Trophic Status of Lake Phewa and Kulekhani Reservoir, Nepal

Eutrophication is one of the growing environmental concerns and is affecting and compromising freshwater bodies across the world making the trophic status assessment of water bodies crucial for their restoration and sustainable use. This paper describes the trophic status of Lake Phewa and Kulekhani Reservoir from Nepal. Sampling was conducted during October 2017 (post-monsoon), April 2018 (Pre-monsoon), July 2018 (Monsoon) and February 2019 (Winter). Trophic State Index (TSI) as given by Carlson (1977) and Trophic State Index Deviation given by Carlson (1991) were estimated to assess trophic status and deviations between the Trophic State Indices. One-way analysis of variance showed significant seasonal variation (p < 0.05) in Secchi depth, total phosphorus (TP), TSI in both the water bodies. Both the water bodies were classified as eutrophic during pre-monsoon and post-monsoon, and hypereutrophic during the monsoon indicating the increased flow of allochthonous inputs from their respective catchments. Non-algal turbidity was found to be the limiting factor for productivity. There is a need for sustainable watershed management in order to reduce the nutrients runoff and accumulation in the water bodies.

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

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.