Stream flow

Question 1

Surface runoff i

Answer 1

is that part of runoff that
travels over the ground surface and
through channels to reach the basin
outlet.

Question 2

Interflow

Answer 2

is the runoff due to the part of
rainfall that infiltrates the surface soil
and moves laterally through the upper
soil horizons toward the streams as
shallow, perched groundwater above
the main groundwater level.

Question 3

Baseflow i

Answer 3

is that part of the runoff due to
deep percolation of the infiltrated water
that has passed into the ground, has
become groundwater and has been
discharged into the stream.

Question 4

Factors (Climatic) affecting Runoff

Answer 4

Precipitation
• Form, type, intensity, duration, time distribution, areal
distribution, frequency of occurrence, direction of storm
movement, antecedent precipitation, soil moisture
– Interception
• Vegetation species, composition, age, and density of
stands; season of the year; size of storm
–Evaporation
• Temperature, wind, atmospheric pressure, vapour pressure
gradients, soluble solids, nature and shape of evaporative
surface
–Transpiration
• Temperature, solar radiation, wind, vapour pressure
gradients, soil moisture, kinds of vegetation.

Question 5

Factors (Physiographic ) affecting Runoff

Answer 5

Physiographic factors
– Basin characteristics
• Geometric factors—size, shape, slope,
orientation, elevation, stream density.
• Physical factors—land use and cover, surface
infiltration condition, soil type, geological
conditions (e.g., permeability), topographical
conditions (e.g. lakes, swamps)
– Channel characteristics
• Carrying capacity—size and shape of cross
section, slope, roughness, length, tributaries
• Storage Capacity—backwater effect

Question 6

Peak flow

Answer 6

– Rational Formula provides a relation between
rainfall and peak runoff
– k, Units coefficient (1 for Imp; 0.0028 for SI)
– Q, peak discharge (cfs or cms)
– c, runoff coefficient;
depends on characteristics of drainage basin.
– i, mean rainfall intensity (in/h or mm/h)
– A, area of catchment (acre or hectare)
Q  kciA

Question 7

Assumptions of rational Formula

Answer 7

– Size of drainage area is normally less than
80 hectares (200 acres)
– Rainfall has a constant intensity i for a
duration equal to the time of concentration
– Runoff coefficient, c, is the same for storms
of various frequencies
– Runoff coefficient is the same for all storms
on a given watershed

Question 8

Streamflow Measurement

Answer 8

• Based on the formula:
– Q = AV
• Selection of gauging station
– Steady flow
– Uniform cross-section
– Unique relation between discharge and stage
• Note that the station only measures river stage
(water depth above some known datum)
• Need to establish a rating curve that relates
stage with discharge
– Can then use curve to convert the stage records to
discharge

Question 9

Streamflow Measurement

Answer 9

Current metering, means of measuring

velocity profile across the river cross section