Division into System Elements

Navigation
| |

In order to divide a water resources system into individual system elements it is vital to consider the problem the model is used for and the existing data basis.

Basically there are two possibilities for the subdivision of the area. This can be either catchment area-based or grid-based. In addition, all hydrological structures relevant to the problem must be identified and represented by a suitable system element, e.g. dams by reservoir, withdrawals by consumer, etc. Often there are several conceivable solutions.

The prework for subdividing a river basin is usually done with a GIS.

Catchment area based subdivision

Criteria for the subdivision can be:

• Area properties (topography)
• Punctual changes of the outflow by
  • Inflows
  • Point sources
  • Withdrawals
• Location of hydrological structures
• Gauge stationing
• Type and geometry of watercourse

The result of this subdivision are digital catchment area boundaries and river sections. If the available data initially results in a rough subdivision, it can be subdivided even further, especially if, due to the problem at hand, certain processes in the water body can no longer be represented with a rough subdivision. In the following a high resolution water resources system is compared to a low resolution system:

As the accuracy of system mapping increases, the importance of hydraulics in waterbodies increases. The parameters of the discharge concentration refer only to the surface discharge in the subarea, or interflow and base discharge. The illustration of the wave runoff in the water bodies is possible.	Simple approaches to the calculation of the runoff formation usually get along better with a rough system illustration. Both the surface runoff in the subareas and the wave runoff that occurs in the water bodies are included in the parameters of the runoff concentration. The illustration of the wave runoff in the waters is hardly possible

Subareas can be defined via a Rainfall-Runoff Model or can be visualised through a hydrograph at the output

The next step is to decide which system elements are to be used to map the catchment areas, depending on the problem and the data basis. Besides the system element sub-basin, which brings the load into the system via a precipitation-runoff simulation, point source can feed the runoff at the exit of the catchment area directly into the system via a runoff hydrograph. The latter is of course only possible if such a discharge hydrograph is available. Then it is the less computationally intensive variant, which in addition (with good quality of the input data) can illustrate most exactly the actually taken place discharge behavior. If, however, for example, a forecast is to be calculated under changed land use conditions or if the discharge hydrograph is not long enough, it is advisable to use the system element sub-basin. In Talsim-NG the selection of the system element for subcatchments can also vary from subcatchment to subcatchment.

Once the system elements are defined, the flow network is created, i.e. the flow relationships between the elements are defined.

Grid-based subdivision

In grid-based subdivision, water is generally passed from one cell to the next according to its flow direction.

The transfer from one cell to the next varies depending on the discharge component:

• Surface runoff is integrated into the runoff formation process of the next cell, i.e. is treated there like additional precipitation.
• Interflow is fed into the storage cascade of the interflow of the next cell.
• Base flow is fed into the storage cascade of the base flow in the next cell.