code: def _dist_corr(dist, phi1, phi2, phi3):
          
          return phi1 * exp(-phi2 * dist) + (1 - phi1) * exp(-phi3 * dist)
docs: Generic distance-decaying correlation function
  
          :param dist: Distance between catchment centrolds in km
          :type dist: float
          :param phi1: Decay function parameters 1
          :type phi1: float
          :param phi2: Decay function parameters 2
          :type phi2: float
          :param phi3: Decay function parameters 3
          :type phi3: float
          :return: Correlation coefficient, r
          :rtype: float