3 Materials and Methods

3.1 SWAT model setup

The SWAT model was developed by the Agriculture Research Service of the United States Department of Agriculture (Arnold et al., 1998). This approach was adopted to simulate the diffuse pollution load in the Kolleru lake catchment. It is a physically-based and semi-distributed model that operates on a daily step and capable of continuous simulation over long periods (Gassman et al. 2007). In this study, the SCS (Soil Conservation Service) (USDA-SCS 1972) curve number was used to calibrate the surface runoff from daily rainfall data, further potential evapotranspiration from Penman-Monteith, and sedimentation from the Modified Universal Soil Loss Equation (MUSLE) (Williams 1976). The model equations are extensively documented on the official SWAT website (http://swatmodel.tamu.edu).
The data used in the SWAT model are in two different formats, i.e., from a spatial and a temporal database. Table 1 outlines the available data for the SWAT simulation. The spatial data includes the DEM (Digital Elevation Model) generated using stereo images of ASTER DEM with a spatial resolution of 30 m. Land-use data were mainly classified into agricultural land (for paddy cultivation), fishponds, urban, barren land (unused or uncultivated land), and forest areas (Fig. 2a). The soil types were categorized into 38 classes (Fig. 2b). The data provide insights into soil depth, drainage, texture, slope, erosion, salinity, etc. (Table 2). The temporal data include hydrological parameters, such as daily precipitation, maximum & minimum temperature, relative humidity, wind speed, and solar radiation. The mainly used rain gauge stations were Bhimavaram, Eluru, Gudivada, Nuzvid, and Tadepalligudem. The catchment weather information used from daily monitoring data for the period 2008-2014. Information on crop patterns, fertilizer application, fish farming, social economics, and industrial pollution was based on previous literature and data collected from local statistic yearbooks (Azeez et a.. 2011), and on-field investigations as well.
The catchment area is composed of 38 different soil types, dominantly with clayey texture. According to this data, 46.7 percent of the catchment is largely extended to the well-drained condition, 19.9 percent is moderately well-drained, while 27.8 percent is composed of imperfectly drained, and 2.4 percent is excessively drained. Very deep soils (55 percent) are predominantly identified within the catchment area, with clay dominance in texture and pore in coarse and medium pores. Present up-slope in the headwaters are covered by shrub vegetation and forest areas. The runoff from the upper catchment passes the agricultural fields of the middle part before entering into the lake. Agricultural land is the dominant land use cover (68%) of the catchment, followed by fishponds (16%), mangrove forests on gently sloped areas (10%), and the urban area does not exceed 3% of the total area.
Using a digital elevation model (DEM) with 30 m × 30 m resolution, SWAT delineated the catchment into 20 sub-basins depending on the flow direction, stream network, and drainage outlets. Slopes were classified into four gradient categories: <3%, 3-5%, 5-10%, and >10%. Hydrologic Response Units (HRU) derived from adjusting thresholds of 12% land-use, 15% soil, and 15% slope. There are 1,281 feature classes (HRU) that were delineated, while each HRU is being independent of the SWAT model, with a similar slope, land-use, and soil characteristics. The model was extensively calibrated against daily discharge, nitrate pollution (NO3_N), and total phosphorus (TP) loads in the Kolleru Lake catchment.

3.2 Workflow to Action Plan

After the “Operation Kolleru,” the lake water still received serious threats by diffuse pollution. Therefore, the state government authorities approved that the lake was not polluted by the fishponds, due to agricultural runoff and urban infrastructure. Kolleru Lake pollution mitigation plans were formulated between 1982 and 2015. The efforts were taken in 2006 to resolve the pollution by fishponds one site. Still, the other sources of pollution left for discussion between researchers, stakeholders, and the state government authorities. Therefore this paper reports about the identification of priority areas of diffuse pollution from 2008 to 2014 (after Operation Kolleru), based on the SWAT model (Fig. 2).
The workflow included four stages: problem definition, preparing a database and SWAT model execution, identification of priority areas, and formulation of pollution mitigation measures. The first stage included the knowledge deficit in this area, discussed with the Kolleru Lake development programs, especially with the Kolleru Lake Forest Department (KLFD), Kolleru Lake Development Committee (KLDC), researchers, and water managers. Researchers and water managers provided the necessary data for understanding and visualizing the pollution levels in the catchment. The second stage devoted to the database preparation and model execution based on the daily time step. The third stage included the identification of priority areas based on the results obtained from the SWAT model. Further, the results and necessary actions will be discussed with the researchers, stakeholders, and state government authorities. The last stage was the implementation of a measures plan protecting lake water against pollution.

3.3 BMPs setups and stakeholders engagements

The first methodological approach has identified the agricultural management priority areas for applying BMPs to facilitate the relevant information to the stakeholders. The central and state government organizations had formulated the Kolleru Lake development programs and aimed to bring an optimized solution to conserve the lake resources (Azeez et al., 2011). One such program is the Kolleru Lake Development Committee (KLDC), which checks the encroachments, regulating or monitoring the pollution level, and clearing the lake weeds every year. This study considers the agricultural runoff attributes the first time for the Kolleru Lake catchment. Thus promotes the awareness of the decision-makers and stakeholders on values, functions of the stream network, and variables of the Kolleru Lake catchment.
Furthermore, the potential outcome of the “Operation Kolleru” program aimed to restore the past glory of the lake. A priority response of an integrated water management plan (IWMP) on the catchment level became possible for an optimal set of the lake ecosystem. However, the IWMP contains an activity to enlighten the stakeholder’s perception towards lake degradation. Stakeholders will become able to include Kolleru Lake ecosystem resource users, will be guided about the crucial significance of the lake functions, values, and resources from which they fulfill their needs. Moreover, the state government agencies should incorporate with the stakeholders to adopt sustainable development activities that would need a priority response.