Runoff & Rainfall-Runoff Relationships Notes

Last reviewed 16 Sept 2026 · 7 min read

Runoff

Runoff is the part of precipitation that flows towards streams, rivers and the sea as surface or subsurface flow.

Component	Path
Surface runoff (overland flow)	Flows over the land surface into channels once rainfall exceeds infiltration and depression storage
Interflow (subsurface storm flow)	Infiltrated water moving laterally through upper soil layers and re-emerging in streams
Baseflow (groundwater flow)	Delayed contribution of groundwater to the stream; sustains flow in dry periods

Direct runoff = surface runoff + prompt interflow; it forms the flood hydrograph.
Natural flow (virgin flow) — runoff unaffected by human diversions and storage; observed flows must be corrected for upstream abstractions and return flows.

Types of streams

Perennial — flow throughout the year; the water table stays above the stream bed (effluent streams fed by baseflow).
Intermittent — flow during wet seasons only; water table above the bed in wet seasons.
Ephemeral — flow only during and immediately after rain; the bed is always above the water table (common in arid zones).

Factors affecting runoff

Climatic: type, intensity, duration and areal distribution of precipitation; direction of storm movement; antecedent rainfall; evaporation and transpiration.

Physiographic (catchment):

Area — total runoff volume increases, runoff per unit area and peak per unit area decrease.
Shape — fan-shaped (compact) catchments give higher, earlier peaks than fern-leaf (elongated) catchments.
Slope — steeper slopes give quicker, higher runoff.
Soil and geology, land use and vegetation (forests reduce and delay runoff; urbanisation increases it), drainage density (stream length per unit area — higher density gives quicker response), storage in lakes and swamps.

Yield of a catchment

The yield is the total quantity of water that can be expected from a stream in a given period (usually a year). The dependable yield is the yield that is equalled or exceeded in a stated percentage of years — commonly 75% dependable yield for irrigation planning and higher dependability (about 90% or more) for hydropower and drinking water.

Rainfall–runoff relationships

Runoff coefficient and correlation

Runoff coefficient $C = R / P$ .
A linear regression $R = a P + b$ (or exponential forms) is fitted to observed annual rainfall and runoff.

Empirical formulas and tables (Indian practice)

FormulaEmpirical runoff relations (R and P in cm)

Inglis and DeSouza (Western India):

Ghat regions: $R = 0.85 P - 30.5$
Deccan plateau: $R = \frac{( P - 17.8 ) P}{254}$

Khosla's formula (monthly):

R_{m} = P_{m} - L_{m}, L_{m} = 0.48 T_{m} (for T_{m} > 4.5 ° C)

$R_{m}$ , $P_{m}$ , $L_{m}$ in cm; $T_{m}$ = mean monthly temperature (°C). Annual runoff $= \sum R_{m}$ .

Tables: Binnie's percentages (Madhya Pradesh), Barlow's tables (Uttar Pradesh catchments classified by type and season), Strange's tables (Maharashtra, Karnataka).

SCS curve number method

FormulaSCS-CN method (SI units, mm)

Q = \frac{( P - I _{a} ) ^{2}}{P - I _{a} + S} for P > I_{a}, I_{a} = 0.2 S

\Rightarrow Q = \frac{( P - 0.2 S ) ^{2}}{P + 0.8 S}, S = \frac{25 400}{C N} - 254

$C N$ (0–100) depends on the hydrologic soil group (A–D), land use and antecedent moisture condition (AMC I dry, II average, III wet).

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Runoff & Rainfall-Runoff Relationships