Sulawesi Is Losing Its Water-Regulating Landscapes, Satellite Record Shows

A new analysis of Indonesia’s enigmatic orchid-shaped island of Sulawesi has quantified, in unusually stark numbers, how three decades of land conversion have eroded the island’s natural capacity to regulate water. The study, published in Environmental Monitoring and Assessment by Munajat Nursaputra of Hasanuddin University, mapped the potential of Sulawesi’s landscapes to deliver water regulation ecosystem services across three snapshot years: 1990, 2006, and 2022. The verdict is sobering. In 1990, nearly six of every ten hectares of the island fell into the very high or high classes of potential water regulation capacity. By 2022, that share had fallen to 44.9 percent, a contraction of higher-capacity land that signals a measurable weakening of the island’s hydrological safety net.

Water regulation is one of the least visible yet most consequential services that ecosystems provide. Forested slopes intercept rainfall, slow surface runoff, and encourage infiltration into soils and aquifers, which in turn sustains baseflow in rivers during dry seasons and buffers downstream communities against floods and landslides. When forests are cleared for agriculture, plantations, or settlements, the landscape loses much of this sponge-like behavior. Rainfall that once soaked gradually into the ground instead races over compacted surfaces, carrying topsoil with it, swelling rivers quickly, and leaving less water stored for the months when it is needed most. Sulawesi, with its rugged central highlands, four peninsulas, and dense network of small watersheds, is particularly sensitive to these shifts because so many communities depend on short, steep catchments for irrigation and drinking water.

To capture this capacity spatially, the study employed a weighted spatial index built on a Simple Additive Weighting approach, a multi-criteria decision technique that combines several input layers into a single score. In this case, the layers included natural landscape characteristics, natural vegetation, and land cover data for each of the three reference years. Each land cover type was assigned a weight reflecting its contribution to water regulation, so that intact natural forest scored highest, while agricultural land and built-up areas scored progressively lower. The weighted layers were then summed to produce a continuous index of potential water regulation capacity, which was classified into categories ranging from very high to very low. This approach allowed the author to compare like with like across a 32-year span and to pinpoint where on the island the losses were concentrated.

The results reveal a clear geographic pattern of decline. Central Sulawesi and South Sulawesi contributed most strongly to the reduction in high-capacity areas, consistent with the intensive forest conversion and agricultural expansion that have characterized those provinces in recent decades. South Sulawesi, home to the densely populated southwestern lowlands and the Makassar urban corridor, has long experienced pressure from rice cultivation, cocoa and other perennial crops, and urban growth. Central Sulawesi has seen extensive land clearing linked to agriculture and, in some areas, plantation development. By contrast, Southeast Sulawesi and North Sulawesi showed partial recovery of high-capacity classes in the later period, suggesting that regrowth of secondary vegetation, protection efforts, or shifts in land management may have locally restored some regulatory function even as the island-wide trend remained negative.

Underlying these capacity changes is a fundamental shift in what covers the land. The study found that vegetated land cover types that strongly support water regulation declined from 75 percent of the island in 1990 to 54 percent in 2022. That is a loss of roughly a fifth of the island’s vegetated surface in a single generation, largely driven by conversion to agricultural and built-up areas. The magnitude of this transition matters because the relationship between vegetation and water regulation is not linear at the margins. Small losses of forest in headwater zones can disproportionately affect downstream flow regimes, and once soils are degraded, the hydrological functions they supported can take decades to recover even if vegetation returns.

The spatial heterogeneity that the study documents is itself scientifically important. Sulawesi is not losing its water regulation capacity uniformly; it is losing it in patches, corridors, and hotspots that correspond to provincial boundaries, watershed networks, and the geography of agricultural frontiers. This heterogeneity is precisely what makes the mapping approach valuable for policy. Rather than treating the island as a single unit with a single trend, the weighted index produces provincial- and watershed-level hotspots where the decline is most acute, and therefore where interventions such as forest protection, restoration, and ecosystem service-based zoning could be prioritized with the greatest expected benefit. In practical terms, a watershed whose index has slipped from high to moderate is a candidate for targeted restoration of riparian buffers and headwater forest, while a province showing widespread contraction of high-capacity classes may warrant stronger land use controls.

