The amount of pre­cip­i­ta­tion that falls dur­ing a storm obvi­ously has an impact on the flood dynam­ics of rivers and streams. When it rains a lot, rivers and streams can flood dra­mat­i­cally. Flood­ing from Trop­i­cal Storm Irene in 2011 is an all too famil­iar example.

Flood­ing in Boiceville as a results of Trop­i­cal Storm Irene

Another impor­tant but less well known influ­ence on flood­ing is the antecedent mois­ture con­di­tion.

To under­stand what antecedent mois­ture con­di­tion is and how it impacts floods we need to briefly dis­cuss the water bal­ance:

     P = RO + ET + ΔS

where,

     P = pre­cip­i­ta­tion,
     RO = runoff,
     ET = evap­o­tran­spi­ra­tion, and
     ΔS = change in ground­wa­ter or soil storage.

This gen­er­al­ized equa­tion is say­ing that all the water that falls as rain either (1) runs off the sur­face and becomes flow in a stream, (2) is evap­o­rated or tran­spired (i.e., used by plants), or (3) is stored in the ground, often in the pore spaces between soil particles.

Soil can be thought of as a giant sponge that can absorb large amounts of water. Antecedent mois­ture con­di­tion is how wet or dry that soil stor­age sponge is when it starts to rain.

If the soil stor­age sponge is already sat­u­rated before the storm hits, only a small per­cent­age of the rain­fall can be absorbed, mean­ing a large por­tion of the rain­fall total will become runoff. For exam­ple, prior to TS Irene in 2011 the antecedent mois­ture con­di­tion was rel­a­tively high, as can be seen in the stream gage hydro­graph at Allaben (below). The orange tri­an­gles rep­re­sent the aver­age flow for that day (approx 20-30cfs). In the week lead­ing up to Irene, flow in the Eso­pus Creek was well above aver­age (blue line, 100–200 cfs), indi­cat­ing that soil mois­ture lev­els were already high when the storm hit.

Hydro­graph of Eso­pus Creek at Allaben prior to TS Irene in 2011.

Con­versely, if the soil stor­age sponge is mostly dry when the storm hits a larger per­cent­age of the pre­cip­i­ta­tion can poten­tially be absorbed, or stored in the soil sponge rather that becom­ing runoff.  Less runoff can some­times mean less dra­matic flooding.

Today, as we await the arrival of Trop­i­cal Storm Isa­ias, antecedent mois­ture con­di­tions are rel­a­tively low, with flow in the Eso­pus at Allaben hov­er­ing near the approx­i­mate aver­age value for early August (20–30 cfs), far less than what it was prior to Irene. There is more room for water in the sponge.

Antecedent mois­ture con­di­tions prior to the arrival of TS Isaias.

This does not mean that flood­ing can’t hap­pen when antecedent mois­ture con­di­tions are low. Even with a dry soil stor­age sponge, the rate of pre­cip­i­ta­tion is also an incred­i­bly impor­tant com­po­nent of flood dynam­ics. If rain falls faster than it can infil­trate into the soil, water will run off regard­less of antecedent mois­ture con­di­tions, which can cause dam­ag­ing flash floods.

The soil stor­age sponge also has a lim­ited capac­ity and can become sat­u­rated quickly.

Please refer to our recent post on the Flash Flood Watch issued for the Ashokan Water­shed for infor­ma­tion on how to pre­pare for a flood.