THE RAINFALL ATLAS OF HAWAI‘I 2011 Final Report Thomas W. Giambelluca1, Qi Chen1, Abby G. Frazier1, Jonathan P. Price2 Yi-Leng Chen3, Pao-Shin Chu3, and Jon K. Eischeid4 1Department of Geography, University of Hawai‘i at Mānoa, Honolulu, HI, USA 2Department of Geography and Environmental Sciences, University of Hawai‘i - Hilo, Hilo, HI, USA 3Department of Meteorology, University of Hawai‘i at Mānoa, Honolulu, HI, USA 4Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO, USA October 2011 The Rainfall Atlas of Hawai‘i 2011 Final Report Acknowledgments The 2011 Rainfall Atlas of Hawai‘i was developed under an agreement between the State of Hawai‘i Commission on Water Resource Management and the U.S. Army Corps of Engineers, Honolulu District under Section 22 of the Water Resources Development Act of 1974. Contract No. W9128A-04-D-0019, Contract Task Order No. 0038 was awarded to Wil Chee Planning, Inc., which subcontracted the work to the University of Hawai‘i at Mānoa, Department of Geography. The development of the 2011 Rainfall Atlas of Hawai‘i was supported by numerous individuals and organizations. Please see the “People” tab of the Rainfall Atlas of Hawai‘i web site (http://rainfall.geography.hawaii.edu) for information about those who contributed to this work. We are grateful to the people listed there, as well as the many other individuals who contributed to Hawai‘i’s rainfall dataset by carefully and consistently making measurements and recording and preserving the data. Those records are an invaluable resource, without which no rainfall map analysis would be possible. Citing the Rainfall Atlas of Hawai‘i Please cite this report and any products from the Rainfall Atlas of Hawai‘i website (including screen shots, copied data, saved graphs, downloaded products, and text) as follows: “Giambelluca TW, Chen Q, Frazier AG, Price JP, Chen Y-L, Chu P-S, Eischeid J., and Delparte, D. 2011. The Rainfall Atlas of Hawai‘i. http://rainfall.geography.hawaii.edu.” 2 The Rainfall Atlas of Hawai‘i 2011 Final Report Introduction The Hawaiian Islands have one of the most diverse rainfall patterns on earth. The mountainous terrain, persistent trade winds, heating and cooling of the land, and the regular presence of a stable atmospheric layer at an elevation of around 7,000 ft. interact to produce areas of uplift in distinct spatial patterns anchored to the topography. The resulting clouds and rainfall produced by this uplift lead to dramatic differences in mean rainfall over short distances. Knowledge of the mean rainfall patterns is critically Why “Rainfall” instead of “Precipitation”? important for a variety of resource management issues, including Precipitation in Hawai‘i includes rain, ground water and surface water various types of frozen precipitation, such development and protection, as snow, sleet, hail, and freezing rain, and controlling and eradicating invasive fog drip. Frozen precipitation is a minor species, protecting and restoring contributor overall. However, fog drip, derived from direct interception of cloud native ecosystems, and planning for droplets by vegetation, is a major source of the effects of global warming. water in the middle-elevation fog zones of Hawai‘i's mountains. We use the term The Rainfall Atlas of Hawai‘i is a set “rainfall” here instead of “precipitation, of maps of the spatial patterns of because, although we include frozen rainfall for the major Hawaiian precipitation as a minor component, the Islands. Maps are available for mean other major precipitation component, fog monthly and annual rainfall. The drip, is not included. maps represent our best estimates of the mean rainfall for the 30-yr base period 1978–2007. However, for many reasons, it is not possible to determine the exact value of mean rainfall for any location. Therefore, for every map of mean rainfall, we provide a corresponding map of uncertainty. Uncertainty tends to be greatest where we have the poorest information about rainfall, for example in remote locations far from the nearest raingage. A Rainfall Atlas of Hawai‘i web site (http://rainfall.geography.hawaii.edu) was developed to make the rainfall maps, data, and related information easily accessible. The maps depict rainfall patterns by color and/or by isohyets (lines of equal rainfall). The interactive map allows users to see the patterns of mean monthly and annual rainfall and corresponding uncertainty, zoom in on areas of particular interest, navigate to specific locations with the help of a choice of different base maps, and click on any location to get the mean annual rainfall and a graph and table of mean monthly rainfall. The locations of stations can also be shown on the interactive map. Clicking on a station gives both station and mapped estimates of monthly rainfall along with station metadata. 