Unraveling the Interplay Between SPI-Based Drought Characteristics and Rainfall Variability: A High-Resolution Dasarian Assessment across Bali’s Seasonal Zones

In the tropical islands of Indonesia, the rhythm of life has long been dictated by the monsoon. For centuries, farmers, fishermen, and communities have relied on the predictable arrival of rain to fill rice paddies and replenish rivers. However, as the global climate warms, this ancient rhythm is becoming erratic. The rain is not just changing in how much it falls, but in how it falls. Instead of a steady, soaking drizzle that seeps into the soil, storms are becoming more intense and unpredictable, while the gentle, steady rains that sustain crops are disappearing. This shift creates a paradox where a region might receive plenty of water in a year, yet still face severe drought because the rain arrives in violent bursts that run off the land rather than nourishing it. Understanding this complex dance between total rainfall and the specific timing of storms is critical for regions like Bali, where water security underpins both the economy and the survival of traditional agriculture.

To make sense of this volatility, researchers from the State College of Meteorology, Climatology, and Geophysics in Indonesia turned their attention to the island of Bali. They sought to move beyond simple yearly totals, which often hide the true nature of the weather, and instead looked at the rain in ten-day chunks. In Indonesia, this ten-day period is known as a "dasarian," a unit of time that has long been the standard for local farmers and meteorologists to track the growing season. By analyzing a continuous, high-quality record of rainfall spanning thirty-five years, from 1991 to 2025, the team examined twenty distinct seasonal zones across the island. Their goal was to see not just how much rain fell, but how the character of that rain had changed over time, specifically looking for signs of drought and the frequency of wet periods that could support agriculture.

The study revealed that the island's climate is far more complex than a simple map of wet and dry areas would suggest. While the total amount of rain falling each year varies significantly across the island, ranging from 1,500 to 3,500 millimeters, the real story lies in the distribution of that water. The researchers found that severe droughts are not happening everywhere at once; instead, they cluster in specific northern and eastern zones. More importantly, they discovered a troubling disconnect between the total volume of rain and the availability of water for farming. In years when the total rainfall was high, it did not necessarily mean there were more days with good, soaking rain. Instead, the rain was often delivered in a few intense, short bursts, leaving long dry spells in between that could wither crops.

A key finding of the research is a fundamental shift in the type of rain the island is receiving. As the climate moves toward wetter conditions, the gentle, light rains that used to be common are vanishing. They are being replaced by medium-intensity storms. This is a critical change because light rain is ideal for agriculture; it soaks deep into the ground without causing erosion. When these light events disappear and are swapped for heavier, more concentrated downpours, the soil cannot absorb the water fast enough, leading to runoff and a loss of moisture for the plants. The data shows that this replacement of light rain with medium rain is a consistent, measurable trend across the island's seasonal zones.

Perhaps the most surprising discovery concerns the most extreme weather events. The researchers looked at the heaviest possible rainfall, defined as more than 300 millimeters in a single ten-day period, to see if these catastrophic storms were becoming more frequent as the climate warmed. They found that the frequency of these extreme events is completely stationary; it has not changed over the last thirty-five years. Whether the island is in a wet phase or a dry phase, the number of these extreme, flood-causing events remains the same. This suggests that these massive storms are not driven by the general wetness or dryness of the climate, but by the island's own geography. The steep volcanic mountains in the center of Bali force the air to rise and cool rapidly, creating these intense bursts of rain regardless of the broader weather patterns.

These findings challenge the way water security is currently managed in the region. For decades, planners have often looked at the total amount of rain in a year to decide if there is enough water for the population and the farms. This study proves that such a broad view is misleading. A year with high total rainfall can still be a disaster for agriculture if that rain comes in a few violent storms followed by long dry spells. The traditional irrigation systems of Bali, known as Subak, rely on a steady, predictable flow of water, not episodic deluges. The research indicates that the island is moving toward a climate where the steady, reliable rain is gone, replaced by a pattern of intense bursts and dry gaps.

The authors conclude that protecting the future of Bali's water resources requires a shift in strategy. Relying on annual averages is no longer sufficient. Instead, water management must focus on capturing and storing water during those intense, short bursts of rain to bridge the widening gaps between them. The study provides a clear, high-resolution picture of how the climate is changing, showing that the island is not just getting wetter or drier, but is fundamentally restructuring the way rain falls. By understanding that extreme storms are fixed by geography while the steady rain is vanishing, local leaders and farmers can better prepare for a future where the rhythm of the monsoon is no longer the same.