Plot Scale Models and Their Application to Recharge Studies - Part 10

By GR Walker and L Zhang

Each booklet in The Basics of Recharge and Discharge series covers a specific technique for detecting, measuring or modelling groundwater recharge and discharge. Part 10 provides an overview of the use of plot scale models in estimating deep drainage.

This booklet explains how such models can add understanding to deep drainage studies. It describes the different broad types of plot-scale models that are available and how they can be most appropriately used.

Throughout the booklet, clear examples demonstrate the potential applications for a range of simulation models. They contrast the features and capabilities of the models and illustrate what can be achieved through modelling. The chapter finishes by describing how to test models and where to find more information on the models most commonly used in Australia.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Oct 9, 2002
• ISBN: 9780643106116

Book preview

1   INTRODUCTION

Estimation of groundwater recharge is important for salinity studies and efficient groundwater resource management. In regions where water supplies rely heavily on groundwater, knowledge of natural recharge is crucial for quantifying safe yields of aquifers to avoid unacceptable declines in water tables (Bouwer, 1989; Sophocleous, 1991). However, the story is different in many areas of Australia. Increased groundwater recharge has caused dryland salinity and many current studies are focused on recharge reduction rather than enhancement. Land management decisions are generally made by individual land owners but the groundwater response leading to salinity occurs at the catchment scale. As such, for salinity studies, it is important to have knowledge of recharge at both paddock scale and at the catchment scale. Catchment groundwater studies help us understand what effect recharge reduction in part of the catchment will have on groundwater levels and salinity. The change in land use required to induce such a recharge reduction will need to be implemented by individual landowners. We need answers to the question of whether there are practical land uses that will have the desired recharge control.

Field techniques alone cannot answer this question. It is difficult to measure recharge or deep-drainage and replicate measurements because of the expense of field work. Often this expense forms a limit to the number of land use options that can be trialed. For similar reasons, there is also a limit to the range of soils and catchments in which field trials can occur. Even with a number of sites it is difficult to use field techniques alone to determine the different recharge rates under different land uses. Recharge is only a small component of the total water balance, and varies considerably from year to year. The best field comparisons of recharge under different agronomic practices have been conducted on long-term rotations (e.g. O’Leary, 1996), but these are not common. The modelling of deep-drainage (water draining below the plant root-zone) or recharge to groundwater has become attractive to extend knowledge of the water balance to different sites and soils and to different climatic seasons. This chapter considers models with a one-dimensional water balance, which they consider water moving vertically in the soil profile but not horizontally, although they do generally estimate rain water run-off. These one-dimensional models are generally used at plot scale, and are often applied to help determine the impacts of different land use options. There are a number of ways in which modelling helps analyse land use and management with respect to the water balance. Modelling can be used