15 Journal of Nepal Geological Society, 2020, vol. 60, pp. 195-205 https://doi.org/10.3126/jngs.v60i0.31267 Land use and land cover changes in the catchments impact the ecosystem in Phewa, Begnas, and Rupa lakes, Nepal Sudip Acharya1, 2 1Key Laboratory of Alpine Ecology, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China 2University of Chinese Academy of Sciences, Beijing 100049, China Author’s email: [email protected] ABSTRACT Lakes are the environmental and socioeconomic assets of Nepal. In the past few decades, land use and land cover (LULC) change and sediment and nutrients loading into the lakes have severely altered the lake ecosystems. Using Landsat satellite images and data from published literatures, this study analyzes the spatiotemporal variations of LULC change and its impact on the environment of Phewa, Begnas and Rupa lakes during 1975–2018. The results showed that from 1975 to 2018, the forest cover and agriculture land in catchments of all three lakes have decreased, while the built-up area has increased. LULC changes in the catchment have bought the significant impact on the lake environment in terms of sedimentation rate, physiochemical parameters and biological environment. From 1975 to 2000, the forest cover in lakes catchment has decreased, whereas the agriculture land has increased. At the same time, sediment and agriculture based nutrient loading into lakes has increased as indicated by changes in the sedimentation rate and the alteration of physiochemical properties of the lakes. From 2000 to 2018, forest and built-up area has increased while the agriculture land has decreased in lakes catchment. From 2000 to 2018, sedimentation rate of lakes has decreased compared to before 2000. The decreased sedimentation rate was attributed to the increase in the forest cover in the lakes catchment. Increased nutrients loading into the lake during 2000 to 2018 have resulted in serious eutrophication problem in lakes. Overall, this study suggests that the LULC change in the catchment has a significant impact on the lake environment. Keywords: Land use and land cover; Lake catchment; Lake ecosystem; Eutrophication Received: 2 March, 2020 Received in revised form: 8 June, 2020 Accepted: 9 June, 2020 INTRODUCTION international visitors to Pokhara (Paudyal et al., 2019). Land use and land cover (LULC) change in the These lakes support high biodiversity, including catchment of lakes has a significant impact on aquatic suitable habitat for many flora and fauna, bird and ecosystems by altering the nutrients input, sediment fish species (Kafle et al., 2008). Also, these lakes play loading and hydrological flow regimes (Jamu et al., a vital role in determining economic activities such 2003; Malthus and Mitchell, 1988; Soranno et al., as tourism in Pokhara. However, LULC of Pokhara 1996). These changes have affected several lakes valley has changed drastically in the past few decades. around the world (Jamu et al., 2003; Li et al., 2009; The urban/built-up area in the Pokhara valley Poraj-Górska et al., 2017). Trophic status and aquatic experienced the most significant transformation from production of lakes have increased (Haas et al., 1988 to 2000, with remarkable changes occurred after 2019). Hydro-chemical parameters of the lake water 2000 (Rimal et al. (2018). The study also revealed have altered (Martin et al., 2011). Thus, the improved a decrease in the agricultural land and an increase understanding of how LULC change impacts the lake in forests (Rimal et al., 2018). At the same time, the ecosystem is crucial for sustainable management and lake’s ecosystem has significantly changed such as long-term protection of the aquatic ecosystem. an increase in mercury concentration, trace elements (Sharma et al., 2015), and sedimentation rates (Ross Pokhara, one of the ancient cities, located at the and Gilbert, 1999), while a decrease in the lake water foothills of >8000 m peaks of the Annapurna Massif quality (Rupakheti et al., 2017), and volume (Watson in central Nepal, is located about 200 km west from et al., 2019). The recent inventory of lakes and water the capital city of Kathmandu. Pokhara valley has a bodies in Nepal by the Department of Forestry (DoF) population of about 261 thousand (CBS, 2012). Every in 2017 found that 95% of lakes have undergone year more than one million national and international serious transformations (DoF, 2017). Adhikari et al. travelers visit Pokhara (Paudyal et al., 2018). In 2016, (2018) suggest that the recent LULC cover change in nine lakes of Pokhara valley were enlisted as Ramsar the catchment has threatened the Phewa lake. Thus, sites of Nepal. These lakes provide the supporting, it is crucial to have an improved understanding of provisioning, regulating and cultural services to local the relationship between LULC change and aquatic inhabitants as well as to the thousands of national and ecosystem. Previous