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Precipitation within the Waikato Region of New Zealand
Dravitzki, S. and McGregor, J. 2011. Extreme precipitation of the Waikato region, New Zealand. International Journal of Climatology 31: 1803-1812.

The authors write that "Trenberth (1999) proposed that globally, extreme precipitation events would account for a larger proportion of annual precipitation as the globe warmed," as was additionally suggested by Trenberth et al. (2003). And they state that "this is also supported by the fourth IPCC report (IPCC, 2007)," which "expects the change in the intensity of precipitation events to be proportional to changes in total precipitation." Thus, they decided to see if any of these projections have occurred over the past century or more in New Zealand's Waikato region, which is an important farming district that also produces 13% of the country's electricity by means of hydro-generation.

What was done
Working with data from 18 meteorological observation stations located in and about the Waikato region, Dravitzki and McGregor developed daily precipitation time series covering the period 1900-2007, where they averaged the precipitation values, as they describe it, "both spatially and temporally to approximate the total volume of precipitation within the region," and where they defined heavy precipitation statistically, using 95th and 99th percentile threshold values, in order to be "consistent with the IPCC climate indices of precipitation extremes," while they also looked for any relationships that might exist between extreme precipitation events and the Interdecadal Pacific Oscillation (IPO), the El Niņo-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM), which relationships have been suggested by others in the past.

What was learned
In the words of the two New Zealand researchers, "since 1900 there have been no significant variations in the total annual precipitation nor in the occurrence or magnitude of extreme precipitation events," and they say that these events "were also uncorrelated to the large-scale IPO, ENSO and SAM, indicating that the seasonal probability of extreme precipitation is independent of these circulations."

What it means
In discussing their findings for the Waikato region of New Zealand, Dravitzki and McGregor write that "the consistency of the precipitation totals suggests that the current economically important water supply is secure within the region." And we would add that their finding of no evidence for the projections of Trenberth and the IPCC over a 107-year period -- when climate alarmists claim the world warmed at a rate and to a level of warmth that were unprecedented over the past millennium or more - suggests that the projections of Trenberth and the IPCC are not what they are cracked up to be, at least for this important part of New Zealand.

IPCC. 2007. Climate Change 2007. Cambridge University Press: Geneva, Switzerland.

Trenberth, K. 1999. Conceptual framework for changes of extremes of the hydrological cycle with climate change. Climatic Change 42: 327-339.

Trenberth, K., Dai, A., Rasmussen, R. and Parsons, D. 2003. The changing character of precipitation. Bulletin of the American Meteorological Society 84: 1205-1217.

Reviewed 1 February 2012