Future Drought projections in a fragile island system: The case of Rapa Nui.

Rapa Nui Island, is the most isolated inhabited place in the world and a popular tourist destination, like other island communities located in the Pacific Ocean. This unique system, which lacks rivers or permanent surface watercourses, is particularly vulnerable to climatic variations that could affect groundwater recharge, which is their main source of freshwater. The increase in water consumption, along with predictions of less precipitation and higher temperatures due to climate change, underscores the need to better understand future drought conditions on Rapa Nui Island. 

Here, we selected and statistically downscale and bias-corrected 11 CMIP5 and 3 CMIP6 Global Circulation Models (GCMs) under the scenarios RCP8.5 and SSP5-8.5, respectively, to study the projections of droughts events in Rapa Nui until the end of the century. To do so, we analyze severe and extreme droughts using the SPI(12) and SPEI(12) indexes estimating potential evapotranspiration (PET) with the Thornthwaite and Hargreaves methods. 

Our results indicate a sustained decrease in precipitation, an increase in temperature, and a higher frequency of drought events with longer durations and greater intensities compared to historical climatological periods (1970-2014). Specifically, by the end of the century, average annual precipitation is projected to decrease by more than 20% (29% under the SSP 5-8.5 scenario compared to 24% under RCP 8.5), while the mean temperature is expected to increase by approximately 2°C for each scenario. Regarding extreme droughts, projections based on the SSP 5-8.5 result in more adverse outcomes, particularly in the far future (2065–2100). For the SPI index, extreme drought frequencies under this scenario are projected to exceed historical frequencies by 61% in the distant future, and by 23% compared to those projected under the RCP 8.5 scenario. 

We conclude that the analysis of drought is highly dependent on the method used to estimate PET. For instance, the projected results using the Thornthwaite method show differences exceeding 17% in the frequencies of extreme droughts by the end of the century compared to the Hargreaves method. Both scenarios project more intense and prolonged droughts than those experienced in the past, emphasizing the urgency of investigating and implementing measures to ensure the population's water supply security and the preservation of the island's biodiversity, always integrating the opinions and respecting the culture of the Rapa Nui people. 

Finally, these results highlight the importance of studying representative values of this variable during the historical period and underscore the relevance of adopting measures to mitigate climate risks associated with drought events in fragile systems such as that of Rapa Nui.