ABSTRACT
The principal drawback of the hydrochemical approach is that it requires frequent, ideally continuous, measurement of discharge and sufficient samples to establish the pattern and extent of variations in solute concentrations. This is not always possible and alternative methods that integrate erosion over a longer time period have therefore been derived. The two most commonly used in karst areas are the micro-erosion meter (Spate et al., 1985) which provides a direct measure of rock surface lowering (see Figure) and rock tablets (Trudgill, 1975). In contrast to the hydrochemical method these techniques are highly site-specific and may only be used to assess erosion rates on bare limestone surfaces, in the soil zone, at the soil-bedrock interface, and in cave streams. Trudgill (1975) suggested that tablets could be used to detect seasonal differences in erosion rates and raised the possibility of making reliable measurements over shorter time scales. However, when Crowther (1983) compared the rock tablet and hydrochemical methods in West Malaysia, he found that the tablets gave estimates two orders of magnitude less than those calculated using hydrochemical data. The most likely explanation is that natural rock surfaces come into contact with larger volumes of water than do isolated rock
tablets, simply because of their greater lateral flow component (see also Erosion Rates: Theoretical Models). Thus, the two methods measure fundamentally different phenomena and the hydrochemical method provides the only reliable means of estimating solutional erosion rates on limestone surfaces. Different problems arise if tablets are placed in cave streams as they will project above the natural surface and as a consequence are likely to erode more rapidly. They are also likely to suffer from abrasion as well as corrosion, although this can be exploited by placing the tablets in nylon cages with differing mesh sizes and comparing the erosional losses suffered.
