Nutrient transfers in smala watersheds: problems of sampling
Water and suspended matter sampling in hydrographic networks is studied through various sampling systems and procedures, and through devices used in fields and in laboratories (fig. 1). Large size particles tend to be underestimated whatever the sampling procedure (manual or automatic) (fig. 2, 3). On the other hand, the sampling for soluble elements and small site particles (less man 50 μm) is very reliable. This is observed in suspended matter concentrations, eg < 2 g.l-1. The largest particles of suspended matter sometimes contribute to nutrient transfer, e.g. phosphorous. This however depends upon the presence of soil microaggregates and nutrient geochemistry. Moreover, natural hydrological variations give rise to another problem for sampling, especially for suspended matter (fig. 7, 9, 10, table 2). Comparatively the variation due to preservation of water is of secondary importance (fig. 11). However, thanks to results from monitoring in experimental watersheds, it is possible to find the best sampling station and the hast time to carry out sampling (fig. 12, 13; fable 3). Duration of survey and intervals between each sample depend, of course, upon the objectives of the study, but also upon watershed characteristics. Considering these characteristics, we propose an example of sampling stategy in the text (table 6, fig. 5).