Storm Hydrograph Analysis

Question 1

what is a hyetograph?

Answer 1

a graphical representation of rainfall depth or snowmelt vs time

Question 2

what is a hydrograph?

Answer 2

a graphical representation of stream discharge vs time

Question 3

what is a rising limb?

Answer 3

represents the period when stream discharge increases in response to a rainfall or snowmelt event (in hydrograph)

Question 4

what is a falling/recession limb?

Answer 4

period when stream discharge decreases after a rainfall or snowmelt event (in hydrograph)

Question 5

what is the event peak flow?

Answer 5

highest flow value reached in response to a precipitation event (in hydrograph)

Question 6

what is the difference between stormflow and baseflow?

Answer 6

stormflow = quick flow = portion of the stream hydrograph that is fed by rapidly-moving runoff

baseflow= slow flow= portion of stream hydrograph that is fed by slowly-moving runoff even in the absence of rainfall or snowmelt (groundwater flow)

Question 7

what are the three graphical hydrograph separation methods for baseflow and stormflow?

Answer 7

• Linsley’s fixed interval method
• straight line method
• recession limb method

Question 8

what is Linsley’s fixed interval method?

Answer 8

N days= A ^(0.2) (mi^2)

Question 9

what is the straight line separation method?

Answer 9

slope of the line = 0.05 pi^3 x A

Question 10

what is lagtime in a hydrograph?

Answer 10

time interval between the peak of precipitation and the peak of event stream hydrograph

Question 11

what is the time to peak in a hydrograph?

Answer 11

time from the beginning of the rising limb to the occurence of peak discharge

Question 12

what is the time base in a hydrograph?

Answer 12

duration of the event stream hydrograph (during which quick flow occurs)

Question 13

what is time concentration in a hydrograph?

Answer 13

time from the end of the precipitation event to the inflection point of the recession limb of the event stream hydrograph

Question 14

what is runoff ratio in hydrograph analysis?

Answer 14

dividing volume of stormflow by volume of water input

Question 15

what is the difference between flashy and subdued storm hydrographs?

Answer 15

flashy: short lag time, high and narrow peak and steep rising limb

subdued: longer lag time, low peak, gently sloping rising limb

Question 16

What do the hydrographs for arid and humid climates look like?

Answer 16

arid: flashy due to hydrophobic soils

humid: more subdued due to mostly subsurface flow, but some saturation overland flow and percolation

Question 17

how do geographic characteristics affect hydrographs?

Answer 17

• watershed area influences the cumulative volume of runoff
• the shape of the watershed influences the time of concentration to the outlet
• the slope of the area -> steeper, the lower the rate of infiltration and faster rate of runoff when soil is saturated
• the length of hillslope and width of valley influences water travel time to the valley and to the watershed outlet

Question 18

how does the shape of watershed affect the hydrograph?

Answer 18

circular: more flashy
elongated: more subdued

Question 19

how does the amount of streams (drainage density) affect stream discharge?

Answer 19

the larger the amount of streams per area, the shorter distance water has to travel to streams and the faster the stream response

Question 20

how do surface depressions, marshlands and wetlands affect stream discharge?

Answer 20

storage will decrease the volume of runoff reaching streams and slow down the travel of water from land to stream

Question 21

what are three geological and soil characteristics that affect stream discharge?

Answer 21

• soil permeability (low permeability -> less infiltration -> more HOF, less SSSF)
• soil depth (deeper, more water can be absorbed)
• bedrock permeability

Question 22

what are 4 examples of human activities that increase streamflows?

Answer 22

• construction of impervious surfaces
• removal of vegetation and soil
• irrigation and drainage ditches
• tillage on wet lands, compacting the subsoil

Question 23

what are 4 measures that can help mitigate high streamflows?

Answer 23

• increasing afforestation
• increasing meadow area at the expense of arable land
• limiting the construction of impervious surfaces
• promoting the implementation of infiltration areas and permeable pavement

Question 24

summarize what each dominant pathway would look like on a hydrograph

Answer 24

infiltration excess overland flow: shortest lag time and time to peak, highest and narrowest flow peaks

saturation excess overland flow: longer lag time and time to peak, lower and wider flow peaks

matrix flow: longest lag time and time to peak, lowest and widest flow peaks

macropore flow: comparable with SOF

pipeflow: comparable with HOF/SOF

Groundwater flow: little to no quickflow, hydrograph is 100% baseflow