Short power point made by AS/A Level students with the aim of explaining Storm Hydrographs and the foundations of the Drainage Basin Hydrological Cycle.

1. StormHydrographs(+DBHC)
AS Level Revision Powerpoint

2. DRAINAGEBASINHYDROLOGICALCYCLEDEFINITIONS(dbhc)
Channel Fall: Precipitation that enters a river channel directly.
Watershed : An imaginary line delimiting one drainage basin from another.
Percolation : The downed movement of water within the rock under the soil.
Interception : Process by which raindrops are prevented from directly reaching the soil surface. Branches, stems, plants, and grass growing close to the surface catch
these.
Overland flow: All the water that enters a river and eventually flows out of the drainage basin. It can be quantified by measuring the discharge of a river.
Evapotranspiration : The amount of moisture removed by the evaporation and transpiration from a vegetated land surface.
Transpiration : The process by which water is lost from a plant through the stomata.
Drainage basin : An area of land drained by a river and its tributaries.
Infiltration: The passage of water into the soil. It takes place relatively quickly at the beginning of a storm, but as the soil becomes saturated the rate falls rapidly.

3. DBHCDEFINITIONSCONTINUED.
Stemflow : The water that runs down branches and stems of plants and trees during and after rain to reach the ground.
Throughfall : The water that drips off leaves during a rainstorm. It occurs when more water falls onto the interception layer of the tree canopy then can remain on the
leaves.

4. Factorsaffecting RiverDischarge
Rock and soil type - permeable rocks and soils (such as sandy soils) absorb water easily, so surface runoff is rare. Impermeable rock and soils
(such as clay soils) are more closely packed. Rainwater cannot infiltrate, so water reaches the river more quickly. Pervious rocks (like limestone)
allow water to pass through joints, and porous rocks (like chalk) have spaces between the rock particles.
Land use - In urban areas, surfaces like roads are impermeable – water can’t soak into the ground. Instead, it runs into drains, gathers speed and
joins rainwater from other drains – eventually spilling into the river. In rural areas, ploughing up and down (instead of across) hillsides creates
channels which allow rainwater to reach rivers faster increasing discharge. Deforestation means less interception, so rain reaches the ground faster.
The ground is likely to become saturated and surface runoff will increase.
Rainfall - The amount and type of rainfall will affect a river’s discharge, antecedent rainfall is rain that has already happened. It can mean that the
ground has become saturated. Further rain will then flow as surface run-off towards the river. Heavy continuous rain, or melting snow, means more
water flowing into the river
Relief - Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate. On more gentle slopes infiltration is more
likely.
Weather conditions - Hot dry weather can bake the soil, so that when it rains the water can’t soak in. Instead, it will run off the surface, straight into
the river. High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge. Long periods of
extreme cold weather can lead to frozen ground, so that water can’t soak in.

5. TheStormHydrograph

6. STORMHYDROGRAPH-KEYWORDS
Hydrograph - A graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow.
Rising limb/appendage - The rising limb of hydrograph also known as concentration curve, reflects a prolonged increase in discharge from a catchment
area, typically in response to a rainfall event.
Falling limb/appendage - The falling limb represents the withdrawal of water from the storage built up in the basin during the earlier phases of the
hydrograph.
Peak discharge - The highest point on the hydrograph when the rate of discharge is at its greatest.
Lag time - the time interval from the center of mass of rainfall excess to the peak of the resulting hydrograph.
Discharge - the rate of flow (volume per unit time) passing a specific location in a river or other channel
Base flow - Base Flow is groundwater seepage into a stream. When the ground's water table is higher than a stream bed, the waters seeps into the
stream.
Peak Rainfall - The peak rainfall is the time of highest rainfall. The peak discharge (the time when the river reaches its highest flow) is later because it
takes time for the water to find its way to the river (lag time) .

7. Differentstormhydrographs
Flashy graphs - These graphs have lots of overland flow and
look like this:
Delayed graphs - These graphs are mostly through flow /
baseflow, and look like this:

8. Factorsaffectingthe hydrograph
The intensity and duration of the storm-If both are high they produce a steep rising limb as the infiltration
capacity of the soil is exceeded.
The antecedent rainfall-Heavy rain falling on a soil which is saturated from a previous period of wet weather
will produce a steep rising limb.
Snow-Heavy snow fall may not initially not show on a hydrograph since being stored in the snow. Water level
in the river may actually decrease during prolonged cold weather or snowfall. When temperatures increase
there is a large amount of water discharge in the river due to the snow melting, this water may take very
little time to get to the river if the ground is frozen.
Porous soil types and or permeable rocks e.g limestone-These produce less steep (flashy) hydrographs because
water is regulated more slowly through the natural systems.
Impermeable rock types- Granite, stone etc, as well as man made materials used for building like tarmac and
concrete. These increase surface run off due to no infiltration.
Size of drainage basin-A small drainage basin tends to respond more rapidly to a storm than a larger one, so
the lag time is shorter.

9. factorsaffectingthe SPECIFICfeaturesofastormhydrograph
Rising/Falling Limb - Both are heavily influenced by the peak rainfall/amount of rain
fallen, so
Baseflow - The amount of water involved in the base flow is highly dependant on what type
of rock is there. If there is impermeable rock like granite and clay, there tends to be
higher densities of surface streams. The higher the density the faster the water reaches
the main river channel, causing rapid increases in discharge.
Peak discharge - The amount of precipitation that falls during the time period can
increase or decrease the peak discharge
Peak rainfall - Completely dependant on the amount of rainfall in the hours before it.
Therefore it can be affected by the weather, so if there’s a storm, a considerable more
amount of rain will fall than if it’s just a shower.
Lag time - Influenced by the the factor of if the ground is saturated or not. Different
materials (e.g clay, tarmac) also affect the speed at which the water reaches the river