Dispersion values (K_h) specify the longitudinal spreading of water quality constituents during transport through river reaches in HEC-ResSim.

Values may either be specified as user-entered constants (allowing for spatial variation in those constants), or may be calculated dynamically at cell faces based on the equation

K_h = 0.011 \frac{v^2 W^2}{dv^*} \quad v^* = v \sqrt{gd}

where

v* is shear velocity,
W is the stream top width,
g is gravitational acceleration,
d is hydraulic depth.

This is the same formulation used in the HEC-RAS 1D water quality model, and its derivation is described in Fisher et al. (1979).