The capabilities of the recharge model ZOODRM

The development of the suite of groundwater models in the ZOOM family has demonstrated the advantages of OO techniques for including advanced numerical techniques such as local grid refinement and the flexibility to add new mechanisms as required. The recharge model was developed as a direct result of the positive experiences of using OO techniques.

The recharge model can simulate five types of recharge processes:

  1. Soil based recharge (direct recharge)
  2. Run-off to surface water systems (indirect recharge)
  3. Routing in the unsaturated zone to springs (indirect recharge)
  4. Urban recharge (e.g. leakage from water mains)
  5. Irrigation recharge (e.g. losses from fields)

The distributed recharge model calculates recharge at appropriate points over the model grid. A daily time step is used for the recharge calculation, with the output supplied as monthly averages. Objects are often developed to represent real world entities, so that in the recharge model, there are both soil objects and wadi objects. The structure of the model is also represented using grid and node objects. The recharge calculation is undertaken within a node object. These node objects are held, in turn, within grid objects and grids can be nested to increase resolution in discrete areas. This facility was developed to provide input data in the correct form for the groundwater flow model ZOOMQ3D, which incorporates local grid refinement in a Cartesian mesh (Jackson and Spink, 2004).

The type of recharge calculation to be made at each node is preselected. Amongst the available recharge calculation methods, two are incorporated in the model: the Soil Moisture Deficit (SMD) method and a soil moisture balance approach that is suitable for semi-arid regions, the Wetting Threshold (WT) method. The latter is based on a sprinkler test carried out close to the Wadi Natuf catchment by Lange et al. (2003) which showed that run-off and recharge only occurred after the system has “wetted up”.

The figure below shows that there are two grids storing node objects; one is used to perform the soil-based recharge, surface run-off processes, and the other is related to unsaturated zone. Run-off is handled by the surface processes grid and routed using a Digital Terrain Model (DTM). The DTM is processed to provide directions of slope on the cardinal points of the compass (i.e. north, south, east and west). The nodes are then linked to create the routing pathways. During the simulation, the run-off calculated by the nodes is routed to the wadi, whereby wadi flow can then leak to the underlying aquifer.

When springs are connected to unsaturated zone nodes a proportion of the recharge arriving at each node is routed to the spring. Unsaturated nodes are connected to spring nodes automatically by the model if they are at a higher elevation than the discharge point. However, there needs to be additional rules for specifying which nodes connect to the spring that define the spring catchment. These relate to the area of outcrop providing the recharge diverted to the spring, the likely maximum distance from the spring a node can contribute flow and, a condition to stop nodal connections from crossing valleys in thin air.A groundwater velocity (VGW) is specified to ensure routed water takes a feasible time to reach the spring.

Lange J, Greenbaum N, Husary S, Ghanem M, Leibundgut C and Schick A P, 2003, Run-off generation from successive simulated rainfalls on a rocky, semi-arid Mediterranean hillslope, Hydrological Processes, 17, 279-296.

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