Research Article Water quality assessment of Loktak Lake, Northeast India using 2‑D hydrodynamic modelling Eliza Khwairakpam1 · Rakesh Khosa1 · Ashvani Gosain1 · Arvind Nema1 Received: 5 July 2020 / Accepted: 25 February 2021 / Published online: 5 March 2021 © The Author(s) 2021 OPEN Abstract The paper comprises of an application of a multi-faceted physically based two-dimensional (2D) hydrodynamic model to simulate the transport phenomena of Loktak Lake, including the water quality of Loktak Lake, for which there is con- sensus that it is deteriorating due to river discharge from sub-catchments carrying sewage loads, soil sediments and agricultural fertilizers, and therefore, has emerged as a serious environmental concern. Accordingly, the study attempts to understand the overall environmental quality of the Loktak system and in particular simulate Loktak Lake water quality (state) variables by coupling through MIKE 21 ECO Lab. The model simulated dissolved oxygen and biochemical oxygen demand throughout the lake. Keywords Hydrodynamic model · Ecological model · MIKE 21 · Loktak Lake · Northeast India 1 Introduction [1]. The lake is well known for foating vegetation, popu- larly known as “Phumdis”. An adjoining 40 km 2 extent of Wetlands can be considered as the most productive eco- Phumdis, known as Keibul Lamjao National Park (KNLP) systems on the Earth, which provides many ecosystem ser- is the only foating national park in the world [2]. This vices. However, they are sensitive to ecological conditions national park is the solitary natural home to the endan- and adaptive systems. Wetlands display various multiplic- gered and endemic species of Manipur’s brow-antlered ity rendering to their geographical location, water regime, deer, commonly known as Sangai. This lake endures rich soil and sediment characteristics, genesis, and dominant biological diversity with 132 species of plants and 428 species. The worldwide areal extent of the wetland eco- species of animals. It also provides irrigation to about system ranges from 748, 100,000 to 778,100,000 ha. Glob- 32,400 ha agricultural area, source of income to about ally, wetland ecosystems provide an assessed monetary 8,700 fshermen, and supports hydro-power generation value of nearly US$15 trillion per year. India, with its var- [3]. The annual benefts from the lake can be estimated to ied climatic conditions and topography, supports unique about 2% of the state’s gross domestic product. and diverse wetland habitats. The tremendous rise of Ecological restoration of a wetland is of enormous population and uncontrolled development have led to environmental asset and scientifc interest these days. major pollution impacts on wetlands, afecting the entire Hydrodynamic (HD) assessment is essential for ecologi- ecosystem. cal restoration as it infuences the chemical and biologi- Loktak Lake, a Ramsar designated, is an internationally cal processes. HD variables such as fow magnitude, travel important wetland system which needs proper manage- time and degree of mixing afect the assimilative capac- ment planning. Acknowledging the ongoing ecological ity of lakes, thereby impacting the ecological condition modifcations, it has been listed in the Montreaux record of lakes. Therefore, HD assessment plays a signifcant role * Eliza Khwairakpam, [email protected] | 1Department of Civil Engineering, Indian Institute of Technology, New Delhi, Delhi, India. SN Applied Sciences (2021) 3:422 | https://doi.org/10.1007/s42452-021-04440-8 Vol.:(0123456789) Research Article SN Applied Sciences (2021) 3:422 | https://doi.org/10.1007/s42452-021-04440-8 in the conservation and management of lakes. Physically- sub-catchments carrying sewage loads, soil sediments and based models have emerged as an essential tool in today’s agricultural fertilizers, which are of serious environmental planning and management strategy. concern. The study also attempts to study environmen- HD models may be categorized into one, two and three- tal quality and simulate water quality (state) variables by dimensional models. In 1D HD models, a river channel is coupling through MIKE 21 ECO Lab for Loktak Lake. Also, generalized into a number of cross-sections perpendicular this study presents frst attempt to study the water quality to the centreline of a channel and the foodplain is consid- of the lake through modelling approach. The paper com- ered as extended cross-sections. Some of the well-known prises of the location and climatology of study area, fol- known 1D HD models include MIKE 11 [4], ISIS [5] and HEC- lowed by a brief description on methodology and results. RAS [6]. Some studies which employed MIKE 11 HD model The paper concluded with output results and its analysis. include [7, 8]. On the other hand, 2D HD models are able to provide results not only regarding the spatial information and depth level but also the fow velocities and dynamics 2 Study area of the system over a modeller defned time frame. Hence, 2D models are extensively used among researchers. In the