If the argument q is used, it overrides RP and provides RP as a function of q (magnitude of variable) as opposed to q as a function of RP. The parameters are estimated by the method of L-moments, as detailed in 'Hosking J. Wallis J. 1997 Regional Frequency Analysis: An Approach Based on L-moments. Cambridge University Press, New York'. The Kappa3 distribution is as defined by This is the Kappa3 distribution as defined in Kjeldsen, T (2019), 'The 3-parameter Kappa distribution as an alternative for use with FEH pooling groups.'Circulation - The Newsletter of the British Hydrological Society, no. 142.