Assessment of Lumped, Quasi-Distributed and Distributed Hydrologic Models of the US Army Corps of Engineers

The conceptual or computational procedure for numerically simulating a process(s) which occur in a watershed is generally called hydrologic modeling. Modeling is commonly used to estimate runoff from a watershed, which is one of the most important aspects of any hydraulic and hydrologic design project. One of the widely used applications of these estimates is to determine the design or flood discharge of a watershed. While useful, any simulation model is a simplification of the real world processes and parameters. Because of these simplifications, there is a range of suitable problems for which models can be applied. Traditionally, hydrologic simulation modeling systems have been classified as 1) Lumped parameters, 2) Quasi-distributed parameter or 3) Distributed systems. In terms of spatial discretization/ resolution, we can organize these hydrologic model categories on an ascending scale of sophistication beginning with lumped models to the physically based distributed model. The lumped models treat the complete basin as a single homogeneous element and develop a single outflow hydrograph. The quasi-distributed models discretize the watershed into homogeneous sub-areas or sub-basins based on the topography or drainage area. The infiltration or rainfall parameters are treated as homogeneous within each sub-basin and the flow is determined. The physically based fully distributed models divide the entire basin into elementary unit areas such as grid cells and flows are passed/routed from one grid cell to another as water drains through the basin. These three model categories are presented graphically in Figure 1. This allows the heterogeneity of the watershed to be incorporated on the scale of a grid cell. Grid resolution is generally chosen so that each grid cell will be fine enough to represent the spatial variation of all the input parameters such as rainfall, infiltration/loss parameters etc.