Runoff True runoff

- draining/flowing off of precipitation from a catchment area through a stream flow in its natural condition (without human intervention)
surface channel.
Natural flow/Virgin flow
- represents the output from the catchment in a given unit of time
- stream flow unaffected by the works of a man (such as reservoir and
Requirements to be first satisfied before the commencement of runoff diversion structures on stream)

- evapotranspiration, initial loss, infiltration and detention storage - have to be derived based on observed flows and data on
abstractions from the stream
Overland flow
Exists storage/ diversion of work on stream
- the excess of precipitation moves over the land surfaces to reach
smaller channels - not true runoff. Unless corrected for the diversion of flow and return
flow
- Involves building up a storage over the surface and draining off the
same Flow on the downstream channel

- lengths and depths are small and the flow is in laminar regime - affected by operational and hydraulic characteristics of structures

Flows (several small channels- bigger channels- larger stream- Hydrograph (annual, monthly, seasonal, flood/ hydrographs due to a
catchment outlet) storm)

Surface run-off - a plot of the discharge in a stream plotted against time cronologically

- flow that travels over the surface as overland flow and through the Annual and seasonal hydrographs
channels as open-channel flow and reaches the catchment outlet
- used in calculating the surface water potential of a stream, reservoir
Interflow/Through flow/Storm seepage/subsurface storm flow/ Quick studies, drought studies
return flow
Flood hydrographs
- part of precipitation that infilters moves laterally through upper crust
of the soil and returns through the surface away from the point of entry - essential in analyzing stream characteristics associated with floods

Geological condition - concerned with the estimation and use of long-term runoffs

- amount of interflow depends on it Water year

Large interflows - advantageous in annual runoff studies (begins from the time when
the precipitation exceeds the average evaporation losses)
- a fairly impervious soil overlying a hard impermeable surface is
conductive to it - In india (June1-May31)

Prompt interflow Complete cycle of climatic changes

- classification of interflow depending upon the time of delay between - expected in a water year and hence the water budget will have the
infiltration and the outflow least amount of carryover

- interflow with the least time lag and delayed interflow RUNOFF CHARACTERISTICS ON STREAMS CLASSIFICATION

