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Khi xem xét các dòng điện ở các vùng nước bề mặt đa dạng như các sông, hồ, cửa sông, và các khu vực gần bờ của đại dương, nó là điều hiển nhiên rằng các biến đổi lớn tồn tại trong quy mô chiều dài và thời gian mô tả những dòng. Quy mô chiều dài đáng kể khác nhau từ kích thước theo chiều dọc của các vi cấu trúc của các dòng phân tầng (ít nhất là một vài cm) kích thước của lưu vực (lên đến vài trăm km), và thời gian quy mô khác nhau. | 5 Hydrodynamic Modeling When considering the currents in surface waters as diverse as rivers lakes estuaries and nearshore areas of the oceans it is evident that large variations exist in the length and time scales describing these currents. Significant length scales vary from the vertical dimensions of the microstructure of stratified flows as little as a few centimeters to the size of the basin up to several hundred kilometers and time scales vary from a few seconds to many years. Although in principle the equations of fluid dynamics can describe the motions that include all these length and time scales practical difficulties prohibit the use of the full equations of motion for problems involving any but the smallest length and time scales. Because of this considerable effort and ingenuity have been expended to approximate these equations to obtain simpler equations and methods of solution. The result is that many different numerical models of currents in surface waters currently exist. The primary difference between these models is usually the different length and time scales that the investigator believes is significant for the specific problem. For example if the details of the flow in the vertical are not thought of as significant one can use a two-dimensional vertically integrated model this may be either steady state or time dependent depending on whether the time variation is considered significant. To investigate flows where vertical stratification due to temperature and or salinity gradients is significant but horizontal flows in a transverse direction are not a two-dimensional horizontally integrated model is relatively simple and may be useful. More complex three-dimensional time-dependent models may be necessary when flow fields vary significantly in all three directions and with time. In whatever numerical model chosen for reasons of accuracy and stability the grid sizes in both space and time must be smaller than the smallest space and time scales .
