code: def write_u2d(self, u2d):
          
          filename = os.path.split(u2d.filename)[-1]
          np.savetxt(os.path.join(self.m.model_ws,self.arr_org,filename),
                     u2d.array,fmt="%15.6E")
          return filename
docs: write a flopy.utils.Util2D instance to an ASCII text file using the
          Util2D filename
  
          Parameters
          ----------
          u2d : flopy.utils.Util2D
  
          Returns
          -------
          filename : str
              the name of the file written (without path)