Understanding how these factors work together is complex when taking into account all the processes and information from natural settings. A hydrologist may need to present all this information together in a mathematical model, on a computer, to help answer questions. Managers, regulators, and the public often want to know how much faith the hydrologist has in the final answers. Modular indicates the program can have modules, or other programs, added to represent certain aspects of the physical area, for example a river or well. Different methods of simulating a process such as variation in geologic properties or evaporation can also be included.
Finite difference indicates the physical system is represented by a mesh of rectangular cells.
The best situation would allow the hydrologist to know everything about the geology, hydrology, and climate in an area and provide answers with no uncertainty concerning the physical system. To collect enough information to understand the whole area would be very costly and impractical. We often want to know what will happen in future years. Rainfall could decrease in some areas and increase in others. The risk of not knowing the real answer can be a serious concern for managers and the public.