1. 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 management
Flood prediction, protection and control
Land and water conservation (soil erosion)
Water pollution control and sanitary engineering
Land drainage and irrigation
Inland water transportation
Water supply engineering (includes groundwater)
Reservoir and hydroelectric power management
Limnology and glaciology
Water recreation management
Inland fisheries management

2. 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. Run- on process is the infiltration of surface water that, as it moves downslope, encounters areas where moisture deficit has not yet been satisfied.

3. 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.

4. 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

5. 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

6. 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

7. Disposal of Rainfall During a Storm

8. 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

10. 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

11. 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

12. 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

13. 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.

14. Situations where saturation overland flow may arise
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

15. 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

16. 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

17. Annual Hydrograph Perennial Streams Ephemeral 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

18. Storm Hydrograph

19. 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.

20. 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

21. Hydrograph Shape
No surface runoff. Small ripples are due to precipitation on channel
No surface runoff but there is interflow or groundwater flow. Upward shift depends on the difference F-SMD
There is surface runoff but no contribution from subsurface
All components of runoff are present. The shape is superposition of case (b) and (c)

22. 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)

23. 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

24. 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.

25. Barns Method Separates surface runoff, 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

26. 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:
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