GROUND WATER RUNOFF/ GROUND WATER FLOW 1. Perennial- the one which always carries some flow. During the
dry season, water table will be above the bed of a stream.
Undergo deep percolation and reach the ground water storage in the 2. Intermittent stream- limited contribution from the ground
soil water
3. Ephemeral stream does not have any base-flow
- another route for infiltration of water contribution, does not have any well-defined channel. Most
rivers in arid zones are ephemeral kind.
Ground water
FLOW CHARACTERISTICS OF A STREAM DEPENDS UPON:
- follows a complicated and long path of travel and ultimately reaches
the surface 1. Rainfall characteristics- magnitude, intensity, distribution (time
and space) & variability
Time lag 2. Catchment characteristics soil, landuse/cover, slope,
geology, shape & drainage density
- the difference in time between the entry into the soil and outflows
3. Climatic factors evapotranspiration
from it (is very large; months, years)
Yield
Ground water flow
- total quantity of surface water that can be expected in a given
-provides dry-weather flow in perennial streams
period from a stream at the outlet of its catchment
Categories of Runoff
Annual yield
1. Direct runoff (Direct strorm runoff/Storm runoff) enters the stream
- end product of various processes such as precipitation, infiltration and
immediately after rainfall. It includes surface runoff, prompt interflow,
evapotranspiration
rainfall on the surface of steams, snowmelt.
2. Base flow delayed flow that reaches a stream essentially as Yield of a stream
ground water flow. It includes delayed interflow.- easily recognized in
the annual hydrograph of a perennial stream as the slowly - always related to natural flow of river
decreasing flow of the stream in rainless period.
Return flow Curve number CN depends upon Soil type, Land use/cover,
antecedent moisture condition
- non-consumptive part that returns to hydrologic system of the basin
FOUR HYDROLOGIC SOIL GROUPS
- available for the suitable use and added to natural flow to estimate
yield 1. Group A (Low runoff potential)
2. Group B (Moderately low runoff potential)
Empirical equations and watershed simulations 3. Group C (Moderately high runoff potential)
4. Group D (High runoff potential)
- alternate methods in the calculation of natural runoff volume
Antecedent Moisture Condition (AMC)
Rainfall-Runoff correlation
-refers to moisture content present in the soil at the beginning of the
- influenced by a host of factors relating to the catchment and climate rain-fall-runoff event under construction
EMPIRICAL EQUATIONS Flow-duration curve of a stream/ Discharge frequency curve
Binnies percentage -plot of discharge against the percent of time the flow was equaled or
exceeded
- Sir Alexander Binnie measured a runoff from a small catchment near
Nagpur 16km2; 1869-1872 (developed curves of cumulative runoff vs Streamflow data
cumulative rainfall)
- arranged in a descending order of discharges using class intervals if
Barlows tables the number of individual values are very large
- first chief engineer of the hydro-electric survey of india 1915;130km2. Slope of flow duration curve
Strange tables - depends upon the interval of data selected
- Strange (1892) studied the available rainfall and runoff Presence of reservoir in a stream
- good, average, bad - modifies the virgin-flow correction curve depending of the nature flow
regulation
1. Runoff volume from total monsoon season rainfall
Virgin-flow correction curve
Double mass curve relationship
- when plotted on a log probability paper plots as a straight line at least
- relationship between cumulative monthly rainfall
over a central region
2. Estimating runoff volume from daily rainfall
Flow-mass curve (Rippls mass curve) (Rippl 1882)
- (for good & bad catchment) add or deduct 25% of the yield
- plot of the cumulative discharge volume against time plotted in
corresponding to the average catchment
chronological order.
Khoslas formula
CALCULATION OF TORAGE-VOLUME
- indirectly based on water balance concept
Demand line
Mean monthly catchment temperature
-constant rate of withdrawal from a reservoir
- used to reflect the losses due to evapotranspiration
-proper demand that can be sustained by the reservoir in that dry
Deterministic watershed simulation period

- technique of predicting runoff which is the catchment response to a A demand line drawn tangential to the Hump
given rainfall input
Tangential to the mass curve at the valley portion
Model
Dry period
- first prepared in the interdependence of various parameters in the
- negative value in column 7
system
Sequent Peak algorithm
Crawford and Linsley (1959)
- recommended as a foolproof method that can be used with
- proposed a watershed simulation model known as the Stanford
confidence in all situations
watershed model (SWM)
-suited for the analysis of data with the help of computer
Continuous simulated models
-first given by Thomas (1963)
- models which stimulate stream flow for a long period of time
- mass curve of cumulative flow volume against chronological time
At least 75 hydrologic simulation (1980s) models are available and
suitable for watersheds Water wasted over the spillway
SCS-CN method -vertical distance between two successive tangents to a mass curve at
the ridges
- simple, predictable and stable conceptual method for estimation of
direct runoff depth based on storm rainfall depth. Refill
- water balance equation of a rainfall in a known interval of time. - the demand line must intersect the mass curve
Insufficient inflow Remote sensing techniques (using imageries)

- non-intersection of demand line and mass curve -offer excellent possibilities for monitoring agricultural drought over
large areas.
Demand rate
Temporal and Spatial Aberrations (in the rainfall)
- varies with time to meet various end uses such as irrigation, power
and water-supply needs -causes of droughts

Mass curve of demand (Variable use line) Drought management

-prepared and superposed on the flow-mass curve with proper - involves development of both short-term and long-term strategies
matching of time.
Short term strategies
Regulation diagram
-include early warning, monitoring and assessment of droughts
-plot at which reservoir is full at first point of intersection of the two
curves Long term strategies

