Abstract
Spatially explicit, wall-to-wall rainfall data provide foundational climatic information but alone are inadequate for characterizing meteorological, hydrological, agricultural, or ecological drought. The Standardized Precipitation Index (SPI) is one of the most widely used indicators of drought and defines localized conditions of both drought and excess rainfall based on period-specific (e.g., 1-month, 6-month, 12-month) accumulated precipitation relative to multi-year averages. A 93-year (1920–2012), high-resolution (250 m) gridded dataset of monthly rainfall available for the State of Hawai‘i was used to derive gridded, monthly SPI values for 1-, 3-, 6-, 9-, 12-, 24-, 36-, 48-, and 60-month intervals. Gridded SPI data were validated against independent, station-based calculations of SPI provided by the National Weather Service. The gridded SPI product was also compared with the U.S. Drought Monitor during the overlapping period. This SPI product provides several advantages over currently available drought indices for Hawai‘i in that it has statewide coverage over a long historical period at high spatial resolution to capture fine-scale climatic gradients and monitor changes in local drought severity.
Highlights
Rainfall ranges in Hawai‘i are greater than those found on some continents, with extreme spatial variation found between wet windward areas and dry leeward locations [1]
Recent efforts to compile and spatially interpolate historical weather station data have resulted in the development of long-term (1920–2012) high-resolution gridded monthly rainfall data for the State of Hawai‘i [3]
Validation Results Validation analysis with the National Weather Service Honolulu Office (NWS) station data indicated that the gridded data performed well statewide across all available Standardized Precipitation Index (SPI) timesteps with R2 values ranging from 0.8 to 0.85, root mean squared error (RMSE) values Dfartao2m0200,.35,910t9o 0.48, and mean biased error (MBE) ranging from −0.02 to 0.07 for the 1, 3, 6, 12, 18, and 24-mont4hoSf P9 I
Summary
Hawai‘i is characterized by extreme variability in rainfall in both space and time [1,2] due primarily to trade winds, land heating, and the archipelago’s complex topography. Recent efforts to compile and spatially interpolate historical weather station data have resulted in the development of long-term (1920–2012) high-resolution gridded monthly rainfall data for the State of Hawai‘i [3] These new products and analyses provide an unprecedented opportunity to understand the effects and impacts of rainfall variability in Hawai‘i over time and space. Farmers, and utilities need high-spatial and -temporal resolution products for monitoring drought conditions and making both short-term and long-term decisions In response to these needs and the shortcomings of available products, the existing gridded monthly rainfall dataset [3] was used to develop a long-term (1920–2012) dataset of the high-resolution (250 m) gridded Standardized Precipitation Index (SPI) [11] for the State of Hawai‘i. At present, only gridded monthly rainfall data have been published for Hawai‘i [3], preventing the calculation of indices like PDSI and SPEI
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