Water-use Zonation under Projected Land-cover Change in Bolon Watershed, Indonesia

Abstract

Land-cover change can weaken watershed hydrological regulation and intensify flood–drought risk, particularly when conversions occur in hydrologically sensitive zones. This study evaluates how observed and projected land-cover dynamics influence water-use zonation in the Bolon River Watershed (North Sumatra, Indonesia) to support spatial planning. A GIS-based workflow was implemented in QGIS using the MOLUSCE plugin (CA–ANN) to model land cover from Sentinel-2 based maps (2017, 2021, 2024) and to simulate future patterns for 2029 and 2033. Water-use zonation was derived by integrating recharge condition, catchment function, and water-availability considerations, and its planning relevance was examined through overlay with the RTRW land-use structure. Results show that, during 2017–2024, forest decreased from 76.63 km² to 58.10 km², wet agriculture from 20.47 km² to 14.41 km², and water bodies from 69.59 km² to 33.69 km², while built-up land increased from 131.78 km² to 152.39 km²; plantations remained dominant, changing slightly from 1,440.74 km² to 1,458.45 km². Projections indicate that by 2033 the watershed remains plantation-dominated (1,558.35 km²) with built-up land increasing to 213.32 km², while forest becomes very limited (3.34 km²). Catchment function is overwhelmingly classified as Low (2029: 177,730 ha; 2033: 177,831 ha) with only a small High class (≈295–296 ha), and recharge condition is dominated by Normal (Natural) (≈98,000 ha) alongside non-trivial Critical areas that increase from 16,913 ha (2029) to 17,583 ha (2033). The 2029 RTRW overlay indicates that most zonation classes intersect Kawasan Budi Daya, including the High water-use zone overlapping with settlement (6,146 ha) and agriculture (3,892 ha), highlighting a planning sensitivity that warrants tighter controls in hydrologically strategic locations. These findings support the use of hydrological zonation as an operational constraint layer in RTRW implementation and permitting to protect watershed function under continuing land conversion.