Mass-curve method -aim at providing drought mitigating measures through proper soil and
water conservation, irrigation scheduling and cropping patterns
- Definite sequence of events and this is its major drawback
Drought-proofing of a region
- Widely used for the analysis of reservoir-capacity demand problems
- calls for integrated approach, taking into account the multi-
Mass-curve analysis dimensional interlinkages

-adequate for small projects or preliminary studies of large storage Salient features of water harvesting
projects
-forms an important component in modification of drought
Time-series analysis components

-sophisticated method for final design Water harvesting

Net-flow volume -process of collecting and concentrating run-off water from run-off
area into a run-on area
- surplus or deficit of storage
-useful in dry land agriculture and form important drought
Residual-mass curve management tool

- have peaks and troughs Collected water

Sequent peak -can be used in groundwater recharge and storage in the aquifer
(recharge enhancement
-next following peak greater than P
WATER HARVESTING
TWO HYDROLOGICAL EXTREMES
1. Rain water harvesting
1. Floods a. Rooftop water harvesting
2. Droughts b. Harvesting of a small ground area surface
2. Flood water harvesting
Drought
a. With storage
-climatic anormaly characterized by deficit supply of moisture b. Without storage

Drought phenomenon Roof top water harvesting

-hydrological extreme like flood and is a natural disaster - productive utilization of water falling on roof-tops of structures

CLASSIFICATION OF DROUGHT (NATIONAL COMMISSION ON Micro-Catchment harvesting (within the field) of rainwater harvesting
AGRICULTURE 1976) (Within field catchment system)

1. Meteorological drought 25% decreased in precipitation - catchment length of 1-30m

Moderate -26-50% seasonal deficiency
- Runoff is stored in soil profile
Severe 50%-above
Drought year- more than 20% area of the country Negarim Micro-Catchments (for trees) from Hebrew word Neger
Drought-prone area- 0.2<P<0.4 (runoff)
2. Hydrological drought marked depletion of surface water
and groundwater -dividing catchment into a large number of micro-catchments in a
Components- Magnitude, duration, severety, frequency of diamond pattern along slope
occurence
3. Agricultural drought- soil moisture and rainfall are inadequate - beneficial to rainfall as low as 150 mm (arid, semi-arid)
using growing season
Deficiency of rainfall principal criteria for defining Macro-Catchment system (within the field) of rainwater harvesting
agricultural drought
Thornthwites water balance technique -Actual evapo- -designed for slightly larger catchment areas wherin overland flow and
transpiration is calculated rill flow is collected behind a bund. (30-200m long; 2:1, 10:1)
AI (aridity index) used as an indicator of possible moisture
-contour bunds made up of filed up stones is used in the system.
stress experienced by crops
Palmer Index (PI) and Moisture Availability Index (MAI) used Floodwater farming (floodwater harvesting) used for larger
to characterized agricultural drought catchments and the flow in the drainage is harvested
Small storage structures

-built across the drainage to store a part of the runoff

Check dams and Nalabunds

-commonly used storage structures

- have an advantage in arresting erosion products

- prevents deepening and widening of gullies

Check dams

-have masonry overflow spillway and the flanks can be either masonry
construction or of earthen embankment

- constructed on lower order streams (up to 3) with median slopes

- proposed where water table fluctuations are high and the stream is
influent

Nalabunds

- constructed along nalas (streams) for impounding runoff flow to

cause small storage. Increase water percolation and improving soil
moisture is its main objective.

- constructed by earthen embankment.

- Spillway stoned lined or rock cut steep channel

Percolation tanks

-similar to nalabunds but a larger dimension

Irrigation tanks in India (Harvesting structures)

Spread water

-infiltrates into the soil and retain as soil moisture and used for growing
cops

Rehabilitation of old irrigation tanks (through de-silting)

- recognized as feasible and desirable activity in drought proofing of a

region.

1875-2004 (india)

- 29 drought years

1918

- worst year. 70% of the country was affected by drought

Natural (virgin) flow in a river basin

- reckoned as surface resource of a basin.

Utilizable water resources

- mean the quantum of water withdrawable from its place of natural

occurence