Surface Water
 Surface water is the water stored or flowing on earth’s surface
 It continuously interacts with the atmospheric and subsurface water
systems.
 Large part of the subject of the hydrology relates to the process
involved in the storage, and movement of water over surface of the
earth.
 Many specialize fields dealing with surface water exists:
 Hydraulic Engineering
 Water fowl and other wildlife
 Flood prediction, protection and
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control
Water pollution control and
sanitary engineering
Inland water transportation
Reservoir and hydroelectric
power management
Water recreation management
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management
Land and water conservation
(soil erosion)
Land drainage and irrigation
Water supply engineering
(includes groundwater)
Limnology and glaciology
Inland fisheries management
Terminology
 Runoff: That part of the precipitation, snow melt, or irrigation water that
appears in uncontrolled surface streams, rivers, drains or sewers. It consists
of precipitation that neither evaporates, transpires nor penetrates the
surface to become groundwater.
Can be classified according to speed of appearance after rainfall or melting
snow as direct runoff (quick flow) or base runoff (baseflow), and according
to source as surface runoff, storm interflow, or ground-water runoff.
 Channel precipitation: That part of streamflow derived from net
precipitation falling directly into the flowing stream
 Overland flow (sheet flow): Is that part of streamflow derived from net
precipitation which fails to infiltrate the mineral soil surface and runs over
the surface of the soil to the nearest stream channel without infiltrating at
any point
 Run-on: When ponded water is allowed to move downslope as overland
flow, an important process called the ‘run-on’ effect manifests itself. Runon process is the infiltration of surface water that, as it moves downslope,
encounters areas where moisture deficit has not yet been satisfied.
 Surface Stormflow: Is the sum of overland flow and channel
precipitation.
 Subsurface Stormflow (interflow): That part of streamflow which
derives from subsurface sources but arrives at the stream channel so
quickly that it becomes part of the storm hydrograph produced
directly by a given rainstorm.
The largest component of stormflow from forests and most wildlands
begins as subsurface flow.
 Stormflow (direct runoff): Sum of surface and subsurface stormflow.
In classical hydrology it was assumed to be entirely overland flow.
 Baseflow (groundwater outflow): Normally thought to be sole
component of streamflow between storm or snowmelt periods, and
presumably the oldest water to be yielded by the basin. Generally
baseflow is the outflow from extensive groundwater aquifers. But
baseflow is also sustained by slow drainage of unsaturated soil in
steep areas. In the East ~70% of total streamflow is baseflow.
 Streamflow: Is the flow of water past any point in a natural channel
above the bottoms and sides of the channel. Quantitatively it is a rate
of discharge measured at a gaging station.
Streamflow is the sum of channel precipitation, overland flow,
subsurface stormflow and baseflow
 Deep seepage: Loss of water from a drainage basin by deep
pathways that do not discharge into the channel above a gaging
station or design site.
Loss may be downward into regional aquifers, or lateral under the
surface water divide. Thus, deep seepage may be either a loss or a
gain to basin streamflow.
 Underflow: Ungaged water moving past a stream channel section in
valley sediment or colluvial material.
 Water Yield: Basin’s total yield of liquid water during some period of
time: WY = P – ET - DS
Disposal of Rainfall During a Storm
 Initially large portion of
precipitation contributes to
surface storage
 As water infiltrates into soil
there is also soil moisture
storage
 Retention: storage held for
long time and depleted by
evaporation
 Detention: short-term
storage depleted by flow
away from storage location
 Precipitation which becomes streamflow may reach the stream by
overland flow, subsurface flow, interflow or combinations of them
More on Surface Storage
 Surface Retention Storage: thin film of water which must wet the
soil surface before flow can begin.
It is seldom more than 0.5 mm
Forest floor retention storage is the throughfall and stemflow that is
retained by the intercepting litter, fermentation and humus layers.
 Surface Detention Storage: rainwater or snowmelt detained
temporarily on the surface by the resistance of surface irregularities
to flow downslope. It offers a large opportunity for infiltration before
the water enters stream.
Detention and retention storages are artificially classified, thus never
precisely separable
Disposal of Rainfall During a Storm
 Hydrologic Response can be
quantified as stormflow (Qs)
divided by storm rainfall (Pg), i.e.,
R = Qs / Pg
 Average R in the Eastern U.S. is 0.2
(computed from 3000 water years),
and varies by physiographic regions
 Cumberland Plateau produce almost
twice as much stormflow compared
to Upper Coastal Plain.
 The plateau is composed of
sandstone caps over limestone, and
shallow soils on steep slopes. Upper coastal
plain has deep sandy soils
 Lower coastal plains have high water tables
which reduce storage capacity
 It is clear that hydrologic response is controlled
more by geology than land cover
Hortonian Overland Flow
 Horton (1933): “Neglecting interception by vegetation, surface runoff is
that part of the rainfall which is not absorbed by the soil by infiltration,
i.e. q = i - f (rainfall excess)”
 Along with overland flow there is depression storage in surface hollows
and surface detention storage proportional to the depth of overland flow
 The soil stores infiltrated water and
then slowly releases it as subsurface
flow to enter the stream as baseflow
during rainless periods
 Hortonian overland flow is
applicable for impervious surfaces in
urban areas, and for natural surfaces
with thin soil layers and low
infiltration capacity as in semiarid
and arid areas
Subsurface Flow
 Hortonian overland flow rarely
occurs on vegetated surfaces in
humid regions as i < f for all
except extreme rainfalls
 Subsurface flow then becomes a
primary mechanism for
transporting stormwater to streams
 In the figure, prior to
rainfall stream surface
is in equilibrium with
water table
 Due to infiltration water
table rises and when
inflow ceases it declines
Subsurface Flow
 All of the rainfall is infiltrated
along surface DE until t=84
min, when the soil becomes
first saturated at D
 As time continues decreasing
infiltration occurs along DE
 The total outflow partly
comprises saturated
groundwater flow contributed
directly to stream and partly
unsaturated subsurface flow
seeping from the hillside above
the water table
Saturation
Excess
Overland Flow
 When subsurface flow
saturates the soil near
the bottom of a slope,
overland flow occurs
as rain falls onto
saturated soil.
