Factors affecting Runoff, Hydrograph and Water Cycle Flashcards
Name two types of Hydrographs
. Flashy Hydrograph
Subdued Hydrograph
Characteristics of Flashy Hydrograph
. Rapid Surface Runoff . Short Lag time . High Peak Discharge . Steep Rising and falling Limb . Roughly Symmetrical Rising and Falling Limb . High Flood Risk
- A drainage basin may only show SOME of these characteristics.
- use the word FLASHY in test. Also describe what causes FLASHY Hydrograph
Characteristics of Subdued Hydrograph
. Slower Surface Runoff
. Longer Lag time
. Shorter Peak Discharge
. Shallower / Gentler Rising and falling Limb
. Rising and Falling tends to be asymmetrical
. Low flood Risk
- A drainage basin may only show SOME of these characteristics
- TRY to use the word SUBDUED in test. Also describe what causes SUBDUED Hydrograph
Name the Physical Factors that affect the hydrograph
. Shape of Drainage Basin . Steepness of Drainage Basin . Drainage Density . Antecedent Conditions . Soil or Rock Type . Vegetation . Quantity and Intensity and type of Precipitation . Water Abstraction
Explain how the ‘ Shape of Drainage Basin’ affects the hydrograph
1.) Drainage Basins that are more CIRCULAR in shape lead to more flashy hydrographs
. This is as for circular drainage basins, each point in the drainage basin is roughly equidistant from the measuring point of discharge on a river. This means that all the water flows into the the river at roughly the same time. This means a higher peak discharge.
.Drainage Basins that are less circular in shape and more long and thin will have more subdued hydrographs.
. This is as each point is not as equidistant from the measuring point of discharge on a river. This means that all water will not flow into the river at the same time resulting in a shorter peak discharge. Additionally, for drainage basins are long, water will have to flow a longer distances to the river meaning that there is a longer lag.
2.) Large drainage basins catch more precipitation resulting in a higher peak discharge. However, they have a longer lag time because precipitation has to travel further
Smaller drainage basins catch less precipitation resulting a smaller peak discharge. However, they have a shorter lag time because precipitation doesn’t have to travel as far.
Explain how ‘steepness of a drainage basin’ affects the hydrograph
. Drainage Basins with steep sides tend to have flashier hydrographs than gently sloped river basins
. The reasons for this is both overflow and throughflow are faster on slopes as gravity is stronger, meaning that water flows faster. Additionally, on a slope, water has less time to infiltrate into to soil meaning that less infiltration , hence throughflow occurs and high amounts of overland flow. Overland flow is faster than throughflow.
. This reduces the lag time and hence increase the peak discharge. This means that the hydrograph will be flashy
Explain how ‘drainage basin density’ affects the hydrograph.
. A drainage basin with a higher drainage density means that they have a lot tributaries to the main river.
. A higher drainage density means a flashy hydrograph
. This is as the lag time is reduced, as there is more tributaries flowing to the river. This hence increases the peak discharge
Explain how ‘antecedent conditions’ affects the hydrograph
. If the drainage basin is already saturated by antecedent rainfall, then overland flow increases, since the infiltration capacity of the soil has been reached.
. Overland flow tends to be the faster flow due to less obstacles on the surface, the lag time is reduced.
. There will be a shorter lag time due to faster flow of water on the surface, and hence a larger peak discharge
. The result is a flashy hydrograph will steep rising and falling limbs, high peak discharge and short lag time
Explain how ‘ Soil or Rock Type’ can affect the hydrograph
. If the soil or rock type within the river basin is impermeable, overland flow will be higher. Overland flow is the fastest flow, due to less obstacles on the surface
. For example, clay or shale rock is impermeable
. Infiltration and hence throughflow into the soil will be prevented due to the impermeable soil and rock meaning a flashy hydrograph
. Impermeable rock or soil means water cannot flow through it or be stored.
. Additionally, overland flow can occur if the soil is hardened during HOT weather or if the soil is frozen during COLD weather
. On the other hand, drainage basins underlain by porous or permeable rock and soil will have a SUBDUED hydrograph. This is as the water soaks into the porous rock or soil. Infiltration and hence throughflow in soil is significantly slower than overland flow, due to more obstacles in the soil.
