code: def save_twi(self, rootpath, raw=False, as_int=True):
          
          self.twi = np.ma.masked_array(self.twi, mask=self.twi <= 0,
                                        fill_value=-9999)
          #  self.twi = self.twi.filled()
          self.twi[self.flats] = 0
          self.twi.mask[self.flats] = True
          # self.twi = self.flats
          self.save_array(self.twi, None, 'twi', rootpath, raw, as_int=as_int)
docs: Saves the topographic wetness index to a file