Catchment Scale Recharge Modelling - Part 4

By TJ Hatton

This paper addresses the need to model recharge to groundwater systems at the scale of whole catchments. It looks at developing the right conceptual model of how water moves through a given landscape for both homogeneous and heterogeneous catchments. One-dimensional recharge models and three-dimensional recharge models are considered. Discussion of which recharge modelling approach to use take in consideration of the availability of data, the nature of the questions being asked, and the expertise of the investigators.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Mar 1, 1998
• ISBN: 9780643106055

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1   INTRODUCTION TO CATCHMENT SCALE RECHARGE MODELLING

What’s unique about this chapter?

In this series, The Basics of Recharge and Discharge, there are chapters which cover recharge modelling at the plot scale, and groundwater models and hydrological methods at the catchment scale. The former provide the means to infer recharge at a point, and the latter provide the means to translate recharge at a catchment to changes in groundwater levels and groundwater discharge. In this chapter, we examine the means available to model and distribute recharge at the catchment scale, and identify the circumstances in which modelling at this scale is essential for deriving appropriate inputs to groundwater models.

Why model recharge at the catchment scale?

It is tempting, and certainly convenient, to think of a catchment as a bundle of small, plot-sized soil profiles for which groundwater recharge can be measured or modelled independently of one another, and then added together or averaged over the catchment to yield the total recharge for the catchment. Indeed, there may be some catchments which operate more or less in this way, and we will look at the features which characterise such catchments later in this chapter. However, it is more usually the case that landscape elements interact in such a way that the water balance of one element is not independent of those upslope. Generally, in landscapes subject to the lateral redistribution of water, either as overland flow or shallow throughflow, it is likely that a different picture of catchment-scale recharge results when this spatial dependence is taken into consideration. Hydrological models which account for these lateral processes are more likely to produce a catchment-scale estimate of recharge consistent with that inferred from inverse modelling of catchment discharge data (Narayan et al., 1996).

Why modelling and not measurement?

In the chapter Groundwater Models and Hydrological Methods we showed how water level, transmissivity and discharge data can be used to infer catchment recharge. In a sense, this is perhaps the most appropriate hydrological method for estimating this quantity at the catchment scale. It is certainly preferred over aggregation of point estimates. So why model catchment recharge at all?

First, there is always uncertainty regarding the inverse method, although it must be said that this uncertainty would also arise out of any predictive modelling approach. More importantly though, the result from the inverse method can only provide a lumped estimate for the entire catchment, and only for the period of hydro-graphic record. As indicated above, different parts of the catchment may operate distinctly with respect to recharge and land use; such distinctions can be very important in catchment management. How much of the lumped recharge was associated with particular landscape features, or land use, cannot be readily determined in this way. Further, it is also hard to project the recharge for the period of record to drier or wetter times.

Predictive modelling is required to project changes in recharge due to landscape position, land use and climate change across a catchment. This involves:

getting the right conceptual hydrological model for the catchment

deciding how to distribute the important hydrological processes and controls across the catchment

deciding how to represent the important hydrological processes relevant to recharge prediction

2   APPROACHES TO CATCHMENT-SCALE RECHARGE MODELLING

Getting the right conceptual model

The key to successful hydrological modelling of any kind is the appropriate conceptualisation of the system of interest. The conceptual model includes the