Hydrographs

Question 0

Time period

Answer 0

Ranges for hours to day to weeks to months and even years

Question 1

Hydrograph

Answer 1

It shows the variation of a river discharge at a given point in the river over a specific period of time

Question 2

River regime

Answer 2

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

Question 3

Baseflow

Answer 3

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

Question 4

Rising limb

Answer 4

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

Question 5

Time lag

Answer 5

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

Question 6

Recession limb

Answer 6

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

Question 7

Before rainfall

Answer 7

Discharge is mainly baseflow

This amount has been ceasing since the last rainfall

Question 8

Onset and beginning of rain

Answer 8

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)

Question 9

As storm continues

Answer 9

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

Question 10

Nearing the end of a storm

Answer 10

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

Question 11

After the storm

Answer 11

Water continues to flow as baseflow until WT drops below channel bed and channel will cease to flow

Question 12

Factors affecting hydrography

Answer 12

DB characteristics: steepness, shape, size and density

Amount of water already present

Rock type

Soil type

Vegetation

Precipitation

Temperature

Human activity

Question 13

Dab characteristics

Answer 13

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

Question 14

Amount of water already present

Answer 14

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

Question 15

Rock type

Answer 15

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

Question 16

Soil type

Answer 16

Sandy soils allow a lot of infiltration but clay soils have lower infiltration

Low infil, higher SR, lower lag time and higher peak discharge

Question 17

Vegetation

Answer 17

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

Question 18

Precipitation

Answer 18

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

Question 19

Temperature

Answer 19

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

Question 20

Human activity

Answer 20

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