Groundwater Flow (Darcy's Law)

Darcy velocity (q = K × i) is a bulk flux — it's the flow rate divided by the entire cross-sectional area, including the solid grains, so it isn't the actual speed of any water molecule. Seepage velocity divides that by an assumed porosity of 0.3 to estimate the true average speed through the connected pore spaces alone, which is the number that matters for predicting how fast a contaminant plume travels.

The calculator uses 0.3 as a representative value for typical sand and gravel aquifers, since porosity isn't one of its three inputs. If your material is markedly different — clays and silts often have higher porosity, while poorly sorted gravels can run lower — the seepage velocity figure will be off by roughly the same ratio as the true porosity differs from 0.3; flow rate and Darcy velocity are unaffected since they don't use porosity at all.

In the formula Q = K × i × A, flow rate scales linearly with all three factors, so increasing either the gradient or the conductivity increases flow proportionally — the calculator doesn't treat them differently. Conductivity, though, is a property of the aquifer material itself (set by grain size, sorting, and fracturing) and typically isn't something you can change in the field the way you can change or measure a different gradient.

The law assumes laminar, steady-state flow through a saturated, homogeneous porous medium with a linear relationship between flow and gradient. It breaks down in fractured rock, karst conduits with open channels, or any setting with high-velocity turbulent flow, where the actual relationship between gradient and flow becomes nonlinear — this calculator has no way to detect those conditions and will still return a number even when the underlying assumption doesn't hold.