Rainfall

Rainfall refers to the amount of precipitation in the form of rain that occurs in a specific area over a given period. Changes in rainfall can have a wide-ranging impact on humans and ecosystems (USGCRP, 2017). On islands, where rainfall is often the primary source of all freshwater, extended dry periods affect drinking water supply (Miles et al., 2020). Drought also affects agriculture, as well as streamflow necessary to maintain aquatic habitat. Heavy or extreme rainfall events can damage crops, increase soil erosion, and lead to in-land flooding that damages infrastructure. Runoff from excessive precipitation can also carry harmful pollutants into nearby water bodies, endangering aquatic species as well as human health.

Monitoring rainfall is important for understanding climate change and variability. Rainfall measurements come from gauges, weather radar, and satellites, and typically include annual and daily totals, the number of dry days or dry spells, standardized drought indices, and counts of heavy‑rain days at thresholds relevant for impacts (IPCC, 2021; McGree et al., 2022).

Palau’s rainfall is regulated by the Intertropical Convergence Zone (ITCZ), the West Pacific Monsoon, and tropical disturbances. Palau experiences a wet season that runs from May to October, though Palau’s proximity to the Pacific Warm Pool results in relatively high rainfall through the year (McGree et al., 2022). Year to year variability is high. Long dry periods tend to be associated with strong El Niño events. For example, during the 1982–1983, 1997–1998, and 2015–2016 El Niño events Palau experienced droughts and acute water shortages which resulted in water rationing (Polhemus 2017; Rupic et al., 2018). Wetter conditions, and more heavy rainfall events, tend to accompany La Niña.

In this section we look at three indicators of change in rainfall. Using observations from the Koror meteorological station we will create plots that show changes in the annual rainfall, dry conditions, and wet days. Dry days are defined as days below 1mm (0.04 inches) threshold. Wet days are defined as days above 1mm (0.04 inches). Heavy rainfall days are defined as days where rainfall is greater than 47mm (1.9 inches), the 95th percentile.