Watershed modelling deals with multiple processes occurring in multiple media. The processes include flow and thermal, salinity, sediment, and biogeochemical transport. The multiple media cover stream/river/canal/open channel networks, lakes/reservoirs, land surfaces, and subsurface media. Analytical and numerical models are commonly employed for simulations to understand sciences or to assess environmental consequences. Why analytical models offer the advantage of clearly and easily explaining the physics involved, the numerical models give practical applications of assessing the impact and interaction among processes and media. This presentation will discuss the issues and difficulties associated with the employment of Green functions to yield analytical models and troubles associated with the use of numeric to generate computational models. Various means of overcoming these difficulties and troubles will be outlined and addressed.