Groundwater research in Nepal for tiger conservation. A reconnaissance study to groundwater dynamics in an alluvial mega-fan in Bardiya National Park (Terai), focusing on the interaction between groundwater and the Karnali river.
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Bardiya National Park (BNP) is located in the lowlands of Nepal, the Terai. It is one of the five national parks of the Terai where the endangered wild tiger Panthera Tigris occurs. The government of Nepal commits to the global agreement of 2009 to double the tiger population by 2022. The survival of the tiger species in Nepal largely depends on the availability of its prey, the deer. The habitat of the deer consists of tall grasses and riverine forests. This type of vegetation thrives by a shallow groundwater table. The groundwater table is therefore regarded as a critical factor in the distribution of the wild tiger. The subsurface of Bardiya National Park is associated with a high permeability as it consists of coalescing alluvial fans, including the alluvial mega-fan of the Karnali river. Therefore, the groundwater head may be dependent on the water level in the Karnali river. The relation between the groundwater and the Karnali river is especially important as the water level in the Karnali river is prone to changes – both natural and human induced. To predict and potentially adapt the groundwater management to these changes, an understanding of the groundwater dynamics is required. Yet, no geohydrological studies have been conducted in BNP. The objective of this reconnaissance study is to understand the groundwater dynamics in BNP, focusing on the interaction between the Karnali river and groundwater. The research consisted of a three months during field study from the end of September to the beginning of December in 2018, followed by a laboratory- and desk study at Utrecht University. The characteristics of the subsurface were examined by observations in the field, an analysis of existing well log data, grain size analyses of soil samples and by pumping tests. Thereafter, the groundwater dynamics were examined by observations in the field, the monitoring of the groundwater head at various locations and the analysis of the isotopic composition of the groundwater samples, river water samples and rain water samples. The subsurface consists of alluvial and limnological deposits, forming at least 2 aquifers in the first 100 m from the surface. The subsurface is well permeable, especially near the Karnali river. There, the transmissivity of the shallow aquifer is between 1.9·103 and 2.8·103 m2/day. In the lower Terai, the shallow aquifer is generally confined by a loamy top layer. Approximately 23% of the annual rain fall may infiltrate through this layer. Thus, the bulk of groundwater recharge by rain water infiltration occurs in the upper Terai, or where the top layer has eroded in the lower Terai. In the post-monsoon of 2018, the Karnali river drained the groundwater. The effect was the largest for the groundwater head in the south of BNP near the Karnali river, where the groundwater head dropped on average 18.2 mm/day in the shallow aquifers during the post-monsoon. At the most northern monitoring location, further from the Karnali river, the groundwater head dropped with only 7.3 mm/day. The isotopic composition of water samples showed that recharge of the groundwater from the Karnali river is also negligible in other periods of the year, but also that the groundwater was substantially recharged by irrigation with Karnali river outside the park boundaries. In conclusion, the Karnali river is controlling the groundwater head near the river. Thus, changes in the Karnali river may have problematic consequences for the depth of the groundwater head and thereby potentially also for the habitat of the wild tiger. At the same, the groundwater head may be managed with relatively simple interventions in the Karnali river. However, this research is not sufficient for making quantitative predictions about the effect of potential changes or interventions. Therefore, I recommend to develop a geohydrological computer model of Bardiya National Park and surroundings, based on the results of this study.