Spatial trend analysis of Hawaiian rainfall from 1920 to 2012

Abstract

Spatial patterns of rainfall in Hawai‘i are among the most diverse in the world. As the global climate warms, it is important to understand observed rainfall variations to provide context for future changes. This is especially important for isolated oceanic islands where freshwater resources are limited, and understanding the potential impacts of climate change on the supply of freshwater is critical. Utilizing a high‐resolution gridded data set of monthly and annual rainfall for Hawai‘i from January 1920 to December 2012, seasonal and annual trends were calculated for every 250‐m pixel across the state and mapped to produce spatially continuous trend maps. To assess the stability of these trends, a running trend analysis was performed on 34 selected stations. From 1920 to 2012, over 90% of the state experienced drying trends, with Hawai‘i Island, and in particular the western part of the island, experiencing the largest significant long‐term declines in annual and dry season rainfall. The running trend analysis highlighted the multi‐decadal variability present in these trends, and revealed that the only region in the state with persistent annual and dry season trends through the study period is the western part of Hawai‘i Island; for most other regions, the drying trends were not significant until the most recent part of the record was included. These results support previous studies that indicate drying across the state over recent decades, and reveal the timing of upward and downward trends as well as important spatial details for natural resource management in Hawai‘i.

Spatial patterns of seasonal crop production suggest coordination within and across dryland agricultural systems of Hawaiʻi Island

“…Yet Hawaiʻi rainfall has fluctuated through the past five centuries (Diaz et al 2016), and leeward Hawaiʻi has experienced strong declines in dry season rainfall over the last decades (Frazier and Giambelluca 2017). Future work should consider how spatial rainfall patterns affected by El Niño and longer-term climate cycles (Frazier, Giambelluca, and Diaz 2012, unpublished manuscript) influence dryland production and island-wide regional socio-political developments.…”

Long‐term streamflow trends in Hawai'i and implications for native stream fauna

“…The running trend analysis highlighted that many of the significant trends did not persist through time but were only significant if the last few decades of record were included in the analysis. Our findings are supported by similar patterns in rainfall trends, which have significantly declined across the Hawaiian Islands from 1920 to 2012 (Frazier & Giambelluca, ).…”

“…The m 3 s −1 per year Sen's slopes were multiplied by 10 and divided by the 1978–2007 mean to give percent change in discharge per decade. The 1978–2007 time‐period was chosen to coincide with the Rainfall Atlas of Hawai'i 1978–2007 reference period (Frazier & Giambelluca, ; Giambelluca et al, ).…”

“…Last, we increased the starting year and calculated the trend for window sizes up to 49 years (1968–2016). To visualize the results, running trend plots in the style of Brunetti et al () and Frazier and Giambelluca () were used.…”

“…Mean surface temperature in Hawai'i has increased 0.163°C decade −1 from 1975 to 2006, with expected increases in potential evapotranspiration (Giambelluca, Diaz, & Luke, ). Recent studies show a decline in total rainfall affecting groundwater recharge and the groundwater contribution to surface flow in many regions of Hawai'i (Bassiouni & Oki, ) as well as a decline in rainfall intensity (Chen & Chu, ; Chu, Chen, & Schroeder, ; Frazier & Giambelluca, ), reducing run‐off to streams (Bassiouni & Oki, ; Leta, El‐Kadi, & Dulai, ; Oki, ). Additionally, recent increases in the frequency of trade wind inversion days in Hawai'i indicates more consistent rainfall on windward coasts, a decline in high‐elevation rainfall, an increase in the number of dry days between storms, and a decrease in leeward rainfall (Cao, Giambelluca, Stevens, & Schroeder, ; Longman, Diaz, & Giambelluca, ).…”