Factors influencing the spatial and temporal variations of surface runoff coefficient in the Red River basin of Vietnam

Lack of sufficient and reliable gauged hydrological data over the Red River basin (RRB)—one of the largest river basins in the world, has been a challenge to water resource planning and management in Vietnam. To address this critical issue, this study mainly aimed to apply the VIC hydrological model to simulate surface runoff at the basin scale over the RRB during the period 2005–2014. The surface runoff coefficient estimated from the VIC model output was analyzed in relation to precipitation pattern and watershed factors in three regions (the Northeast, Northwest, and Red River Delta (RRD) region) of the RRB to investigate key factors influencing the spatial and temporal variations of surface runoff coefficients. The results showed that the spatiotemporal variation of surface runoff coefficient was strongly impacted by precipitation pattern, which was indicated by the increased surface runoff coefficients associated with both the increased precipitation amount and precipitation intensity for all regions in the RRB, most obviously in the wet season. In addition, the combined effects of various watershed factors (terrain elevation, land use and land cover, soil type, and soil moisture) in different regions of the RRB also largely contributed to the spatiotemporal variation of surface runoff coefficient. The highest surface runoff coefficients were found in the uppermost areas of the Northeast and Northwest regions and the middle areas of the Northwest region where the high terrain elevations over 1500 m and the dominance of savanna, shrubland, and deciduous broadleaf forest were identified as the key factors contributing to the variation of surface runoff coefficients. These findings could provide a better understanding on the spatial and temporal variations of surface runoff over the RRB toward supporting governmental agencies in making policies and decisions for soil and water resources conservation.