12. An Evaluation of Hydrological Modeling Using CN Method and Satellite Images in Ungauged Barsa River Basin of Pasakha, Bhutan

Bhutan is poised to a rugged terrain country that exhibits variant of hydrology behaviors that demands to be effective utilization of different types of hydrological databases. Due to the complicated network of drainage systems, proper forecasting needs to be studied and analyzed, so that effective management of floods/drought events on a daily basis can be implemented in the coming future. However, hydrological data in Bhutan are scarce information exclusively for a developing country, wherein, minimal gaging stations are set up currently to analyze the hydrological behaviors. This paper attempts to analyze the majority of the ungauged basins. While doing analysis, the hydraulics structure aligned to complicated hydrological behaviors precisely incorporated for the stormwater management works and allied river mitigations. This study is also to evaluate the surface runoff from the Barsa (River) Basin in Pasakha using precipitation data and curve number (CN) by performing infiltration methods. Pasakha is one of the most vibrant industrial hub areas as all kinds of industries are located, which is seemingly felt to be protected from flood from Barsa River. HEC-HMS 3.4 has been employed to prepare the Hydrological Soil Group (HSG) map and Land use map of the area. Curve Number zoning of the area has been assigned to generate the runoff amount from the HEC-HMS. HSG map has been mapped by performing numerous standard infiltration tests throughout the basin to get the infiltration rate at each basin and interpolating the obtained results by employing the Geostatistical Kriging interpolation method in ArcGIS. A land use map has been prepared by using Toposheet 78F05 (1:50,000), Phuentsholing base map (CAD data), and Recent Google images. Rainfall data for the year (1996–2019) has been acquired from the Metrological Department, Ministry of Economic Affairs. Curve Number has been manually calculated using SCS-TR55 table, those HSG map and Land use map, which later has been used as feed data in HEC-HMS for simulation of surface runoff as discharge. The study area, pasakha is grouped as A and B based on the HSG classification table, and assigning the value is validated by visiting each basin. It has been noticed that W350 has the highest runoff potential after simulation as the soil is grouped as B, a higher impervious area with an average flow of 11.6 m³/s. The lowest runoff potential is found to be W500 at 2.7 m³/s and the average surface runoff overall was 7.4 m³/s.