<p>The word "hydrological" is meant to indicate that the model only includes a sort of bulk description of the water: where it is, how much is there, and where it flows. It generally only includes length and time scales of interest to the system in question. So, for ground water models, this may be tens of meters and longer in horizontal direction, and hours and longer in time.</p>

<p>On the other hand, "hydrodynamic" is generally understood to resolve the governing equations for water flow, i.e., the Navier-Stokes equations (possibly posed in a porous medium). This must then deal with the length and time scales of eddies, which are substantially smaller and shorter than the ones above, and consequently requires a lot more computational power.</p>

CC BY-SA 4.0
2019-12-28T12:36:22.430Z
2019-12-28T12:36:22.430Z
Wolfgang Bangerth
<p>In the description of a Software it says that it computes the net rainfall, runoff formation and runoffconcentration on a surface hydrologically. With an extra module, the runoff formation can also be computed hydrodynamically. </p>

<p>What is the difference? How does both work, roughly speaking? </p>

CC BY-SA 4.0
2019-12-26T14:15:25.230Z
Which software? Doesn't it have a reference documentation (print or online) describing the different options? (I don't mean RTFM, I'm just curious)
Jean-Marie Prival
<hydrology><models>
hydrologic vs. hydrodynamic modelling of surface runoff
bluesky
# hydrologic vs. hydrodynamic modelling of surface runoff

(Asked by: username_0 on 2019-12-26)

In the description of a Software it says that it computes the net rainfall, runoff formation and runoffconcentration on a surface hydrologically. With an extra module, the runoff formation can also be computed hydrodynamically.

What is the difference? How does both work, roughly speaking?

**Tags:** hydrology, models

> **Comments:**
> **username_1**: Which software? Doesn't it have a reference documentation (print or online) describing the different options? (I don't mean RTFM, I'm just curious)  

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## 1 answers

### Answer by username_2 on 2019-12-28. Score: 4 | Accepted answer

The word "hydrological" is meant to indicate that the model only includes a sort of bulk description of the water: where it is, how much is there, and where it flows. It generally only includes length and time scales of interest to the system in question. So, for ground water models, this may be tens of meters and longer in horizontal direction, and hours and longer in time.

On the other hand, "hydrodynamic" is generally understood to resolve the governing equations for water flow, i.e., the Navier-Stokes equations (possibly posed in a porous medium). This must then deal with the length and time scales of eddies, which are substantially smaller and shorter than the ones above, and consequently requires a lot more computational power.


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