researches on Phewa, Begnas, 195 Sudip Acharya and Rupa lakes have paid more attention to the altitude of 742 m. The lake has the surface area, annual and seasonal limnological condition. There is maximum depth, and average depth of 5 km2, 24 m and a lack of studies and historical data on the complete 7.5 m, respectively (Rai, 2000b). The eastern shore limnological condition and how the LULC, recent of the lake is surrounded by Pokhara valley. In 1933, climate change and human activities affect the lake the lake was dammed for irrigation and hydropower ecosystems are poorly understood. generation and the construction detail is described in Ross and Gilbert (1999). Begnas lake (Fig.1), This study reviews the limnological condition, the second largest lake of Pokhara valley, lies at an presents the time series analysis of LULC change in altitude of 650 m. The lake has 3.28 km2 surface area, catchments of Phewa, Begnas and Rupa lakes. The and the maximum and average depth of 20 m, and 6.6 main aim of the study is to examine how the LULC m, respectively (Rai, 2000a). The lake was dammed in change and climatic variability affect lake ecosystems 1988 for irrigation purpose. Rupa lake (Fig. 1) is the in the Pokhara valley. The study provides an overview third-largest lake of Pokhara valley. The lake lies at of Phewa, Begnas and Rupa lake, which may be helpful an altitude of 600 m and has 1.3 km2 surface area and for the development of environmental management has a maximum and average depth of 6 m, and 3 m, plans, protocols and local level governance. respectively (Rai, 2000b). The major part of the lake is surrounded by the forest and agricultural fields. The STUDY AREA sedimentation rate in Rupa lake is three times higher than that of the Begnas lake (Acharya et.al., 2020). Phewa lake (Fig.1) is the second largest lake in Nepal The Phewa, Begnas and Rupa lakes are the natural and the largest lake of Pokhara valley located at an freshwater lake fed by rainwater and small rivers. Fig. 1: Maps showing (a) study site locations (Phewa, Begnas and Rupa lake) in Nepal (b) Location of Phewa, Begnas and Rupa lake in Pokhara valley (c) The catchment of Phewa lake (d) The catchment of Begnas and Rupa lake. 196 Land use and land cover changes in the catchments impact the ecosystem in Phewa, Begnas, and Rupa lakes, Nepal METHODOLOGY The data of lakes status were obtained from the published The climatic data of Pokhara meteorological station literature and reports. Cloud free Landsat images of was obtained from the Department of Hydrology 1975, 2000 and 2018 (8-operational land imager) were and Meteorology of Nepal. To capture the local downloaded from the United States Geological Survey observations on how the lakes have changed and (USGS) earth explorer (https://earthexplorer.usgs. to identify the possible mechanisms controlling gov/) geoportal. The acquired Landsat images were the changes, key informant interview and informal of the post-monsoon period (October to December), discussions with the relevant stakeholders and local to obtain the less cloud cover. The obtained Landsat peoples were carried out during the field visit in images were found to be geometrically correct and February 2019. aligned with each other at the sub-pixel level, and no pre-processing was done. After the extensive field RESULTS visit and observation in February 2019, 6 types of Phewa lake classification schemes (forest, agriculture land, water bodies, built-up area, degraded land, and wetland) for In 1973, a limnological investigation of the lakes Phewa and 5 type of classification scheme (forest, around the Pokhara valley was conducted (Maeda and agriculture land, water bodies, built-up area, and Ichimura, 1973) and it categorized the Phewa into a wetland) for Begnas and Rupa lakes were applied subtropical monomictic lake with seasonal thermal (Table 1). The obtained images were analyzed using stratification (Chaudhary et al., 2015; Maeda and the ArcGIS 10.4 software. Vector polygons for Ichimura, 1973). The pH, water temperature, and predefined land classes were generated using Google electrolytic conductivity of Phewa lake ranges from Earth imagery as a reference. The vector polygons 7 to 8, 18.1 to 26.9°C and 20 to 50 µS, respectively were merged and labelled into predefined land classes. (Maeda and Ichimura, 1973). The alkalinity and The unsupervised classification was applied to get the visibility were 0.06 mval/l and 0.04 mval/l (Maeda LULC map of 2018. Similar approach was used to and Ichimura, 1973). The lake was extremely generate the LULC of 1975 and 2000. To minimize the dynamic with advection mixing occurring during the error during land use classification and to make sure monsoon period (Lohman et al., 1988). Lohman et al. the classified pixel represents the ground reality, it is (1988) categorized the Phewa lake as a mesotrophic necessary to check the accuracy of the Landsat image.