River Discharge Flashcards
Define river discharge.
The volume of water passing a measuring point or gauging station in a riven in a given time.
What is river discharge measured in?
Cubic metres per second (cumecs)
What is river discharge affected by?
Precipitation (the more, the higher), hot weather (the higher, the lower due to evaporation) and removal of water (abstraction)
What are hydrographs?
Hydrographs show variations in a rivers discharge over a short period of time, usually during a rainstorm.
What is peak discharge?
The highest point on the graph showing when the river discharge is at its greatest.
What is lag time?
This is the delay between peak rainfall and peak discharge. This delay happens because it takes time for the rainwater to flow into the river. A shorter lag time can increase peak discharge because more water reaches the river during a shorter period of time.
What is the rising limb?
This is the part of graph up to peak discharge. The river discharge increases as rainwater flows into the river.
What is the falling limb?
This is the part of the graph after peak discharge. Discharge is decreasing because less water is flowing into the river. A shallow falling limb shows water is flowing in from stores long after it’s stopped raining.
What is the hydrograph influenced by?
Drainage basin characteristics, precipitation, amount of water already present in the drainage basin, rock type, soil type, vegetation, temperature, land use and urbanisation.
Drainage basin characteristics?
Size- Larger drainage basins can catch more precipitation, so they have higher peak discharge compared to smaller basins. Smaller basins have short lag times because precipitation has less distance to travel, so reaches the main channel quicker.
Slope angle- In steep sided upland river basins the water reaches the channels much more quickly than in gently sloping lowland river basins, producing a steeper rising limb and shorter lag times.
Shape of the basin- rainfall reaches the river more quickly from a round basin than from an elongated basin.
Amount of water already in the drainage basin?
'’Antecedent moisture’’- if the grounds already waterlogged then infiltration is reduced and surface water increases. Surface run off is much fast than through flow or base flow, so rainwater reaches the river more quickly, reducing lag time.
Rock type?
Impermeable rocks don’t store water or let water flow through them. this reduces infiltration and increases surface run off, reducing lag time. Peak discharge also increases as more water reaches the river in a shorter period.
Soil type?
Sandy/ porous soils allow a lot of infiltration, but clay soils have very low infiltration rates. Low infiltration rates increase surface runoff, reducing lag time and increasing peak discharge.
Vegetation?
Vegetation intercepts precipitation and slows its movement to the river channel, increasing lag time. Interception is highest when theres lots of vegetation.This varies with seasons. In summer there are more leaves on deciduous trees so interception rate is high and peak discharge lower The more vegetation there is in a basin, the more water is lost before it reaches the river channel, reducing peak discharge.
Precipitation?
Intense storms will generate more precipitation and so greater peak discharges than light rain showers. The type of precipitation e.g. snow thats fallen in a winter storm can melt, and the meltwater can flow into the river in spring, giving a very long lag time. (When the snow is being ‘stored’ it may not show on the hydrograph until melting occurs)
