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.
