*3.3. Approximants*

In this section, we explore the application of approximants for the transfer function. In general, an approximant is a function, series, or other expression which is an approximation to the solution of a problem. Here, we trial the application of a conceptual model, specifically a linear reservoir model with a time delay, to approximate transfer functions, namely:

$$TF(t) \sim 1 - \exp(-a(t - t\_6)), \ t > t\_5 \tag{45}$$

where a, *t*5 and *t*6 are fitted parameters. Such models have previously been used [18] for recharge through a deep vadose zone. Such conceptual models have been used widely in surface hydrology to calibrate surface flow models. The linear reservoir model forms a good approximation where the output (recharge) is a linear function of the storage (the mass of water in the unsaturated zone) [18].
