Antecedent moisture condition (AMC)
Antecedent rainfall is the rainfall that occurs prior to the start of a storm event. It increases soil moisture levels and affects rates of infiltration into the soil.
The Antecedent Moisture Condition is a parameter used in the loss calculations of the Horton (ILSAX) hydrological model to specify the wetness or dryness of catchment at the start of a storm. It is used to set the starting levels for infiltration relationships, and can have a significant effect on the flowrates generated by DRAINS. Thus you need to carefully consider the values that you choose. Sensitivity analysis can be used to assess the effects of choosing different AMC values.
An AMC number corresponds to a starting point on an infiltration curve, as shown in the figure below. The curve defines the rate at which rainwater can penetrate into the soil. During a storm event, this will decrease, due to the soil becoming wetter, soil swelling and other effects.
In research on DRAINS and related models, it has proved to be reasonably accurate to relate the AMC value of 1 to 4 to the rainfall in the previous 5 days, as follows:
As with the related soil type, non-integer values such as 1.5 or 2.45 can be specified. DRAINS interpolates between the parameters corresponding to the integer AMC values.
When using DRAINS with actual storms recorded on rain gauges, the rainfall in the previous 5 days can be easily found from available records. When using it with probabilistic design storms, an AMC should be selected on the basis of the expected rainfall prior to a design event. This can be found by analysing the records of daily rainfalls prior to large storm events at or near the location of interest.
One way to do this is to analyse daily rainfall records and on a spreadsheet calculate the rainfalls for the 5 days preceding each day. Daily rainfalls can then be ranked and the antecedent rainfalls for the highest 100 rainfalls, say, can be analysed. From the mean or median antecedent rainfalls and classification numbers a most-likely value of AMC can be selected.
The spreadsheet shown below performs this procedure for a gauge at Albany, determining average values for AMCs and for rainfalls in the 5 days preceding the 100 largest daily storms.
In areas where rainstorms are frequent and prolonged, a high AMC is required; in areas that are usually dry, and runoff is likely to come from thunderstorms, a low AMC would be appropriate.
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