 4)*Groundwater* Water stored in *Permeable rock underground*, Soil water •5) *Channel storage*: Water in river or stream

Inputs + outputs Outputs ( TEER)

Inputs *Input* - *Precipitation* - vary to quantity, duration, intensity and distribution over the year *Output* (3) •1)*Runoff* - Rivers entering the sea at the mouth. 2) *Evaporation* 3)*Transpiration* - Water loss- *vegetation* *Evapotranspiration* =*Evaporation + transpiration*

The Water Balance

Water Surplus: *excess water* *precipitation exceeds evapotranspiration* • Deficiency: *Reduction of water available* *Evapotranspiration exceeds precipitation* . • Recharge: *After a period of deficiency* *precipitation* *replace the lost water* in the soil.