 Saturation overland
flow occurs most often
at the bottom of hill
slopes and near stream
banks.
Situations where
saturation overland
flow may arise
a)
b)
c)
d)
Convergence of
Subsurface flow paths
Downslope reduction
in hydraulic gradient
associated with slope
break
Local area of thin soil
Formation of perched
saturated zone above
low-conductivity
layer with constant
slope and soil
tickness
Saturation Excess Overland Flow
 In Hortonian flow the soil is saturated from above by infiltration, while in
saturation overland flow it is saturated from below by subsurface flow.
 The velocity of subsurface flow is so slow that not all of a watershed can
contribute subsurface flow or saturation overland flow to a stream.
 Variable source areas
contribute to flow
(partial areas)
 They tend to expand
and shrink and may
constitute 10% of
watershed area during
a storm in a humid,
well vegetated region
Streamflow Hydrograph
 Graph or table showing flow rate or stage (height of water above a
datum) as a function of time
 It is an integral expression of the physiographic (the study of
physical features of the earth's surface) and climatic characteristics
that govern the relations between rainfall and runoff of a particular
drainage area
 Total flow, seasonal distribution of flow, daily flow, peak flow,
minimum flow, frequency of various critical flow rates are computed
from hydrograph
 Two types of hydrographs are particularly important
Annual hydrograph: shows the long term balance of precipitation,
ET, and streamflow in a watershed
Annual
Hydrograph
 Perennial Streams
 Continuous flow regime
 Typical of humid climate
 Spikes are caused by rain
events (direct runoff or quick
flow)
 Slowly varying flow in
rainless periods is baseflow
 Total flow is the basin yield
 Ephemeral streams
 Common in arid climates
 Long dry periods
 No baseflow
Storm Hydrograph
Theoretical Hydrograph
 Consider a uniform rainfall block
 i is rainfall intensity, f is infiltration
rate, ds is depression storage rate, dr
is surface detention rate, q is
surface runoff, and f0 is initial
infiltration capacity
 At the beginning f > i , thus q = 0
from t = t0 to t = t1
 Once f < i, irregularities on the
surface will be filled first as
depression storage, ds
 Then, as a very thin layer water will start moving over the surface
producing surface detention, dr.
 After these, q starts and increases as f decreases.
Hydrograph Shape
 Shape of the hydrograph changes for different conditions of rainfall and
soil characteristics
 Four essential forms can be identified according to (i) rainfall rate i,
(ii) infiltration rate f, (iii) total infiltrated water F, and (iv) soil moisture
deficiency, SMD
 SMD is the amount of water necessary to bring soil to field capacity.
Water movement in soil starts once SMD is satisfied
Hydrograph Shape
a) No surface runoff. Small ripples are due to precipitation on channel
b) No surface runoff but there is interflow or groundwater flow. Upward
shift depends on the difference F-SMD
c) There is surface runoff but no contribution from subsurface
d) All components of runoff are present. The shape is superposition of
case (b) and (c)
Hydrograph Shape
 The shape of the hydrograph also changes with the orientation of storm
in the basin
 If the storm is in the upstream part then a late hydrograph is observed
(a), if the storm is oriented near the outlet than an early peak occurs (b)
 Different orientation of basins with the same shape also affect the early
or late occurrence of peak flow: (c) and (d)
Hydrograph Shape
 Consider the hypothetical watershed below divided into 4 sections
 Runoff from each section arrives at the gaging station G at different
times: in 1 hr from A, 2 hrs from B, 3 hrs from C, and 4 hrs from D
 Consider a 5 hr rainfall with i = 1 cm/hr covering watershed uniformly
 If we neglect infiltration then all will be effective and produce runoff
Baseflow (BF) Separation
(a) Straight line method: It is
applicable in ephemeral
streams. An improvement is
to use an inclined line. For
small forested streams
baseflow during a storm
can be assumed to be
increasing at a rate of 0.05
cfs / sq. mile per hour.
(b) Fixed base method: Surface runoff is assumed to end at a fixed time N after the
hydrograph peak. Baseflow before surface runoff began is projected ahead to the
time of the peak. A straight line is used to connect this projection at the peak to
the point on the recession limb at the time N after the peak.
(c) Variable slope method: BF curve before the surface runoff began is extrapolated
forward to the time of peak discharge, and the BF curve after SR ceases is
extrapolated backward to the time of the point of inflection on recession limb.
Barns Method
 Separates surface runoff,
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interflow, and baseflow.
Hydrograph is plotted on a
semi-log paper.
Extend the linear part in
recession limb backward just
below the point of inflection (J)
and connect to B.
Area above BJH is direct runoff,
below BJH is baseflow
Plot direct runoff and repeat the
same procedure
Area above MIL is surface
runoff, below MIL is interflow
Master Depletion Curve
 A different recession curve is obtained from each hydrograph and no one of these
different recession curves is representative for the watershed
 To find a representative depletion curve or recession curve as many single
hydrographs as possible should be studied.
 normal depletion curve or master baseflow recession curve is a characteristic
graph of flow recessions compiled by superimposing many of the recession
curves observed on a given stream.
 Recession curve:
Q(t )  Q0 e  (t t0 ) / k
 By nothing the periods of
time when the streamflow
hydrograph is coincident
with the normal recession
curve, the points where
direct runoff begins and
ceases can be identified