The current study area is Loktak Lake, a fresh-water wet- 2D model, modelling fuid is divided into cells. In these land system, located in Manipur, Northeast India (Fig. 1). cells, each variable is assumed to be constant. All the input The lake has been designated under Ramsar Convention and output parameters are assumed to be uniform within due to its international importance. Also, it is listed under a cell. Some of the well-known 2D models are MIKE 21 [9], Montreaux record due to its ecological modifcations [1]. LISFLOOD-FP [10], TELEMAC-2D [11], SMS [12], Delft FLS The lake has a catchment area of about 5,040 km 2 while [13] and SOBEK [14]. In the 2D model, the vertical varia- the lake extends to around 287 km2. The land use land tion of fow velocity is usually neglected for two-dimen- cover type of the catchment area has been categorised sional approximation. Thus, 2D models are adequate for as forest, degraded forest, settlement, agricultural areas, shallow lakes generally. MIKE 21HD simulates fows and jhum cultivation areas, Phumdis and water bodies. The fuctuations in water level according to change of forcing pH of top layer soil ranges from 4.5 to 5.2 as indicated by functions in bays, estuaries, coastal areas and lakes. This feld study. The annual average rainfall is 1392 mm with an module has been used for HD assessment in various case approximate 150 rainy days [25]. The average temperature studies [15, 16]. However, 3D HD models involve complete varies from about 24 °C (summer) to 14 °C (winter). Hydro- representations of fow processes in all three-dimensions. logically, the catchment consists of nine sub-catchments They are capable of providing more accurate results, but namely, Khuga, Western, Nambul, Imphal, Kongba, Iril, also accompanied by complex model structures and time- Thoubal, Heirok and Sekmai. However, Sekmai and Heirok consuming. Some of the extensively used 3D models are sub-catchments no longer contributes during lean season MIKE 3 [17], FLUENT (Fluent Incorporated Company) [18], due to the construction of barrage for irrigation purposes. Delft 3D [13]. Water quality models can be used for simulating and predicting environmental pollution in water resources. 3 Materials and methods Water quality models range from simple theoretical to experimental and to physically based. The selection of 3.1 Brief description of models model type to be employed for a study area is highly infu- enced by data availability and the objective/s of the study. 3.1.1 MIKE 21 HD Streeter and Phelps model was developed as the first water quality model for analysing water quality of Ohio MIKE 21[9] is a professional hydrodynamic fow transport state located in US [19]. Subsequently, there have been modelling platform and ofers capability ranging from various upgradation in the development of water quality fow simulation, sediments, waves and water quality in models such as WASP [20], QUAL [21], QUASAR (Quality estuaries, seas, bays, coastal areas, lakes and rivers. It is Simulation Along Rivers) [22], MIKE [23] and EFDC (Envi- particularly useful for problem domain comprising of shal- ronmental Fluid Dynamics Code) [24]. low water bodies where stratifcation can be neglected. The primary objective of this research is to develop 2D MIKE 21 HD simulates fows and fuctuations of water level HD model of Loktak Lake using MIKE 21 HD modelling according to change of forcing functions in bays, estuaries, platform and to assess environmental quality. This paper lakes and coastal areas. The fows and water levels of the presents a leading attempt for assessment of Loktak Lake study area are determined on a rectangular grid by provid- based on physically based model. The water quality of ing certain information such as bathymetry, wind data and the lake is deteriorating as a result from river run-of from boundary conditions. The efects of sources and sinks such Vol:.(1234567890) SN Applied Sciences (2021) 3:422 | https://doi.org/10.1007/s42452-021-04440-8 Research Article Fig. 1 Location map of Loktak Lake, bathym- etry and its source (S1:Thongjaorok; S2:Awang Khujairok; S3:Nambo; S4: Nambul; S5: Kongba; S6: Imphal; S7:Thoubal and Iril) (M1:KLNP; M2:Sendra; M3:Ningthoukhong; M4:Nambul; M5:Thanga; M6:Ithai) as river discharge, soil water recharge, precipitation, and parameters. Rivers draining into the lake represented as S1 evaporation are included in the hydrodynamic equations. to S7 were modelled using MIKE 11 ECO Lab. Daily fows Further details of MIKE 21 HD module can be found in the and water quality variables were simulated for the time literature [9]. period January 2011 to December 2016. Details can be found from the previous work, Khwairakpam et al. [27, 3.1.2 MIKE 21 ECO Lab 28]. The models were based on feld data collection, vis- ual inspections and observed data procured from Loktak ECO Lab is a module in MIKE, used to model aquatic eco- Development Authority (LDA) and State Pollution Control system to simulate eutrophication, ecological conditions, Board (SPCB), Manipur.