. Sandstone is an example of a porous rock
Explain how ‘vegetation’ affects the hydrograph
(* Vegetation flashcard; deforestation and afforestation flashcards and agriculture ALL link together so COMBINE information)
. The thicker and denser the vegetation in a drainage basin, the larger the interception of precipitation
. This slows movement of rainwater to the ground (throughflow) meaning there is less overland flow, less groundwater flow and less throughflow into the river
. Water is also lost due to EVAPORATION and TRANSPIRATION from the vegetation surfaces reducing the amount water flow into the river.
. This subdues the storm hydrograph, increasing the lag time and reducing the peak discharge
Explain how ‘Quantity, Intensity and type of precipitation’ affects storm hydrographs
1.) Heavy intense storms with a large quantity of water entering the drainage basin over a short time increases the peak discharge.
. This is as large amount of rainfall quickly saturated the ground to its field capacity and no more water can infiltrate the soil, increasing surface run off.
. Therefore, storm events are less effective are RECHARGING water stores than prolonged rainfall
2.) Type of precipitation also affects storm hydrograph. The lag time will be greater if the precipitation is snow rather than rain. This is as snow takes time to melt before the water enters the river channel.
. However, when there is rapid melting of snow at the same time, the peak discharge could possibly be high
Name the human factors that affects the storm Hydrograph
. Deforestation and afforestation
. Agriculture
. Urbanisation and growth of other impermeable surfaces
Explain how ‘Deforestation and Afforestation’ affects the hydrograph
(* Vegetation flashcard; deforestation and afforestation flashcards and agriculture ALL link together so combine information)
1.) Deforestation reduces interception rates allowing rainwater to hit the surface directly increasing throughflow and overland flow
. Since there is a lack of vegetation roots, it means a lack macro and micro channels in the soil. This means the infiltration rates, hence throughflow in soil won’t be as high, since there is more obstacles in the form of soil for the water.
. This leads to rapid overland flow and flashy hydrographs with short lag time and high peak discharge
. Additionally, deforestation exposes the soil to greater rates of erosion. The erosion of soil results in an increase in sediment in the river, which can cause the river to rise in height. This REDUCES the bankfull capacity of a river and can lead to a greater chance of flooding.
2.) Afforestation has the opposite effect making it a useful flood prevention measure. It will lead to a subdued hydrograph with a longer lag time and smaller peak discharge.
KEY THING TO REMEMBER
. If there is lots of infiltration and hence throughflow into the soil, it may result in a subdued hydrograph
. If there is overland flow on the surface, it will result in a flashy hydrograph
Explain how ‘agriculture’ affects the hydrograph
(* Vegetation flashcard; deforestation and afforestation flashcards and agriculture ALL link together so combine information)
1.) Ploughing breaks up the topsoil allowing greater infiltration, hence allowing more throughflow. Since throughflow is slower than overland flow, this subdues the hydrograph, resulting in a longer lag time and hence a smaller peak discharge.
However, contour ploughing is where furrows are created that run down a slope. This can act as small stream channels, where water flows faster due to gravity. This will lead to flashier hydrographs with a short lag time and hence a high peak discharge
Additionally, if wet soils are ploughed, it can cause impermeable smears in the subsoil called PLOUGH PANS. This reduces the rate of infiltration and percolation, hence reducing groundwater flow and
throughflow. This leads to increase rates of overflow, which leads to faster flow into the river, resulting in a flashy hydrograph with a short lag time and hence a high peak discharge
2.) Growing crops leads to a subdued hydrograph. Crops intercept precipitation, which leads to a LONGER lag time and hence a smaller peak discharge with a shallow rising limb. The reason for the LONGER lag time and SHORTER peak discharge is that water stored on the crops as INTERCEPTION storage is either absorbed by the plant (leading to vegetation storage) or falls slowly to the surface as throughfall.
Additionally, the roots of the crops create macro and micro channels in the soil, which increases the rate of infiltration and hence throughflow in the soil. This is as its easier for the water to flow, due to less obstacles in the soil. An increase in throughflow means less overland flow. Since throughflow in the soil is slower than overland flow, due to less obstacles or the surface, the hydrograph will be subdued with a longer lag time and hence a shorter peak discharge.
- ) Terracing hill slides slopes also leads to subdued hydrographs
- ) Irrigation causes runoff if infiltration capacity of soil is reached
- ) Pastoral farming can trample the ground, decreasing infiltration and increasing overland flow
