Shrinkage of Nepal’s Second Largest Lake (Phewa Tal) Due to Watershed Degradation and Increased Sediment Influx

Abstract

Phewa Lake is an environmental and socio-economic asset to Nepal and the city of Pokhara. However, the lake area has decreased in recent decades due to sediment influx. The rate of this decline and the areal evolution of Phewa Lake due to artificial damming and sedimentation is disputed in the literature due to the lack of a historical time series. In this paper, we present an analysis of the lake’s evolution from 1926 to 2018 and model the 50-year trajectory of shrinkage. The area of Phewa Lake expanded from 2.44 ± 1.02 km2 in 1926 to a maximum of 4.61 ± 0.07 km2 in 1961. However, the lake area change was poorly constrained prior to a 1957–1958 map. The contemporary lake area was 4.02 ± 0.07 km2 in April 2018, and expands seasonally by ~0.18 km2 due to the summer monsoon. We found no evidence to support a lake area of 10 km2 in 1956–1957, despite frequent reporting of this value in the literature. Based on the rate of areal decline and sediment influx, we estimate the lake will lose 80% of its storage capacity in the next 110–347 years, which will affect recreational use, agricultural irrigation, fishing, and a one-megawatt hydroelectric power facility. Mitigation of lake shrinkage will require addressing landslide activity and sediment transport in the watershed, as well as urban expansion along the shores.

Hydrogeochemical Analysis of Phewa Lake: A Lesser Himalayan Lake in the Pokhara Valley, Nepal

“…The growing population and urbanization in the catchment are causing municipal discharge, discharge from hotels/restaurants and subsurface flow from septic tanks leading to sedimentation and siltation, and water quality deterioration (Rai, 2000a;Ross and Gilbert, 1999). Based on the rate of areal decline and sediment influx, 80% storage capacity of Phewa Lake has been reported to be lost by the next 110-347 years (Watson et al, 2019). Phewa Lake is one of the better-studied lakes in the Pokhara Valley (Adhikari and Khadka, 2017;Ferro and Swar, 1978;Gurung et al, 2006;Gurung et al, 2010;Hickel, 1973;Jones et al, 1989;Kato and Hayashi, 1982;Pradhan and Kim, 2017;Rai, 2000a;Rai, 2000b;Regmi et al, 2017;Ross and Gilbert, 1999;Rowbotham and Dudycha, 1998;Swar and Fernando, 1979a;Swar and Fernando, 1979b;Swar and Fernando, 1980;Watson et al, 2019).…”

Land use and land cover changes in the catchments impact the ecosystem in Phewa, Begnas, and Rupa lakes, Nepal

“…The study also revealed a decrease in the agricultural land and an increase in forests (Rimal et al, 2018). At the same time, the lake's ecosystem has significantly changed such as an increase in mercury concentration, trace elements (Sharma et al, 2015), and sedimentation rates (Ross and Gilbert, 1999), while a decrease in the lake water quality (Rupakheti et al, 2017), and volume (Watson et al, 2019). The recent inventory of lakes and water bodies in Nepal by the Department of Forestry (DoF) in 2017 found that 95% of lakes have undergone serious transformations (DoF, 2017).…”

Water quality and land use/cover changes in the Phewa Watershed, Gandaki Province, Nepal

“…Nep J Environ Sci (2019), 7, 31-39 https://doi.org/10.3126/njes.v7i0.34473 rest of the months, the lake receives water from the tributaries and groundwater seepage (Watson et al, 2019). The lake is fed by two perennial streams: namely the Harpan Khola and Andheri Khola, as well as several other seasonal streams, and the water of the lake exits through an outlet at the Dam site area throughout the year.…”