Surface Water Balance for Recharge Estimation - Part 9

By L Zhang, GR Walker and M Fleming

This addition to The Basics of Recharge and Discharge series deals with the surface water balance approaches that form the traditional basis of hydrological investigations. It explores both field methods and modelling methods for measuring or estimating the different parts of the water balance, including rainfall, evaporation, run-off and soil water storage.

The authors discuss the concepts required to understand a surface water balance result or to set up an experiment. As water balance studies can be both time-consuming and expensive, this report will give readers a better understanding of water balance approaches and the considerations before going into a water balance study for the purpose of deep drainage.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Oct 9, 2002
• ISBN: 9780643106109

Book preview

1   INTRODUCTION AND SCOPE

The Basics of Recharge and Discharge series presents a range of approaches to the estimation of deep-drainage, groundwater recharge and discharge. This series has been strongly motivated by the occurrence of dryland salinity across southern Australia. Dryland salinity has resulted from increased recharge and its long-term control will be dependent on the ability to sufficiently decrease recharge across the landscape or increase discharge. A better understanding of recharge for different regions is thus likely to benefit salinity remediation. Different techniques for estimating recharge are likely to be effective for different regions and different objectives. This series describes different techniques and their advantages and disadvantages. Nearly all techniques were developed to support the assessment of sustainable yield of aquifers and thus, the series also provides a useful background for this more general water resources issue.

This chapter deals with the surface water balance approaches that form the traditional basis of hydrological investigations. Surface water balance approaches entail those methods which estimate the different components of the hydrological cycle and tries to close the water balance. We will generally consider field-based methods, but will show some examples of integrating them with modelling methods. The field methods try to measure or estimate the different parts of the water balance, including rainfall, evaporation, runoff and soil water storage. Such methods have been refined over a long period for various purposes, including catchment water yield, agronomic production, forest hydrology and erosion studies. The aim of the closing the water balance is to ensure that there are no other sources or sinks of water and that all measurements are consistent. The emphasis on the different components will change according to the purpose of the experiment. Hence, what may be essential as a method for one purpose may not be so important for another.

There are surprisingly few water balance studies of deep-drainage in Australia. As there are no methods other than lysimetry for directly measuring deep-drainage as part of the water balance, deep-drainage estimates have often been no more than that quantity required to close the water balance. Thus, for these studies the error in the estimation of deep-drainage has been dependent on the accumulation of the errors in the estimation of the other components relative to the magnitude of deep-drainage flux. For wetter areas where deep-drainage fluxes are higher, confidence is generally greater than in drier areas. Given that salinity occurs in sub-humid to semi-arid areas, water balance methods are problematic, without an emphasis on deep-drainage.

Temporal variability of rainfall can be a problem. It is well known that Australia has a highly variable climate when compared to other continents. This has important implications for the design of field experiments. How do we obtain a longer-term estimate of deep-drainage when the measurements may occur during a dry period or a very wet period? The components of the water balance such as runoff and recharge accentuate this variability by being processes that are related to exceptionally wet periods, when rainfall exceeds either evapotranspiration and water storage deficits or the instantaneous infiltration