Hydrographs Flashcards
Time period
Ranges for hours to day to weeks to months and even years
Hydrograph
It shows the variation of a river discharge at a given point in the river over a specific period of time
River regime
Aka annual hydrograph
Variability of a river’s discharge in the course of a river and it’s response to ppt, DB characteristic, temp and vegetation
Baseflow
The starting and finishing line of discharge which is a river’s discharge produced when groundwater slowly seeps into the river bed
Main contributor of a river’s flow during dry seasons or periods of little or no ppt
Usually a constant amount unless following periods of intense rainfall or rapid snowmelt
Amount of baseflow depends on permeability of overlying rock, amt of percolation and infiltration
Lower magnitude than surface overland flow as low velocity does not allow it to fluctuate
Rising limb
Limb to reach peak discharge and indicates the river’s highest flow in the channel
Steepness depends on amount of SR that occurs and also the intensity and duration of the storm
Time lag
Delay between peak rainfall and peak discharge
Takes time for water to travel from where it hits the ground to the gauging station, resulting in the time lag
It is calculated by subtracting the time taken to reach peak rainfall from time take to reach peak discharge
Varies from minutes to day and even week and months depending on Db, ppt, temp and vegetation
Recession limb
Measures the rate at which discharge decreases; higher the rate, the steeper the limb
Produced mainly by water flowing through the soil into channel from baseflow
When SR ceases to flow, water will continuer to flow in the form of baseflow and the amount of baseflow determines the steepness of the limb
Ghee percolation and infiltration contributes to more baseflow and hence steep per limb
Before rainfall
Discharge is mainly baseflow
This amount has been ceasing since the last rainfall
Onset and beginning of rain
Infiltration into the soil is high (time before steep rising limb indicates before IC reached)
Large part of ppt enters the soil surface(infiltration) and into the soil (percolation) to add to lateral flows underground (baseflow and through flow)
As storm continues
Overland flow begins as IC is reached and exceeded
Discharge rises sharply to a peak with contribution from both SR and throughflow
The steeper the rising limb, the faster the response to rainfall
Peak discharge occurs when the river reaches the highest level. Time between peak rainfall and peak discharge known time lag
Nearing the end of a storm
Discharge generally decreases -> recession limb
However ReL is gentler the RiL as RiL is mainly contributed by Sr while ReL is by throughflow and baseflow
After the storm
Water continues to flow as baseflow until WT drops below channel bed and channel will cease to flow
Factors affecting hydrography
DB characteristics: steepness, shape, size and density
Amount of water already present
Rock type
Soil type
Vegetation
Precipitation
Temperature
Human activity
Dab characteristics
Size: large Db can catch more ppt, so they have a higher peak discharge compared tow smaller basin. But smaller basin have a shorter lag time as ppt has less distance to travel
Steepness: steep sided have a shorter lag time as water flows more quickly downhill
Shape: circular basins tend to have a more flashy hydrograph than a long narrow basin. This is so us all points in the water shed is roughly the same distance from the point of discharge management
Density: basins with more streams have a higher drainage density as they can drain quicker and have a shorter lag time
Amount of water already present
If soil is already waterlogged, the soil cannot absorb anymore moisture
Infiltration decreases and SR Increaes
SR is faster than baseflow/throughflow so rainwater reached the river quicker and hence shorter lag time
Rock type
Impermeable rocks do not store water or allow water to pass through
This reduce infiltration and increase SR and decreases lag time
Peak discharge increases as more water reaches the river
Soil type
Sandy soils allow a lot of infiltration but clay soils have lower infiltration
Low infil, higher SR, lower lag time and higher peak discharge
Vegetation
Intercepts rainfall and slows its movement to the river channel and hence increases lag time
Interception Is highest when there is dense vegetation and deciduous trees
The more vegetation, more water is lost thru transpiration and evap even before reaches the river channel and reduce peak discharge
Precipitation
Intense rainfall will generate more ppt so there is a greater peak discharge than light rainfall
Type of rainfall also affects lag time
Snow melt longer lag time
Temperature
Hot dry and freezing conditions results in hard gourd, reduces infil, Increaes SR, reduces lag time and Increaes peak discharge
High temp can increase eavpotranspiration, reducing peak discharge
Human activity
In urban areas, large amounts if soil, is covered by impermeable substance ( concrete) . water cannot infiltrate, Infil reduces, SR increases, lag time reduce and peak discharge Increaes
Man made D systems can also affect graphs as water flows down the drain into river, it can evap, or infiltrate the soil, lag time Increaes and peak discharge decreases
