%smith93, assumes r=r80
function dfdr=fdrop_sm(u,r)
u14=u.*(1+.04/.4*log(1.4));
a1=10.^(.0675*u14+2.43);
a2=10.^(.959*sqrt(u14)-1.476);
f1=3.1;f2=3.3;
r1=2.1;r2=9.2;
dfdr=a1.*exp(-f1.*(log(r/r1).^2))+a2.*exp(-f2.*(log(r/r2).^2));