3 The Rainfall Atlas of Hawai‘i 2011 Final Report Rainfall maps can also be downloaded in various forms. Our analysis produced digital maps called rasters or grids. On these maps, the islands are divided into 8.1-arcsecond spatial units, or approximately 234 × 250 m (770 × 820 ft). Rainfall and uncertainty are estimated for each spatial unit. GIS (Geographic Information System) users can obtain mean and uncertainty maps as raster files, as well as Figure 1. The spatial unit for rainfall analysis in the 2011 Rainfall Atlas of Hawai‘i is 8.1 arcseconds, or shapefiles of isohyet lines 0.00225° longitude and latitude, equivalent to about and station points. 234 m (east-west) by 250 m (north-south). Alternatively, image files showing rainfall patterns by color and/or by isohyets can be downloaded. Rainfall measurements taken at over 1,000 stations were used as the principal source of information in the development of the rainfall maps. Files containing estimated mean monthly and annual rainfall and uncertainty for each station used in the analysis are available for download. A file with information on each station, including the name, observer, location, elevation, and period of record, is also available. This report and the Rainfall Atlas of Hawai‘i website update and supersede the original Rainfall Atlas of Hawai‘i (Giambelluca et al., 1986; herein referred to as the “1986 RF Atlas”). The following pages describe the input data, explain the methods used, and examine the results of the new rainfall analysis. A more detailed description of the methodology is given in the Appendix to this report. History of Rainfall Mapping in Hawai‘i The earliest known rainfall observations in Hawai‘i were taken by Dr. Thomas Charles Byde Rooke in 1837 at Nu‘uanu Avenue and Beretania Street in Honolulu. By the end of the 19th century, rainfall was being monitored at 106 stations. That number increased to 422 by 1920. As data accumulated and the number of observation sites expanded, various efforts were made to map the spatial patterns 4 The Rainfall Atlas of Hawai‘i 2011 Final Report of rainfall. Table 1 lists some of the more prominent of those efforts. With each successive analysis, the resulting maps were refined and improved, taking advantage of a growing database and better understanding of the processes controlling rainfall. Differences among these maps reflect this refinement and improvement, as well as fluctuations in rainfall over time. Figures 2-8 show some examples of analyses of monthly and annual rainfall for O‘ahu. Table 1. Prior Rainfall Maps of Hawai‘i Statistic Interval Coverage Citation Mean Annual O‘ahu Voorhees (1929) Mean Annual O‘ahu Nakamura (1933) Mean Annual Major islands Feldwisch (1939) Mean Annual Maui Stearns & Macdonald (1942) Median Monthly O‘ahu Halstead & Leopold (1948) Mean Annual East Maui Leopold (1949) Mean Annual Major islands Stidd & Leopold (1951) Mean Monthly, Annual Major islands Mordy & Price (1955) Median Monthly, Annual Major islands Taliaferro (1959) Mean Annual Major islands Blumenstock & Price (1967) Median Annual Major islands Dept. Land & Nat. Res. (1973) Median Annual Major islands Meisner et al. (1982) Mean, Median Monthly, Annual Major islands Giambelluca et al. (1986) Mean Monthly, Annual Major islands Daly et al. (2006) Figure 2. Map of median January rainfall for the island of O‘ahu, developed by Halstead and Leopold in 1948. The base period of the statistics is 1936–1946. 5 The Rainfall Atlas of Hawai‘i 2011 Final Report Figure 3. Map of mean January rainfall for the island of O‘ahu, developed by Mordy and Price in 1955. No common base period was used. Periods of record ranged from 10 to 68 years. Figure 4. Map of median January rainfall for the island of O‘ahu, developed by Taliaferro in 1959. The base period of the statistics is 1933–1957. Figure 5. Map of median annual rainfall for the island of O‘ahu, developed by Meisner et al. in 1982. The base period of the statistics is 1916–1975. 6 The Rainfall Atlas of Hawai‘i 2011 Final Report Figure 6. Map of mean annual rainfall for the island of O‘ahu from the 1986 RF Atlas developed by Giambelluca et al. The base period of the statistics is 1916–1983. Figure 7. Map of mean annual rainfall for the island of O‘ahu, developed by the PRISM Group in 2006. The base period of the statistics is 1971–2000. Figure 8. Map of mean annual rainfall for the island of O‘ahu, developed from the new 2011 Rainfall Atlas of Hawai‘i. The base period of the statistics is 1978–2007. Some key differences can be seen in the methods used for these different analyses. One important issue involves the use of raingage measurements taken during different periods of time. Over the years, many gages were set up and operated for various numbers of years and subsequently discontinued.