Friday, November 13, 2015

Newly published research on evaluating the persistence of prolonged drought periods for the Island of Ireland.

An evaluation of persistent meteorological drought using a
homogeneous Island of Ireland precipitation network

R. L. Wilby, S. Noone, C. Murphy, T. Matthews, S. Harrigan and C. Broderick

Int. J. Climatol. (2015)
 Published online in Wiley Online Library ( DOI: 10.1002/joc.4523

ABSTRACT: This paper investigates the spatial and temporal properties of persistent meteorological droughts using the homogeneous Island of Ireland Precipitation (IIP) network. Relative to a 1961–1990 baseline period it is shown that the longest observed run of below average precipitation since the 1850s lasted up to 5 years (10 half-year seasons) at sites in southeast and east Ireland, or 3 years across the network as a whole. Dry spell and wet spell length distributions were represented by a first-order Markov model which yields realistic runs of below average rainfall for individual sites and IIP series. This model shows that there is relatively high likelihood (p=0.125) of a 5-year dry spell at Dublin, and that near unbroken dry runs of 10 years or more are conceivable. We suggest that the IIP network and attendant rainfall deficit modelling provide credible data for stress testing water supply and drought plans under extreme conditions.

This piece of research uses the homogeneous Island of Ireland Precipitation (IIP) network of Noone et al. (2015) to evaluate the occurrence and persistence of meteorological droughts from 1850-2010. For this research a drought is defined as half year periods or longer at site or regional scales that have below average precipitation. This examination uses seasonal rainfall and persistence metrics not applied in the homogenization process and allows for an independent quality assurance test of the IIP network.

The key findings show that the Island of Ireland has been prone to runs of seasonal rainfall deficits with prolonged dry spells in the 1850’s,1880’s and 1970’s that are far more persistent than any experienced in the last 40 years. These episodes could provide useful information that could be used to stress test the effectiveness of water supply and drought plans. These results should be of particular interest to Irish Water as they continue to invest in water infrastructure and design stress testing. This research has shown that there is a high likelihood (p=0.125) of a prolonged continuous 5 years (10 season) dry spell at Dublin. Population growth and out of date infrastructure mean that the Irish water system is already functioning on the brink of its capacity.

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