Development of a Watershed Hydrological Model Based on Pre-Development Monitoring Data and Future Projections

National Agriculture and Food Research Organization (NARO), Western Region Agricultural Research Center

FY 2026–2029 Grant-in-Aid for Scientific Research (KAKENHI)

KABEYA, Naoki (Kyushu Research Center)

In forested watersheds, forest soils support the hydrological cycle by retaining rainfall and gradually releasing it to groundwater and streams. This function helps stabilize streamflow and is a principal factor in sustaining water resources. However, in recent decades, rapid conversion of forests to agricultural land and the expansion of plantation agriculture in tropical regions have raised concerns about their impacts on these hydrological systems. Accurately assessing how forest development alters watershed hydrology remains challenging, largely because pre-development monitoring data are rarely available. The Forestry and Forest Products Research Institute (FFPRI) has conducted continuous hydrological observations, including rainfall and streamflow measurements, in a watershed in Cambodia for more than 20 years. These observations constitute a globally valuable dataset that documents environmental changes from before forest conversion through the present day. This study builds upon these long-term observations to clarify the relationship between forest development and water resources.

The objective of this study is to develop a watershed hydrological model that can reproduce water movement within a watershed using long-term monitoring data collected since before forest development occurred. The model will be used to evaluate and predict the impacts of land-use conversion, climate change, and population growth on future hydrological processes and water resources, with projections extending to approximately 2050. Through these efforts, the study aims to provide a scientific basis for balancing forest conservation and regional development, thereby contributing to sustainable watershed management.

This study will be conducted in the Stung Chinit watershed, Kampong Thom Province, Cambodia. First, in addition to utilizing long-term hydrological datasets such as rainfall and streamflow records, field surveys will be carried out in forests, rubber plantations, orchards, cashew plantations, and cassava fields to characterize vegetation-specific water-use patterns. Second, satellite image analysis and field investigations will be used to reconstruct land-use changes over the past two decades. Interviews with local residents will also be conducted to understand changes in water use and livelihood activities associated with land-use transformation. Based on these findings, multiple future land-use scenarios will be developed, including: a business-as-usual scenario, a development-oriented scenario, and a conservation-oriented scenario. Furthermore, a watershed hydrological model (SWAT: Soil and Water Assessment Tool) will be applied to reproduce historical hydrological processes. By integrating future climate change scenarios, the model will be used to project changes in water resources through 2050. Ultimately, the results will be shared with local communities and governmental agencies to support the development of a sustainable long-term vision ("grand design") for watershed management.

Kabeya N. et al. (2008) Preliminary study of flow regimes and stream water residence times in multi-scale forested watersheds of central Cambodia. Paddy and Water Environment, 6(1):25-35.

Shimizu Y. et al. (2020) Effect of in-stream impoundment on water quality of a suburban stream. Limnology, 21(3):393-402.

Kabeya N. et al. (2021) Long-term hydrological observations in a lowland dry evergreen forest catchment area of the lower Mekong River, Cambodia. JARQ, 55(2):177-190.