Quoted from: http://www.colby.edu/chemistry/EastPond/East_Pond/DYRESM_Model.html 

DYRESM, the Dynamics Reservoir Simulation Model, from the Center for Water Research, CWR, at the University of Western Australia is a “one dimensional hydrodynamics model for predicting the vertical distribution of temperature, salinity, and density in lakes and reservoirs satisfying the one-dimensional approximation.”1   This approximation is accurate over simulation times of several weeks to tens of years but smaller times scales do not allow for an accurate approximation of the data because of the influence of destabilizing factors.  These factors are wind stress, surface cooling and plunging inflows.  The approximation is therefore deemed valid when these factors do not act on a body of water for a prolonged period of time.  Because these factors are normally associated with extreme events like storms and floods if the simulation is based on periods of time greater then those of the extreme events the approximation can well define the dynamics of the body of water.  The longer the period of time the less significance the extreme events will have.  “The model thus provides a means of predicting seasonal and inter-annual variability of lakes and reservoirs as well as sensitivity testing to long-term changes in environmental factors or watershed properties.”2