Water, a prime natural resource, is the most basic need for mankind and both human life & the health of the environment depend on it. Recognizing the importance and scarcity of resources to meet biological needs and maintaining various economic and growth activities is a major issue. Constantly increasing population, industrialization, concentrated agricultural actions, industrialization, and urbanization resulted in a massive decline in freshwater availability during recent years. For industrial use, agricultural use, domestic use and to maintain a healthy and hygienic ecosystem, regular water quality monitoring of the water resources is essential. Using composite parameters to assess water quality can be a complex practice and raise any concerns about the overall water quality [1]. It is a quite difficult task to evaluate water quality because of various samples having accumulation of different constituents. Experimentally calculated parameter results and existing instructions are traditionally compared to evaluate water quality.
Taunsa Panjnad (TP) Link Canal having designed discharge 12000 cusecs at its head lead off from left flank of Mighty River Indus at Taunsa Barrage. Panjnad head works faced serious shortage of water supply as a result of operationalization of Indus-Basin Treaty between Pakistan and India. Taunsa-Panjnad Link Canal was constructed in early 1970’s under Indus Basin Replacement Works by WAPDA [2] and upon completion this link canal was handed over to Taunsa Barrage Division. This 62 km link canal diverts irrigation water from Mighty River Indus to river Chenab to cop up with the scarcity of water at Panjnad head works. TP Link Canal is fulfilling the desire water requirements since its construction and supplying water at Panjnad head works for 1.4 Million acres of fertile lands of District Bahawalpur and Rahim Yaar Khan. Similarly, such channel is maintaining water supply chain in southern Punjab and playing a prime factor role of sustainability of social as well as economic life of two Districts of southern Punjab. Likewise, TP link canal also provides supply to Rangpur canal at Head Muhammad Wala for District Muzaffargarh. In addition, such long channel has earthen prism and it runs in sand dunes in its course of alignment. Moreover, three cross regulators are provided in canal to maintain water levels in various reaches for stability of earthen prism of channel against sloughing as well as scouring.
Another challenge is to choose a suitable water quality model (WQM) because of the development of various WQMs such as QUAL series, SIMCAT, MIKE-11, HEC-RAS, WQRRS, WASP, MONERIS, etc. for the water quality assessment during the recent years and also each WQM has a different formulation and has its pros & cons. Critical parameters that must be simulated, the availability of input data, the model's use, the model's availability (public domain), and supporting materials (such as manuals) can be considered as a criterion in the selection of an appropriate WQM [3]. Water quality model selection is usually governed by the following factors: research aims, application of conclusions, target-related attributes of the system, required level of information (space and time), relevant chemical and physical processes, the calibration needs, accessibility of input information, past model experiences & scientific community approval, serviceability, susceptibility to the process of interest, time and resources at hand [4].
There are a number of well-known and extensively used software packages that incorporate physics-based equations, including the Hydrologic Engineering Centre-River Analysis System (HEC-RAS), developed by the US Army Corps of Engineers which is not only most extensively applied in the study-based literature but also in application as well [5]. Due to utilization of graphical user interface (GUI) in HEC-RAS 5.0.6, which standardizes several aspects of data entry, provides an efficient display of model outputs, and facilitates communication between sub-components of model, authors selected it for their research project. For water quality modeling data required includes; geometric data, flow data and water quality simulation data. In steady flow modeling, geometric data and flow data are both utilized which can be obtained from respective irrigation department. Water quality simulation data includes boundary conditions, initial conditions, meteorological data sets, and observed conditions. Meteorological datasets can be get from meteorological department while literature work and different testing can be utilized for the rest of the data required. Also modelled results and observed data are compared visually and quantitatively for the calibration and verification process of model.
HEC-RAS offers three types of modeling; Water temperature modeling, Nutrient simulation module (NSM-I) modeling and arbitrary constituent modeling. Authors opted NSM-I modeling for their water quality assessment research and it is also to be noted that water temperature modeling is compulsory for NSM-I modeling. With the help of NSM-I [6], due to biochemical reactions, kinetic processes and corresponding time rates of change of concentration can be determined separately. The NSM-I modeling consists of only those processes that deposit or transfer material onto the sediment bed, but the constituents which are already in the bed cannot be modelled through such modeling. The rate at which a kinetic process progresses is determined by the input of kinetic data from the users. The one of the benefits of NSM-I modeling is that aquatic water quality simulation can be conducted with the aid of simplified processes and minimum state variables because of the design of NSM-I. It simulates carbonaceous biological oxygen demand (CBOD), dissolved oxygen (DO), simplified nitrogen and phosphorus cycles, resulting in the additional state parameters such as organic nitrogen, ammonium nitrogen, nitrate nitrogen, nitrite nitrogen, organic phosphorus, orthophosphate, and algae.
In the present paper, the HEC-RAS (version 5.0.6) model was used to analyze the water quality of the Taunsa-Panjnad link canal. WQM was chosen in compliance with the proposed methodology [3]. HEC-RAS is an integrated system of software that was conceptualized for collaborative usage in a multi-tasking environment. It mimics the hydraulics of water flow in natural rivers and other channels. The main aims of the present study are to simulate the algae, DO, CBOD, ammonium nitrogen, organic nitrogen, nitrite nitrogen, nitrate nitrogen, organic phosphorous, and orthophosphate for a selected reach of the Taunsa-Panjnad link canal using the HEC-RAS model during twelve weeks’ period, to determine the model's performance during water quality simulation and to assess the water quality in the understudy reach of the Taunsa-Panjnad link canal.
The study's objectives are as follows:
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To determine the current state of water quality by evaluating chosen water quality parameters such as algae, DO, CBOD, ammonium nitrogen, organic nitrogen, nitrite nitrogen, nitrate nitrogen, organic phosphorous, and orthophosphate.
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To compare the findings with the international research.
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To forecast the concentrations of specified water quality parameters from the simulation model, including algae, DO, CBOD, ammonium nitrogen, organic nitrogen, nitrite nitrogen, nitrate nitrogen, organic phosphorous, and orthophosphate.
The purpose of this work is to investigate and simulate several water quality parameters along the under-study length of the Taunsa-Panjnad link canal, as well as to forecast the water quality status.