static void outputWaterCut(const Opm::Watercut& watercut,
                            const std::string& output_dir)
 {
     // Write water cut curve.
     std::string fname = output_dir  + "/watercut.txt";
     std::ofstream os(fname.c_str());
     if (!os) {
         OPM_THROW(std::runtime_error, "Failed to open " << fname);
     }
     watercut.write(os);
 }