Visual OTTHYMO models flows generated from rainfall (or snow melt) on a drainage system. The drainage system first receives water from rainfall or snow melt and transform it to flow. The flow is then routed from upstream to the outlet. Structures may exist to 1) merge multiple flows together or 2) split one single flow to multiple parts.
Visual OTTHYMO conceptualized the drainage system as a collection of hydrologic processes. A hydrologic process is a unit process to 1) generate flow from rainfall, 2) route flow, 3) merge flow or 4) split flow. Each hydrologic process is modeled with a Hydrologic Object, a visual object represented with an icon on canvas or a feature on map. The hydrologic objects are then connected to simulate the sequence of hydrologic processes to simulate the whole drainage system.

An example of the conceptual model is given in the figure above. The drainage system consists of five (5) catchments, three (3) channels and two (2) confluence points. The system can be broken down into five catchments, three channels and two confluence points where catchments transform the rainfall to flow, channels route flow, and confluence points merge flow. Each of the components is a hydrologic process and can be represented by a Hydrologic Object (the icon at the right bottom corner of each component). The drainage system can be simulated with these hydrologic objects in Visual OTTHYMO by creating the hydrologic objects first, and then linking them as a connected system.

The behavior of a hydrologic process (e.g. flow generation) may be different. The behavior can be characterized with parameters (e.g. area and slope) and algorithms (e.g. different unit hydrograph). Visual OTTHYMO uses different Hydrologic Objects for different algorithms of same hydrologic process. For example, the flow from a catchment can be calculated using the Nash unit hydrograph or William unit hydrograph. So two different types of hydrologic objects, NasHyd and WilHyd, are provided in Visual OTTHYMO. As different algorithms may require different parameters, the parameters of different hydrologic objects will also be different.