Modelling the quality

As the last post of 2015, let’s look into water quality modelling, in this post we are going to look at the importance of quality modelling and introduce a modelling package.

Importance of quality modelling 

Water availability is dependent on quality and quantity. During the last decades, the water courses have been deteriorated due to anthropogenic discharges of nutrients. The European Directive and the Water Framework Directive both refer to affluent loads and the problem of eutrophication. Mathematical modelling is a useful tool to estimate the pollutant loads into an aquatic environment, to establish relation between what pollutants has what kind of effect on water quality, and to predict the response of the aquatic environment under different scenarios. These simulations are useful tools for management as they assist the decision makers to come up with strategies that considers the basin conditions as well as what the effects could be in case of accidental discharges (Oliveira et al, 2012).

The quality of the freshwater is threatened by population growth, development and expansion of agricultural and industrial activities, and changes in hydrological cycle as a result of Climate Change. The water scarcity hand in hand with decreasing water quality has forced the countries to look into remediation options for river water quality, and hence effective water management strategies to ensure sufficient water supplies with high quality is required (Vieira et al,2012).

QUAL2K

QUAL2K is water quality model for rivers and streams. The model includes many elements such as considering a one dimensional model, assuming the channel is well-mixed both laterally and vertically, assuming steady state, non-uniform for flow hydraulics, heat budgets, and water-quality kinetics that are dependent on heat and mass inputs (Chapra, Pelletierand Tao, 2012).

The model divides the river into smaller sections of “reaches” and “elements”. Reaches represent a length of the river with constant hydraulic characteristics such as slope, bottom width etc. Then each reach can be divided into smaller equally-spaced units called “elements”. The model’s computations are carried out for each element. Then flow balance calculations are carried out for each element by considering a steady-state flow balance equation, then considers the hydraulic characteristics and the presence of weirs. Each element is then considered as an idealized trapezoidal channel and uses the Manning Equation to express the relationship between flow and depth (which we will not get into because I enjoyed other subject more than fluids dynamics!). The model also has a temperature model component and calculates the net absorbed radiation as well. Then the mass balance and flow biochemical reactions are calculated (Chapra,Pelletier and Tao,2012).

In order the get the correct result, a series of computations should be done since they are interdependence. Meaning that a variable such as surface mass transfer coefficient (s) depends on another variable (SOD), which itself is dependent on ammonium and methane concentrations, and so on. Hence, an iterative technique is used in QUAL2K to calculate the fluxes of nitrate, ammonium, and methane, etc. (Chapra, Pelletier and Tao,2012).

Pretty complicated modelling package! It has integrated different fields of water chemistry, fluid dynamics and fluid thermodynamics! Join me in 2016, where I will look into some case studies that have used this package and the results they got!

Wishing you all a fantastic year ahead!

So long,