Rainfall Trends through Time: A Running Trend Analysis of Hawaiian Rainfall

Abstract:

Significant declines in rainfall have been observed in Hawai‘i since 1920. However, these trends have not been uniform in space or time. It is important to understand past rainfall variations to provide the context for future changes. This is especially important for isolated oceanic islands where resources are limited, and understanding the potential impacts of climate change on freshwater supplies is crucial. Maps of seasonal and annual rainfall trends have been produced for the State of Hawai‘i for the period 1920-2012. However, since rainfall time series can be highly variable and these trend values may be sensitive to the time period chosen, a running trend analysis was performed to assess the stability of these trends through time. Running trend plots were generated for selected stations chosen to represent unique climate regions based on exposure (windward or leeward) and elevation (low, mid or high). The running trend analysis indicates that in many areas, these drying trends are only significant when the last few years of the record are included in the trend period. The only region with persistent trends through time is the Kona region of the Island of Hawai‘i, with significant long-term drying trends, particularly in the dry season. This analysis also highlights the influence of natural variability in short-term trends. These results support previous studies that indicate drying across the state, and reveal the timing of upward and downward trends as well as important spatial details for water resource managers in Hawai‘i.