Ponding

08/17/25

“What happens when water has no place to drain?”

Water will collect over land when there are insufficient infiltration capabilities. If this water has no place to drain, it will coalesce into a sizable puddle. This Ponding process can produce small bodies of water that can carry pollutants, become mosquito nesting grounds, and even damage building structures if it collects on a small roof.

Potential Maximum Retention

08/16/25

“How can we quantify the maximum amount of water a watershed can hold before runoff?”

Watershed geologies can hold a certain amount of moisture before runoff occurs. We can measure this through the Potential Maximum Retention, a hydrological quantity that represents the maximum amount of water a watershed can absorb before starting runoff.

 Initial Abstraction (Hydrology)

08/15/25

“How can we quantify the initial rainfall losses before runoff begins?”

Rainfall will lead to water hitting the ground. Before anything happens, some of the rainfall will get intercepted by vegetation or infiltrate into the ground and get lost from the system. Hydrologists represent this value as the Initial Abstraction. Initial abstraction is an important quantity in hydroshed and stormwater studies.

Hydraulic Head

08/11/25

“How can we conceptualize the total potential of a fluid’s pressure?”

We know that a fluid that is higher than another will usually flow downwards if they’re connected. However, if the lower fluid has a high enough pressure or velocity, then it can overpower the higher fluid and move upwards. One way to conceptualize this is to imagine that the pressure and kinetic energy of each fluid adds extra height, and whichever fluid has the highest theoretical “height” will govern the direction of flow. This is the concept behind Hydraulic Head, as illustrated by the equation h = P/(rho_w * g) + z + v^2/(2*g), where h is the hydraulic head, P the fluid pressure, rho_w the fluid density, g the gravitational constant, z the fluid elevation, and v the fluid velocity.

Stormwater Discharge

08/10/25

“What happens when stormwater runoff enters a regulated body?”

Stormwater runoff is the water from rain that flows over the ground instead of seeping into it. Some of this rain may enter a regulated body, such as a stream or a storm drain. This water, known as Stormwater Discharge, may contain pollutants it picked up during its journey on the surface. Infrastructure operators should treat stormwater or prevent it from getting polluted before it enters water bodies. Engineers and planners must design public facilities around predicted peak stormwater discharge values and holding times.

Unit Peak Discharge

08/09/25

“How can we get a base estimate of a small watershed’s discharge potential?”

Runoff events vary by storm and watershed. However, we may want to compare the discharge potential of different watersheds without having to account for variations in different storms. So, what if we were to calculate how much peak discharge each watershed would produce for a 1-inch rain event, regardless of its area? Well, this is the idea behind Unit Peak Discharge, which is measured in flow rate per area per runoff amount. Calculating unit peak discharge is essential in estimating peak discharge using the TR-55 graphical method.

Runoff Reduction Volume

08/06/25

“How do we classify the amount of stormwater runoff that can be captured and kept out of the stormwater system?”

Rain brings stormwater runoff. And when stormwater runoff is too large to control, it can lead to damaging flooding and pollution. But suppose systems, natural or human-made, are designed clever enough. In that case, they can capture some of this stormwater and prevent it from entering stormwater handling infrastructure through absorption, infiltration, evaporation, or reuse. Whatever amount of stormwater is handled through this manner is classified as Runoff Reduction Volume. Interventions like rain gardens and bioswales can increase the expected runoff reduction volume.