$//<![CDATA[ \begin{array}{l}\displaystyle {\rm Re} = \frac{u(l) \cdot d}{\nu(l)} = \frac{4 \rho_s q_s}{\pi d} \frac{1}{\mu(T, p)}\end{array} //]]>$

The along-hole variation of Darcy friction factor  $//<![CDATA[ \begin{array}{l}f\end{array} //]]>$ is usually not exceeding 2-3 %  and in many practical applications can be considered as constant along hole $//<![CDATA[ \begin{array}{l}f = f_s = \rm const\end{array} //]]>$. 

The absolute value is staying between  $//<![CDATA[ \begin{array}{l}f = 0.04\end{array} //]]>$ for the very small flow rates (< 100 cmd) and  $//<![CDATA[ \begin{array}{l}f = 0.015\end{array} //]]>$ for the very high flow rates  (> 1,000 cmd). 

See also

Physics / Fluid Dynamics / Pipe Flow Dynamics / Darcy–Weisbach equation / Darcy friction factor