1. Introduction

In recent decades the effective pollution abatement measures to the water quality are sizeable (Hering et al., 2010; Barton et al., 2005; Hettige et al., 1996) But, in developing countries like India, the water quality pollution levels are so high, creating existential threats to biodiversity, as well as threatening economic progress and sustainability of human lives (World Economic Forum, 2019). India is one of the foremost agriculture-based economies in the world, with high fertilizer applications and the excessive nutrients from agricultural lands, leading to prominent diffuse pollution to the surface water quality (Central Pollution Control Board, 2016; Bassi et al., 2014). The high alarming rate of increasing pollutant load of surface water from industrial accompanies is known from the concentration based discharge control of point sources, which is already an important task to control and to achieve water quality targets (Wang et al., 2004). Additionally, the reduction of diffuse pollution sources is required.
Although urbanization and demographic changes are substantial influences within the lake’s catchment, land-use changes cause extreme disturbances of the catchment’s ecosystems and the lake itself. Most studies demonstrate that land-use changes (Tu 2009; Zampella et al., 2007) as a driving factor for the environmental, including the physical and chemical characteristics of surface water bodies and their internal structure. Improper management of natural resources, coupled with an ever-increasing population, is responsible for introducing many impairments of water quality threats. Most of the freshwater resources are under stress caused by urbanization, and large-scale industrialization processes are a worldwide concern (Fang et al., 2019; Holopainen et al., 2016; Liao et al., 2012).
The complexity of several ecosystem functions in the surface water bodies adversely affected foremost water quality in freshwater lakes which, in turn, and among others, influence ponds, rivers, streams and slowly enter into the groundwater (Gilboa et al., 2014; Thevenon et al., 2011; Banadda et al., 2010). Diffuse pollution caused by agricultural activities can be carried into adjacent water bodies by surface runoff and erosion (Taylor et al., 2016; Guo et al., 2010). Such excess of nutrients accelerates eutrophication and algae blooming in freshwater ecosystems. Besides, point sources are another significant reason for the deteriorating water quality in surface water bodies. However, the spatial and temporal distribution of diffuse pollutants is a challenge. It is important to monitor these distributions even for a large catchment area, due to changing climate, land-use, and strong relations to anthropogenic activities (Shen et al., 2013; Randhir and Tsvetkova 2011). Therefore, it is essential to identify the critical pollution sources of a catchment and to apply the best management practices (BMPs) to protect lake water quality.
The Kolleru Lake catchment in India has been taken as a case study for understanding and modeling of the Spatio-temporal variability in the pollutant loads, which will be a prerequisite for better management of agricultural, industrial, and water resources.
In recent decades, the Soil and Water Assessment Tool (SWAT) (Arnold et al., 1998) has become widely used to model the management of agricultural catchments for identifying polluted areas. SWAT is a useful tool for the estimation of both nitrogen and phosphorus (N & P) emissions and the degree of eutrophication. Both information a necessary prerequisite for the selection of BMPs from small scale areas (Coffey et al., 2013; Shang et al., 2012; Kang et al., 2006) to large scale catchments (Abbaspour et al., 2015; Yalew et al., 2013). The U.S. Environmental Protection Agency (EPA) recognized the SWAT model and incorporated it into the EPA’s BASINS (Better Assessment Science Integrating Point and Non-point Sources) (Abbaspour et al., 2015). Apart from that, several studies were extended into the SWAT-based optimization tool for obtaining cost-effective strategies for sustainable management (Liu et al., 2019; Wallace et al., 2017). However, due to continuous simulations and operations on a daily time step, it is a useful tool for the identification of pollutant sources.
The main objective of this study serves a better understanding of diffuse pollution sources in the Kolleru Lake catchment, a typical flood balancing catchment between the Krishna and the Godavari basins. Here the first study is conducted to estimate diffuse pollution in Kolleru Lake for the catchment level. Further, the study assimilated the critical sub-basin measures on the Hydrological Response Unit (HRU) level priority areas, to conclude the planning of BMPs. Furthermore, suggestions are provided for the implementation of better lake management practices in the catchment.