History of Rainfall Mapping in Hawaiʻi
The earliest known rainfall observations in Hawaiʻi were taken by Dr. Thomas Charles Byde Rooke in 1837 at Nuʻuanu 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 of rainfall. The table below lists some of the more prominent of those efforts.
Prior Rainfall Maps of Hawaiʻi
| Statistic | Interval | Coverage | Citation |
|---|---|---|---|
| Mean | Annual | Oʻahu | Voorhees (1929) |
| Mean | Annual | Oʻahu | Nakamura (1933) |
| Mean | Annual | Major islands | Feldwisch (1939) |
| Mean | Annual | Maui | Stearns and Macdonald (1942) |
| Median | Monthly | Oʻahu | Halstead and Leopold (1948) |
| Mean | Annual | East Maui | Leopold (1949) |
| Mean | Annual | Major islands | Stidd and Leopold (1951) |
| Mean | Monthly, Annual | Major islands | Mordy and Price (1955) |
| Median | Monthly, Annual | Major islands | Taliaferro (1959) |
| Mean | Annual | Major islands | Blumenstock and Price (1967) |
| Median | Annual | Major islands | Department of Land and Natural Resources (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) |
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.
Below are some examples of previous analyses of monthly and annual rainfall.
 |
Map of median January rainfall for the island of Oʻahu, developed by Halstead and Leopold in 1948. The base period of the statistics is 1936–1946. |
 |
Map of mean January rainfall for the island of Oʻahu, developed by Mordy and Price in 1955. No common base period was used. Periods of record ranged from 10 to 68 years. |
 |
Map of median January rainfall for the island of Oʻahu, developed by Taliaferro in 1959. The base period of the statistics is 1933–1957. |
 |
Map of median annual rainfall for the island of Oʻahu, developed by Meisner et al. in 1982. The base period of the statistics is 1916–1975. |
 |
Map of mean annual rainfall for the island of Oʻahu from the original Rainfall Atlas of Hawaiʻi developed by Giambelluca et al. in 1986. The base period of the statistics is 1916–1983. |
 |
Map of mean annual rainfall for the island of Oʻahu, developed by the PRISM Group in 2006. The base period of the statistics is 1961–1990. |
 |
Map of mean annual rainfall for the island of Oʻahu, developed from the new 2011 Rainfall Atlas of Hawaiʻi. 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. Because rainfall can vary significantly on time scales of years to decades, the “era” of a particular gage, i.e., the time during which it operated, can have a big influence on the estimated mean rainfall. When mapping rainfall, means calculated from different eras can produce spurious spatial patterns. This problem has been addressed in various ways. Mordy and Price (1955) acknowledged this issue, but decided not to address it. They simply used all the data available for stations with 10 years of record or more. Taliaferro (1959) calculated medians for a 25-year base period (1933–1957) “based on actual and extrapolated data”. No details were provided on the method of extrapolation. Meisner et al. (1982) adjusted rainfall medians to a common 60-year base period (1916–1975) using a statistical technique called ridge regression. In the original Rainfall Atlas of Hawaiʻi, Giambelluca et al. (1986) adopted Meisner’s approach, using a base period of 1916–1983 for all islands except Molokaʻi, where a 1931–1983 base period was used. In the 2011 Rainfall Atlas of Hawaiʻi, each station in the selected network was “gap-filled” using a variety of statistical techniques to produce complete or nearly complete records for a 30-yr base period, 1978–2007.
Another difference among the previous maps is in the choice of a normal statistic. The average, or mean, used in the 2011 Rainfall Atlas of Hawaiʻi, was commonly used in the past. But several maps were done using the median, the value for which half the observations were higher and half lower. The mean is more meaningful for hydrological purposes, because it is related to the total amount of rainfall over the base period. The mean can be strongly influenced by a relatively few extreme values, hence, some prefer the median as a more representative measure of the central tendency of rainfall.
In most prior rainfall analyses done for Hawaiʻi, point rainfall values were analyzed manually by drawing lines of equal rainfall (isohyets). In areas with a dense and well-distributed network of stations, the analyst must use expert knowledge to resolve apparent conflicts among station values to produce smooth isohyets. Similarly, in areas lacking sufficient measurements, expert knowledge is called upon to estimate the patterns based on presumed relationships between rainfall and topography or patterns of vegetation. In the 2011 Rainfall Atlas of Hawaiʻi manual analysis was not used. Instead, raingage data were supplemented with other predictors, in the forms of rainfall maps derived from radar, a dynamical weather model, and a previously done analysis incorporating relationships with terrain (PRISM, Daly et al., 2002), and statistical techniques were used to merge these different predictors to produce the final maps. Please refer to the Methods page or our final report for the details of this procedure.
References
Blumenstock, D.I. and Price, S. 1967. Climate of Hawaii. In Climates of the States, no. 60-51, Climatography of the United States, U.S. Department of Commerce.
Daly, C., Smith, J., Doggett, M., Halbleib, M., and Gibson, W. 2006. High-resolution climate maps for the Pacific Basin Islands, 1971-2000. Report submitted to National Park Service Pacific West Regional Office. PRISM Group, Oregon State University.
http://www.botany.hawaii.edu/basch/uhnpscesu/pdfs/sam/Daly2006AS.pdf
Department of Land and Natural Resources. 1973. Climatological stations in Hawaii. Report R42, Division of Water and Land Development, DLNR, State of Hawai‘i. 187 pp.
Feldwisch, W.F. 1939. Progress report (1939)--Water resources. Territorial Planning Board, Territory of Hawaiʻi, Honolulu.
Giambelluca, T.W., Nullet, M.A., and Schroeder, T.A. 1986. Rainfall Atlas of Hawaiʻi, Report R76, Hawai‘i Division of Water and Land Development, Department of Land and Natural Resources, Honolulu. vi + 267 p.
Halstead, M.H. and Leopold, L.B. 1948. Monthly median rainfall maps, what the are--how to use them. Report No. 2, Meteorology Department, Pineapple Research Institute and Hawaiian Sugar Planters' Association. 18 pp.
Leopold, L.B. 1949. Average annual rainfall of East Maui, T.H. Hawaii Sugar Planters' Record 53(2): 47-59.
Meisner, B.N., Schroeder, T.A., and Ramage, C.S. 1982. Median rainfall, State of Hawaii. Circular C88, Division of Water and Land Development, Department of Land and Natural Resources, State of Hawaiʻi.
Mordy, W.A. and Price, S. 1955. Average monthly rainfall maps. Meteorology Department, Pineapple Research Institute, and Experiment Station, Hawaiian Sugar Planters' Association.
Nakamura, W.T. 1933. A study of the variation in annual rainfall of Oahu Island (Hawaiian Islands) based on the law of probabilities. Monthly Weather Review 61: 354-360.
Stearns, H.T. and Macdonald, G.A. 1942. Geology and ground-water resources of the island of Maui, Hawaii. Bulletin 7, Hawaii Division of Hydrography. 344 pp.
Stidd, C.K. and Leopold, L.B. 1951. The geographic distribution of average monthly rainfall, Hawaii. On the rainfall of Hawaii, a group of contributions. Meteorological Monographs 1(3): 24-33.
Taliaferro, W.J. 1959. Rainfall of the Hawaiian Islands. Hawaii Water Authority, State of Hawaiʻi, Honolulu. 396 pp.
Voorhees, J.F. 1929. A quantitative study of rainfall of the island of Oahu. App. A, Report of the Honolulu Sewer and Water Commission to the Legislature of the Territory of Hawaiʻi, 15th Regular Session.
