1.2 Discharge relationships within drainage basins

Define discharge

The volume of water passing a certain point in a river at a particular time.

Calculation for discharge

Discharge (m3/second) = velocity (m/s) x cross section area (m2)

What is a storm hydrograph?

It is a graph that shows how the discharge of a river changes over time after a storm. At a particular point on a river

Hydrographs:

• Definition of rising limb

• Definition of descending limb

• The period during which discharge increases

• The period during which discharge decreases

Hydrographs:

• Definition of lag time

The time period between the end of the rainfall and the peak of the discharge

Hydrographs:

• Definition of baseflow

The normal discharge of the river

Hydrographs:

Definition of peak flow

The maximum discharge in the river

Physical factors affecting discharge

• Basin size, shape, relief

• Rock/soil type

• Drainage area and drainage pattern

• Surface water stores

Size of drainage basin:

• Effect on the shape of the hydrograph

• Reasons

• Small basin = Shorter lag time and higher peak

  Large basin = longer lag time and lower peak

• Water reaches the channel more rapidly in a smaller basin as shorter distance to travel

Shape of drainage basin:

• Effect on the shape of the hydrograph

• Reasons

• Circular basin = shorter lag time and higher peak

  Elongated basin = longer lag time and lower peak

• Less time for water to reach the channel in circular basin as all the boundaries are an equal distance from the channel

Relief:

• Effect on the shape of the hydrograph

• Reasons

• Steeper basin = shorter lag time, higher peak, steep rising and falling limb

• Water reaches channel faster as it travels quickly downhill

Permeability of rock type:

• Effect on the shape of the hydrograph

• Reasons

• Impermeable rock = shorter lag time, higher peak

• Impermeable rocks are saturated faster than porous and pervious rocks. Saturation-excess overland flow is more common

Soil texture and structure:

• Effect on the shape of the hydrograph

• Reasons

• Sandy soil = slower lag time, lower peak

• Larger pore spaces in sandy soils than clay soils. Infiltration is most rapid in sandy spaces

Drainage area and drainage pattern:

• Effect on the shape of the hydrograph

• Reasons

• higher density, faster water reaches the channel = short lag times, high peak discharges

• Drainage density is the number of surface streams in a drainage basin. High densities found in areas of impermeable rock and clays

Surface water stores:

• Effect on the shape of the hydrograph

• Reasons

• Higher river discharge

• Lakes and glaciers could melt during a warm weather spell and more water would enter the river

Vegetation:

• Effect on the shape of the hydrograph

• Reasons

• Forested basin = slower lag time, lower peak

• Vegetation intercepts a large proportion of rainfall. Deciduous trees cause higher discharge in a forested basin in winter as less interception

Meteorological Characteristics

• Type of precipitation

• Rainfall intensity, amount and duration

• Distribution of rainfall across the drainage basin

• Prior precipitation/soil moisture

• Factors affecting evapotranspiration

Length of precipitation event:

• Effect on the shape of the hydrograph

• Reasons

• After prolonged precipitation = shorter lag time and higher peak

• Ground is saturated in water = more saturation → excess overland flow.

Intensity of precipitation:

• Effect on the shape of the hydrograph

• Reasons

• Intense rainfall = shorter lag time and higher peak

• Rain is falling faster than infiltration rate so there is infiltration-excess overland flow

Type of precipitation:

• Effect on the shape of the hydrograph

• Reasons

• After snowfall = slower lag time, lower peak

• Snow does not reach channel but is stored on ground surface. As snow melts the meltwater will reach the channel quickly as infiltration is prevented if the ground is still frozen.

Factors affecting evapotranspiration:

• Effect on the shape of the hydrograph

• Reasons

• High temps increase rate of evapotranspiration = reduce river discharge levels

• Cool temps freeze the ground, restrict infiltration, increase surface run-off = high river discharge levels

• Other factors (humidity) influence rates of evapotranspiration

Human Characteristics

• Urbanisation (land use)

• Deforestation

Urbanisation:

• Effect on the shape of the hydrograph

• Reasons

• Urbanised catchment = faster lag time, higher peak

• Higher surface run-off in urban areas as more urban surfaces and sewers take water rapidly to rivers

Deforestation:

• Effect on the shape of the hydrograph

• Reasons

• Deforested catchment = faster lag time, higher peak

• Less interception and evapotranspiration, more surface runoff in a deforested catchment

PRACTICE QUESTION

1a) Match the graph with the drainage basin

A = X

B = Z

C = W

D = Y

Calculation for drainage basin discharge

Drainage basin discharge = precipitation - (evaporation + transpiration) +/- changes in store

Hydrographs:

- What is direct channel input?

- What is antecedent discharge?

A very small amount of rainfall that falls on the river itself.

The level of discharge before the storm (summer = small, winter = large)

Explain how water gets to the river using the flows

Rain begins:

• Most discharge will reach channel by BASEFLOW

• As discharge increases, a larger proportion is THROUGHFLOW

• Near to peak discharge, a larger proportion of water is OVERLAND FLOW

• After peak discharge has passed, the descending limb is less steep because discharge no longer contains SURFACE RUNOFF

• The discharge returns to baseflow level.

Explain two factors that might cause variations in river discharge throughout the year (5)

Glacial melt:

• river will be frozen in cold temperatures so river discharge will be low but once it unfreezes it will increase

Vegetation:

• summer there is more vegetation which decreases river discharge as it intercepts the rain. less vegetation = less interception and transpiration = soil becomes saturated quicker

Farming seasons:

• more crops in farming season more vegetation = decrease river discharge = more interception and transpiration

What are river regimes?

the annual pattern of discharge by a river responding to the region’s climate.

What factors affect river regimes?

• Temperatures

• Rainfall

• Evaporation

• Morphology of basin

• Vegetation

• Infiltration capacity of rock

Two types of variations that can be produced by changes in discharge

• Temporal Variations (over time):

  • Short-term fluctuations e.g. due to heavy rainfall

  • Medium-term fluctuations e.g. seasonal variations

  • Long-term fluctuations e.g. climate change or change in land use

• Spatial Variations (from place to place)

  • Changes geographically due to climate and factors affecting discharge

Definition of soil moisture recharge

Occurs when precipitation exceeds potential evapotranspiration

There is some refilling of water in the dried up pores of the soil

Definition of soil moisture surplus

the period when soil is saturated and water cannot enter, and so flows over the surface

Definition of soil moisture utilisation

the process by which water is drawn to the surface through capillary action

Definition of soil moisture deficit

the degree to which soil moisture falls below field capacity

GRAPH TO LEARN