An example of model building hypotheses in a predominantly top-down approach. (a) A System Diagram that identifies the boundary of the system and incoming and outgoing mass and energy fluxes. The mass fluxes are precipitation, transpiration, interception, surface runoff, baseflow, and snow/ice sublimation. The energy fluxes are incoming short-wave radiation and outgoing latent heat fluxes. (b) A System Architecture Diagram that identifies the sub-system components to be modeled as a directed graph of nodes (state variables) and links (processes); here the state variables are canopy water holding capacity, soil moisture, and snow. (c) A Process Parameterization Diagram that specifies the state-flux relationships determining mass and energy fluxes linking the states. (d) A Parameter Specification Diagram that illustrates the fixing of parameters to appropriate values based on their conceptual and physical properties. (e) A cartoon illustrating how the hierarchical progression of model building hypotheses translates into information (read change of uncertainty) regarding the system output (total streamflow as summation of surface runoff and baseflow; qrunoff + qbase). When the hypotheses are properly specified, the simulated output trajectories should closely track the observations, and bracket them with an acceptable level of uncertainty.

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