Dear researchers
Since there are three types of CN value (e.g., dry, average, and wet), there are three types of S value according to the AMC (AMCI: dry; AMCII: average; AMCIII: wet ) which relies on the 5-day antecedent rainfall (P5). To apply S values, a threshold is required, for instance, if P5 < 13 mm we use SI while if P5 > 28 mm we use SIII. How can I determine these thresholds (e.g., 13, 28) for a given area?
Thanks in advance.
A combination of hydrologic soil group, hydrologic condition, land use and treatment class (cover) constitutes the hydrologic soil cover complex. Each such combination by a number called as CN on the basis of runoff potential. Higher CN indicates higher runoff potential. Runoff CN values for different complexes AMC II and Ia = 0.2S are generally give in standard tables.
Ia = Initial abstraction during the period between beginning of rainfall and runoff
in equivalent depth over the watershed / catchment, mm
S = potential maximum retention, mm
Soil Map of country is used to classify any watershed / catchment accordingly to hydrologic soil groups. Moisture content prior to rainfall event plays an important role in determining the runoff. Therefore, moisture conditions are generally expressed in term of AMC I, II, III. Based on the total rainfall of the preceding 5 days.
Rainfall limits ( threshold values) for estimating AMC
AMC 5 days total antecedent rainfall (mm) Runoff producing potential
Dormant Growing
I Less than 12.5 Less than 35 Lowest
II 12.5 to 27.5 35 to 52.5 Moderate
III more than 27.5 more than 52.5 Highest
See if you might have access in your area to weather data collected and mapped such as this. As suggested by Dr. Singh, not all soils respond to rainfall and hold water equally, so some adjustment by soil type might be appropriate.
https://www.wpc.ncep.noaa.gov/qpf/p120i.gif?1622128348
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