Modelling the effects of urbanization on nutrients pollution for prospective management of a tropical watershed: A case study of Skudai River watershed

Highlight

• An insight on the amount of TN and TP pollutants generated under rapid urban development.

• Effects of TN and TP pollution inflows on the trophic states of tropical rivers.

• Future urban development poses a threat to overall water quality in the study area.

Abstract

Nutrient pollution is considered as a primary factor of water quality deterioration in urban-dominated watersheds in which an informed decision on the management strategies are required to improve the water quality condition. The Hydrological Simulation Program Fortran (HSPF) model is used to evaluate the impacts of pollution by these nutrients using the Skudai River watershed in Malaysia as a case study. A developed land-use/land-cover (LU/LC) scenarios were used to evaluate these impacts. Statistical methods were employed to assess the extent of these impacts and their significance in shifting the trophic state of the rivers in the watershed. The study shows that when urban development increases from 18.2 to 49.2%, the total nitrogen (TN) and total phosphorus (TP) loads increase from 3.08 to 4.56 × 10 ³ kg/yr and from 0.13 to 0.27 × 10³ kg/yr, respectively. Streamflow and stream concentrations (NH₃N, NO₃N, and PO₄-P) produce varying responses as the watershed land-use changes (from 1989 to 2039). As the rivers in the watershed shift their trophic state with respect to the level of anthropogenic disturbance within their catchments, the TN and TP concentrations at the estuaries are likely to change from oligotrophic to eutrophic state. This is an indication that the Johor Strait and the coastal rivers will be exposed to eutrophication, subsequently resulting in harmful algal bloom. This condition can be prevented by integrating water quality management alongside urban development because it is observed that a control of non-point source (NPS) pollutants from 1% of the urban development will decrease TN and TP concentration in Skudai River by 0.023 mg/L and 0.004 mg/L respectively.

Introduction

The level of nutrient pollution produced from the urban areas is among the leading causes of stream impairment in most urbanized watersheds (Duan et al., 2012, Edwards, 2016, Li et al., 2017). For example, the conversion of vegetative and wetland areas to impervious surfaces (buildings, roads, etc.) changes the hydrologic regimes, ecosystem distribution, and nutrient dynamics (Hogan et al., 2014; Bhaskar et al., 2016). The impervious surface amplifies runoff volumes and increases transport of nutrient pollutants from areas of non-point sources (NPSs). These changes adversely affect the water quality and aquatic habitat, their functions in stream ecosystems (Rafferty et al., 2013).

Urban development affects the environment (Netusil et al., 2014; Gashi et al., 2016), and therefore evaluating its influence on the nutrient dynamics will provide useful information on how to protect the environment and ensure human safety (Shen et al., 2015). There are relatively few studies on effects of urbanization in humid tropical regions. Humid tropic regions are characterised with extreme weather conditions (more rainfall and high temperatures) as compared to temperate climates (Ghaffarianhoseini et al., 2019; Ayoub et al., 2020). Depending on the land-use type, the tendency for excessive sediment and nutrient pollution entering the rivers will be more severe (Abd Wahaba et al. 2019). Land-use is a driver of nutrient pollutants and thus important to be considered in urban planning and watershed management and hence, the amount of nutrient pollutants produced due to sudden or gradual urban expansion can be readily determined and applied to support the management of water quality in urbanized settings (Bello et al., 2019). Therefore, understanding the impact of urban development on nutrient pollutants at watershed scale in a tropical region like Malaysia is very important. According to Sala (2000), the effects of land-use changes on the water cycle may outweigh those associated with climate change, and this assertion was supported by a study conducted in Puget Sound Basin (Sun et al., 2016). The result shows that urbanization has a much larger effect than climate change on both the magnitude and the seasonal variability in the studied area. As observed by Tang et al. (2011) and Lee et al. (2017), rapid urbanization is directly associated with an increase in excess nutrients in Malaysia and hence changes in characteristics of the watersheds over a period due to human activities should be considered for water quality improvement. This will help in determining the impact of the potential anthropogenic activities and mitigation actions alongside the future development (Häder et al., 2020).

Other water quality studies at watershed scale explore the effects of historical land use changes on surface runoff and NPS pollution loading (Wu et al., 2012; Wang et al., 2016), but their integration into future perspectives are often ignored. Therefore, in some cases the effect of excess nutrients in a potential urban development remains unknown due to the inadequacy of information and restricted urban planning (Alberti et al., 2007), even though unique information and an outlook with regard to the water quality conditions can be derived (Lee et al., 2009). Nutrients in themselves are important for the function of aquatic ecosystems, but their availability in excess will result in distortion of the ecosystem (Baron et al., 2013).

In this study, a water quantity and quality model of a tropical river watershed is developed using the Hydrological Simulation Program Fortran (HSPF) model. HSPF has been used in modelling several watersheds considering various response parameters that includes; the hydrology (Biru and Kumar, 2018; Lee et al., 2020), sediments (Sarkar et al., 2019), and nutrients (Roostaee and Deng, 2019). Most of the studies have shown the robustness of the HSPF model in determining the effects of various best management practices at a watershed scale (Risal et al., 2021). Also, HSPF model have the ability to detect a small change in the land-use (as input data) and provides the likely responses of the watershed (Alukwe et al., 2014).

This study will provide information on the variation of TN and TP from NPS areas in a tropical watershed system under rapid urbanization settings that will act as a basis for management support and mitigation actions to curtail future water quality problems. Recently, Malaysia planned its future economic development corridors, out of which Johor Bahru, athe capital of Johor state (part of the Skudai River watershed) is one of them. The aim was to modernize the economic infrastructures and urban development areas that will attract investors. As a result, a rapid transformation is taking place in most part of the district. The Skudai River watershed represents the general characteristics of a tropical watershed considering its historical and planned urban development over the span of 5 decades. Part of Johor Bahru township falls within the watershed and its district is considered as one of the major urban area in Malaysia and most urbanized in Johor state (Rizzo, 2021). But the impacts of the rapid urban development and other anthropogenic activities that may lead to water quality deteriorations are not yet determined.