Structure of Pegase

Introduction

Pegase is an integrated model watershed/River dedicated to calculating in a deterministic way the river water quality according to releases and pollution load, for various hydrological situations, in steady-state or non-stationnary mode over years. It also provides provisional calculation of measures foreseen to enhance water quality (waste water treatment, release reduction…) before their actual implementation.

Pegase can proceess several thousands of rivers simultaneously and the watershed processed can reach several tens of thousands of km². Pegase can also process fine simulations on a subset of the whole watershed (e.g. a single river with a watershed of a few km²).

Pegase belongs to a new generation of models, which implement a detailed representation of system dynamics, particularly the explicit calculation of the evolution of various biomasses. Furthermore, the structure of te Pegase model is su!ch that most of the parameters have a physical, chemical or biological meaning (e.g. maximal growth rate) and can be calibrated separately using well targeted experiments results. One of the advantages is that Pegase can therefore be used wityh few modification and calibration for various hydrological situations and various hydrographic networks.

Structuration of the model

Considering the process to represent, Pegase is structured as a series of 5 submodels implementing the following features:

• The Release submodel represents in a structured way urban releases, industrial releases, the role of Waste Water Treatment Plants, releases due to livestock handling and the diffuse soil load. It calculates inputs to the modelled river network
• The hydrological and hydrodynamic model calculates the flow and other hydrodynamic parameters (velocity, stage…) in all the nodes of the river network and at all time, from the morphometric characteristics of the water courses (slope, width, dams…) and flows measured in several gauging stations
• The thermal submodel calculates the water temperature, based on daily average of measured water temperature in some points, the altitude and the eventual thermal discharges
• The Ecosystem submodel calculates:
  • using sunlight (irradiance at surface) as main input data, the growth of phytoplanktonic biomasses
  • the growth of barcterial biomass
  • the flux of organic matter (soil load, urban releases, industrial releases et internal production by the ecosystem)
  • the flux of nutriments
  • finally, the oxygen balance (production by plants, consumption through respirations, reaeration at surface)
• The meta-pollutant submodel calculates the evolution of micropollutants and macropollutants released, using an appropriate representation of the sedimentation

Processes modelled

Amongst the main processes anfeatures included in the Pegase model, we find:

• explicit calculation of the primary production (phytoplanktonic biomass) through a multi-species submodel
• explicit calculation of the assimilation mechanisms (calculation of des bacterian biomass and its activity)
• explicit calculation of the micropollutants (heavy metal, pesticides…)
• calculation of the physico-chimical quality indices (e.g. french SEQ-Eau)

Use of the model

Pegase can be used in 2 modes:

• The "steady-state" mode, in which hydrometeorological conditions (flows, temperatures, insolations) are considered as constants. This mode allows hydrometeorological simulations representing a characteristical regime (wet years, percentile of flow…) which can easily viewed and compared with other simulations
• The "non-stationary" mode, in which hydrometeorological conditions vary on a daily basis and allow calculating the evolution over the year of concentrations over the whole modelled hydrographic network

The use of the model has been facilitated in the recentyears, by the addition of a dedicated GUI, realised in the frame of the program "Pegase Opera" (to make the Pegase model more operational) with the support of Pegaseusers, and resulting in the software suite PegOpera.