Investigating the groundwater hydrology in Chitwan National Park, Nepal

Abstract

In the Chitwan National Park, Nepal, a decrease in grassland area due to woodland intrusion is considered to threaten the the Royal Bengal tiger population. A possible explanation for the decrease in grassland area might be a change in groundwater dynamics over the last years. This study aims to gain an understanding of the groundwater dynamics that are at work in Chitwan National Park. To do so, the relationship between the subsurface and the water dynamics is investigated with the use of multiple ?eld and laboratory methods. The study period runs from the end of September till mid-December, during the post-monsoon period. Recharge is found to occur primarily in the Churia Hill range, where the permeable Upper Siwalik and the Middle Siwalik formations are present. Discharge areas are also found in the Upper and Middle Siwalik, in the form of springs. As the Rapti River is found to be draining the study area, this is also regarded to as a discharge area. Hydraulic conductivity of the top layer that was found throughout the study area ranges from approximately 7 m/day in the Churia Hill range to 0.04 m/day at the piedmont of the Churia Hill range. The top layer is present in the oodplain of the Rapti River as well, where it functions as a con?ning top layer for the deeper aquifer system. Except for the area adjacent to the East Rapti river, the aquifer is in con?ned conditions. Upward ow from the aquifer into the top layer occurs in the oodplain and at the piedmont of the Churia Hill range. A wide range of transmissivities is found for the aquifer system, indicating that the aquifer system is very heterogeneous. Average transmissivity for the deeper aquifer system ranges from 3.0*103 m2/day to 4.0*103 m2/day Drainage rate is found to be the highest shortly after the monsoon period, in October, and then steadily decreases throughout November en December. Change in hydraulic head over the whole study period is around -2.5 cm/day. The grasslands in CNP that are found in areas close to the East Rapti River but elevated relatively high above the river level (approximately > 5m) are expected to be the most vulnerable for woodland intrusion. Grasslands located further away from the East Rapti river are expected to be safe from woodland intrusion.