The study’s methodology also illustrates a broader trend in ecosystem service science: the use of relatively simple, transparent, and repeatable indices rather than data-hungry process models. Complex hydrological simulations can capture the physics of rainfall-runoff partitioning in detail, but they demand extensive calibration data that are often unavailable in tropical regions. A Simple Additive Weighting index, by contrast, requires only consistent land cover and landscape datasets, which are increasingly available from national mapping agencies and satellite programs. The trade-off is that the index measures potential capacity rather than actual water fluxes; it indicates where the landscape is structurally capable of supporting rainfall retention, baseflow, and watershed resilience, not how many cubic meters of water are being stored in any given year. For regional planning purposes, however, this potential-based framing is often exactly what is needed, because it identifies where the hydrological infrastructure of the landscape is being dismantled before the consequences appear in stream gauges.

The findings sit within a wider body of evidence on deforestation in Indonesia and its consequences. Sulawesi has been identified in previous research as a priority island for conservation because of its exceptional endemism and the rapid loss of its forests, with documented impacts on primate habitat and unique ecosystems. Studies of deforestation patterns in Sulawesi between 1990 and 2018 have shown persistent pressure on forest cover, and work on mangrove decline in South Sulawesi has highlighted how coastal ecosystem services have been degraded alongside terrestrial ones. The new analysis extends this picture into the hydrological domain, showing that the same land conversion processes that threaten biodiversity are also dismantling the island’s capacity to regulate water, with direct implications for flood risk, dry-season water supply, and agricultural stability for millions of people.

What makes the study particularly timely is the policy context. Indonesia has experimented with forest moratoria, peatland protection, and payment-for-ecosystem-service schemes, and researchers have debated how cost-effective these instruments are in reducing emissions and protecting services. Mapping exercises like this one provide the spatial evidence base that such policies require. If restoration funds are limited, they should flow to the watersheds where the loss of regulation capacity is steepest and where downstream exposure is greatest. The study’s outputs, which identify provincial- and watershed-level hotspots, are designed to support exactly that kind of prioritization, and the author notes that they could inform ecosystem service-based zoning as part of provincial spatial planning.

There are also lessons here for how tropical islands elsewhere are monitored. Sulawesi’s story, a 15-percentage-point decline in high-capacity land over three decades, driven overwhelmingly by the replacement of vegetation with agriculture and settlements, is a pattern that repeats across Southeast Asia, Madagascar, the Caribbean, and many other island systems where steep terrain concentrates both water services and land use pressure. The demonstration that a weighted spatial index built from freely available land cover data can track these changes consistently over more than three decades offers a template for other data-limited regions. As climate change intensifies both droughts and extreme rainfall in the tropics, the landscapes that quietly regulate water are becoming more valuable, not less. Sulawesi’s shrinking high-capacity core is a quantitative reminder that this natural infrastructure is finite, spatially specific, and, once lost, slow and expensive to rebuild.

Subject of Research: Land cover change and water regulation ecosystem services on Sulawesi Island, Indonesia

Article Title: Spatial heterogeneity of landscapes and land cover in Sulawesi Island in relation to water regulation ecosystem services

Article References: Nursaputra, M. (2026). Spatial heterogeneity of landscapes and land cover in Sulawesi Island in relation to water regulation ecosystem services. Environmental Monitoring and Assessment, 198(10), Article 1086. https://doi.org/10.1007/s10661-026-15892-z

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15892-z

Keywords: ecosystem services, water regulation, land cover change, spatial heterogeneity, Sulawesi Island, deforestation, Simple Additive Weighting, watershed resilience, forest restoration, Indonesia, landscape ecology, remote sensing