= Travel coordinate for a conservative transport perturbation
{title2=$a(s,t)=\phi(\xi(s)-t)/V(s),\quad\xi(s)=\int_0^sV(r)^{-1}\,dr$}
For $a_t+(V(s)a)_s=0$ with $V>0$, the <flux> perturbation $b=Va$ solves $b_t+Vb_s=0$. The travel <coordinate> transforms it into unit-speed transport. Thus the <flux> perturbation is constant on <characteristics> while <area> <amplitude> changes as $1/V$. In a supplied power-law river, $A_{\rm eq}'=M/V$, giving travel <time> $[A_{\rm eq}(l)-A_{\rm eq}(0)]/M$. At a zero-speed head, this <time> can remain integrable even though the <amplitude> formula is not uniform.
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