ausdr = 500 - s + 0.05/0.00025 Log[s/490];
f[i0_,rho_] = If[1000-i0<rho, i0, 1000-rho (1 - Log[rho] + Log[1000-i0]) ];
abb = Plot3D[f[i0,rho],{i0,0,300},
{rho,50,500},AxesLabel->{"i0","b/a","i_max"},ImageSize->{400,300}];