1. Definition
The Antecedent Moisture Condition (AMC) describes the soil moisture content before a rainfall event. It is a crucial factor in hydrology because it directly affects how much rainwater will infiltrate into the soil and how much will run off as surface flow.
In simpler terms: AMC tells us how wet or dry the ground already is before it rains.
2. The Three Standard AMC Classes
The Natural Resources Conservation Service (NRCS, formerly SCS) defines three AMC classes:
| AMC Class | Soil Condition | Typical Scenario | Effect on Runoff |
|---|---|---|---|
| AMC I | Dry | Soil is dry, low recent rainfall | Low runoff, high infiltration |
| AMC II | Average | Normal conditions, moderate soil moisture | Standard runoff (reference value) |
| AMC III | Wet | Soil is already saturated or very moist | High runoff, low infiltration |
For the Curve Number (CN) method, AMC II is used as the reference. AMC I and III are adjustments for unusually dry or wet conditions.
3. Adjusting Curve Numbers for AMC
The CN provided in standard tables corresponds to AMC II. To calculate CN for AMC I or III:
Example: If CN_II = 75:
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CN_I ≈ 56 → dry soil
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CN_III ≈ 89 → wet soil
4. Considerations
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AMC is not fixed; it depends on recent rainfall and season.
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CN increases with wetter conditions (AMC III) → more runoff.
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CN decreases with drier conditions (AMC I) → less runoff.
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AMC allows hydrologists to account for soil wetness variability in runoff calculations.
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