Runoff curve number
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The runoff curve number (also called a curve number or simply CN) is an
empirical Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...
parameter used in
hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...
for predicting direct
runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock market ...
or
infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...
from
rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
fall excess. The curve number method was developed by the
USDA The United States Department of Agriculture (USDA) is the federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, and food. It aims to meet the needs of com ...
Natural Resources Conservation Service, which was formerly called the ''Soil Conservation Service'' or ''SCS'' — the number is still popularly known as a "SCS runoff curve number" in the literature. The runoff curve number was developed from an
empirical analysis In philosophy, empiricism is an epistemological theory that holds that knowledge or justification comes only or primarily from sensory experience. It is one of several views within epistemology, along with rationalism and skepticism. Empir ...
of runoff from small catchments and hillslope plots monitored by the USDA. It is widely used and is an efficient method for determining the approximate amount of direct runoff from a
rainfall Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
event in a particular area.


Definition

The runoff curve number is based on the area's hydrologic soil group,
land use Land use involves the management and modification of natural environment or wilderness into built environment such as settlements and semi-natural habitats such as arable fields, pastures, and managed woods. Land use by humans has a long h ...
, treatment and hydrologic condition. References, such as from USDA indicate the runoff curve numbers for characteristic land cover descriptions and a hydrologic soil group. The runoff equation is: :Q=\begin 0 & \text P \leq I_a \\ \frac & \text P>I_a \end where :Q is
runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock market ...
( in) :P is
rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
fall ( in) :S is the potential maximum soil moisture retention after runoff begins ( in) :I_a is the initial abstraction ( in), or the amount of water before runoff, such as
infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...
, or rainfall interception by vegetation; historically, it has generally been assumed that I_a = 0.2S, although more recent research has found that I_a = 0.05S may be a more appropriate relationship in urbanized watersheds where the CN is updated to reflect developed conditions. The runoff curve number, CN, is then related :S = \frac - 10 CN has a range from 30 to 100; lower numbers indicate low runoff potential while larger numbers are for increasing runoff potential. The lower the curve number, the more permeable the soil is. As can be seen in the curve number equation, runoff cannot begin until the initial abstraction has been met. It is important to note that the curve number methodology is an event-based calculation, and should not be used for a single annual rainfall value, as this will incorrectly miss the effects of antecedent moisture and the necessity of an initial abstraction threshold.


Selection

The NRCS curve number is related to soil type, soil infiltration capability, land use, and the depth of the seasonal high water table. To account for different soils' ability to infiltrate, NRCS has divided soils into four hydrologic soil groups (HSGs). They are defined as follows. *HSG Group A (low runoff potential): Soils with high infiltration rates even when thoroughly wetted. These consist chiefly of deep, well-drained sands and gravels. These soils have a high rate of water transmission (final infiltration rate greater than per hour). *HSG Group B Soils with moderate infiltration rates when thoroughly wetted. These consist chiefly of soils that are moderately deep to deep, moderately well drained to well drained with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission (final infiltration rate of per hour). *HSG Group C: Soils with slow infiltration rates when thoroughly wetted. These consist chiefly of soils with a layer that impedes downward movement of water or soils with moderately fine to fine textures. These soils have a slow rate of water transmission (final infiltration rate per hour). *HSG Group D (high runoff potential): Soils with very slow infiltration rates when thoroughly wetted. These consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious materials. These soils have a very slow rate of water transmission (final infiltration rate less than per hour). Selection of a hydrologic soil group should be done based on measured infiltration rates, soil survey (such as th
NRCS Web Soil Survey
, or judgement from a qualified soil science or geotechnical professional. The table below presents curve numbers for antecedent soil moisture condition II (average moisture condition). To alter the curve number based on moisture condition or other parameters, see Adjustments.


Values


Adjustments

Runoff is affected by the soil moisture before a precipitation event, the ''antecedent moisture condition'' (AMC). A curve number, as calculated above, may also be termed AMC II or CN_, or average soil moisture. The other moisture conditions are dry, AMC I or CN_, and moist, AMC III or CN_. The curve number can be adjusted by ''factors'' to CN_, where CN_ factors are less than 1 (reduce CN and potential runoff), while CN_ factor are greater than 1 (increase CN and potential runoff). The AMC factors can be looked up in the reference table below. Find the CN value for AMC II and multiply it by the adjustment factor based on the actual AMC to determine the adjusted curve number.


Initial abstraction ratio adjustment

The relationship I_a = 0.2S was derived from the study of many small, experimental watersheds . Since the history and documentation of this relationship are relatively obscure, more recent analysis used model fitting methods to determine the ratio of I_a to S with hundreds of rainfall-runoff data from numerous U.S. watersheds. In the model fitting done by Hawkins et al. (2002) found that the ratio of I_a to S varies from storm to storm and watershed to watershed and that the assumption of I_a/S=0.20 is usually high. More than 90 percent of I_a/S ratios were less than 0.2. Based on this study, use of I_a/S ratios of 0.05 rather than the commonly used value of 0.20 would seem more appropriate. Thus, the CN runoff equation becomes: :Q=\begin 0 & \text P \leq 0.05S \\ \frac & \text P>0.05S \end In this equation, note that the values of S_ are not the same as the one used in estimating direct runoff with an I_a/S ratio of 0.20, because 5 percent of the storage is assumed to be the initial abstraction, not 20 percent. The relationship between S_ and S_ was obtained from model fitting results, giving the relationship: : S_=1.33^ The user, then, must do the following to use the adjusted 0.05 initial abstraction ratio: # Use the traditional tables of curve numbers to select the value appropriate for your watershed. # Calculate S_ using the traditional equation:S = \frac - 10 # Convert this S value to S_ using the relationship above. # Calculate the runoff depth using the CN runoff equation above (with 0.05 substituted for the initial abstraction ratio).


See also

*
Hydrological modelling A hydrologic model is a simplification of a real-world system (e.g., surface water, soil water, wetland, groundwater, estuary) that aids in understanding, predicting, and managing water resources. Both the flow and quality of water are commonly stud ...
*
Runoff model (reservoir) A runoff model is a mathematical model describing the rainfall– runoff relations of a rainfall ''catchment area'', drainage basin or ''watershed''. More precisely, it produces a surface runoff hydrograph in response to a rainfall event, repre ...


References


External links


SCS TR-55 Peak Discharge and Runoff CalculatorCurve Number Calculator
Online Free Curve Number Calculator

{{Rivers, streams and springs Hydrology Soil physics