Lumped Modeling
The most basic approach for more complex modeling of runoff from a basin is called the "lumped" method. This type of hydrologic model views a particular drainage area as a single unit and uses basin-averaged hydrologic and meteorological inputs. The output from a lumped model is usually a hydrograph at the basin outlet.
One example of a lumped model currently in use in the United States is the Sacramento Soil Moisture Accounting model called SACSMA, which is a spatially lumped continuous accounting model. This continuous model uses mean basin precipitation, evaporation, temperature, basin topography, and soil characteristics as inputs. Outputs include direct runoff, surface runoff, baseflow, interflow, and evapotranspiration.
Continuous means that the model variables that account for soil moisture are updated on a day-to-day basis. This allows for creation of an ongoing model condition of soil moisture.
The SACSMA model is ideally suited for the simulation of large drainage basins – greater than 1,000 square kilometers. Because the model simulates storm runoff as well as baseflow, it is also useful for water supply estimates. Additional inputs to this model include the location of important features such as reservoirs, lakes, and river junctions.
In the past, lumped modeling methods were required due to data collection methods and software limitations. Lumped models are still useful for producing flood guidance. They require less data input and less computational power than more modern methods. However, as new geo-spatial technologies become available, lumped models are being replaced by methods using more detailed spatial information to examine the basin on a finer scale.