A decade drought monitoring through enhanced and standardized vegetation index in isolated karst environment: Nusa Penida Island, Bali, Indonesia

Abstract

Nusa Penida is one of the regions in Indonesia that vunarablely experiences extreme drought. The worst drought occurred in December 2019, coinciding with the Covid-19 pandemic outbreak. At that time, drought caused major crises in many local primary economic aspects, such as clean-water crises and local crop failure. The situation worsened after the global pandemic lockdown was imposed. Many tourism-related hospitalities (i.e., hotels, resorts, and restaurants) also have crashed. Isolated karst islands such as Nusa Penida Island pose significant challenges for water and agricultural land management. Geologically, this area consists of hilly carbonate rock terrain, a lack of proper aquifers, low precipitation, and tend to had high concentrations of dissolved Ca and Mg in its water. These conditions make many areas less preferable for settlements and traditional agriculture. In this study, we assessed the area using geospatial data for drought monitoring. We analyzed weekly-22 years of data (18/02/2000-18/02/2022 or 507 weeks data) to understand the period pattern of seasonal droughts in Nusa Penida over the past decade. This study used compiled satellite raster-images data to compute the Enhanced Vegetation Index (EVI) to express the Standardized Vegetation Index (SVI), which is sensitive to vegetation change during drought. The results showed that the longest droughts in Nusa Penida occurred from December 2000 to October 2001 (20 weeks) and Feb 2002 to Jan 2023 (21 weeks). The worst droughts occurred in December 2019 (SVI: -1.903 to -1.236), 2006 (SVI: -1.872 to -1.374), and 2002 (SVI: up to -0.136). There are patterns in which droughts in Nusa Penida tend to occur towards the end of the year, including at the end of 2022. Additionally, we detected some evergreen areas in Nusa Penida that require further investigation. These areas potentially act as natural permanent aquifers even during droughts. Furthermore, this study provides more insights for other comprehensive research, especially to enhance mitigation policies for near-future drought periods in Nusa Penida.