Fig. 6 Response surface in the -K_s plane obtained by the three-set combination approach. Other parameters include: _sk = 0.4, _rk = 0.05, _k = 12 m^-1, K_sk = 2.8 x 10^-5 m s^-1, and q = 1 x 10^-6 m² s^-1. Subscript k stands for the true value of the corresponding parameter