How this calculator works
Subtract downstream hydraulic head from upstream hydraulic head and divide by separation. A positive value indicates head decreasing in the chosen point-1-to-point-2 direction. Negative values are valid and reverse the inferred flow direction in a uniform isotropic medium. Use a common elevation datum for both heads and distance measured along the relevant path. A head is an energy per unit weight, expressed as a length.
Formula
i = (h1 - h2)/L = delta h/L; delta h = iL; h1 = h2 + iL; h2 = h1 - iL.
Worked example
Heads of 12 m and 10 m separated by 100 m give i = 0.02, or a 2% head slope. Reversing the heads gives i = -0.02.
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Frequently asked questions
Is gradient a percentage?
It is a dimensionless ratio. Multiply by 100 to express it as a percentage.
Is head the water elevation alone?
In a piezometer, the measured water level represents elevation plus pressure head. Other flow settings may also require a velocity head.
Can equal heads determine distance?
No. A zero head difference with zero gradient is consistent with any positive distance.