Limitations of the Linear Reservoir Method

The Linear Reservoir method is a simple method for calculating overall water balances in the saturated zone. As such, it is unsuitable when a detailed spa­tial distribution of the water table is required. However, even given the sim­plicity of the method, the following simplifications and limitations should be noted:

·         In the Linear Reservoir Method, the same river links are used for both of the base flow reservoirs. This is a limitation in the sense that occasion­ally you may find that fast response and the slow response baseflow may contribute to different parts of the stream.

·         When calculating the unsaturated flow, the bottom boundary condition is input from a separate file - not calculated during the simulation. This means that changes in the water levels in the reservoirs will have no effect on the UZ boundary condition.

·         The numerical solution used for the Linear Reservoir module assumes that the inflow to each of the linear reservoirs is constant within a time step. Strictly speaking, this is not correct as outflow from each reservoir changes exponentially during a time step. The calculation procedure uses the mean outflow from the upper reservoir as inflow to the down­stream reservoir. In this way, there is no water balance error but the dynamics are somewhat dampened. If this is a problem, smaller time steps can be chosen, which will lead to a more accurate solution, as the changes in flow become smaller during each time step.