Open Access

ARTICLE

Integrated Assessment of Hydrologic Response and Spatial Hazard Vulnerability for Tropical Watershed Management

Lanie A. Alejo^1,*, Orlando F. Balderama¹, Rex Victor O. Cruz²

1 College of Engineering, Isabela State University, Echague, Isabela, Philippines
2 College of Forestry and Natural Resources, University of the Philippines, Los Baños, Laguna, Philippines

* Corresponding Author: Lanie A. Alejo. Email:

(This article belongs to the Special Issue: Application of Remote Sensing and GIS in Environmental Monitoring and Management, 2nd Edition)

Revue Internationale de Géomatique 2026, 35, 509-532. https://doi.org/10.32604/rig.2026.084091

Received 16 April 2026; Accepted 31 July 2026; Issue published 25 September 2026

Abstract

The Abuan Watershed in Isabela, Philippines faces increasing hydrologic and environmental pressures from rainfall variability, land-use change, and hazard-sensitive landscape conditions. This study integrated Soil and Water Assessment Tool (SWAT) modeling with GIS-based vulnerability assessment to evaluate watershed response and spatial vulnerability to flood, drought, and soil erosion. A 5 m × 5 m IFSAR-derived digital elevation model, land-use/land-cover data, soil maps, and hydroclimatic and streamflow records supported the analysis. Streamflow performance was acceptable, with calibration values of R² = 0.61, nRMSE = 22%, and PBIAS = 3.1%, and validation values of R² = 0.70, nRMSE = 14%, and PBIAS = 10%. Under baseline conditions, the 63,922-ha watershed received 2169.1 mm of annual precipitation, of which 47% became surface runoff, 23% evapotranspiration, and 30% percolation. Wetter scenarios produced the clearest hydrologic response, increasing runoff by up to 15.99% and soil erosion by up to 13.02% under combined land-use and rainfall-change conditions. Under the projected 2050 rainfall scenario, reforestation caused only minor runoff and percolation changes but reduced simulated soil erosion by up to 1.16 t ha⁻¹ yr⁻¹ relative to the urbanized condition. Flood vulnerability was concentrated in downstream agricultural and settlement areas, whereas drought and soil-erosion vulnerability were greater in cultivated and rainfed uplands. Priority actions therefore include flood-risk reduction and runoff moderation in downstream lowlands, together with vegetation recovery, reforestation, and soil-conservation measures in cultivated hilly and rolling terrain. The integrated SWAT–GIS approach provides a practical, spatially explicit basis for watershed prioritization and intervention planning for future watershed management.

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Keywords

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Supplementary Material

Supplementary Material File

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