Towards prediction of extreme rainfalls in New Zealand

摘要

Dimensionless rainfall depths are estimated for New Zealand, treated as three regions, for durations from 1 to 72 hours and return periods from 10 to 1,000 years. Data from individual gauges or sites are modelled using the General Extreme Value distribution. On average some 70% of sites follow an Extreme Value type 1 distribution, 27% EV2 and 3% EV3. Regional data are modelled by the Kappa distribution and show that dimensionless depths decrease slightly with duration, with the largest values for each return period occurring in the South Island East Region, intermediate values in the North Island Region and smallest values in the South Island West Region. As this study is exploratory, derived values of dimensionless rainfall depth may be employed as a rapid check on High Intensity Rainfall Design System (HIRDS V3) estimates up to return periods of 100 years; and for greater return periods as indicative values in design. At locations where only the mean annual rainfall is known, rainfall depths may be calculated for specified return periods from 100 to 1,000 years using our dimensionless depth values, and donor sites with suitable rainfall records.