Solute Variations in Small Catchment Streams: Some Comments

Abstract

The study of temporal variations in solute concentrations in streams has attracted the attention of many workers during the last twenty years. They have demonstrated that solute concentration can exhibit marked seasonal and stormbased variation and several important controls on solute levels have been proposed. In order to document variations in solute concentrations over short periods of time, in response to hydrometeorological controls, an intensive sampling programme is required. Results obtained from continuous monitoring and frequent sampling in some Devon streams are presented to illustrate the nature and extent of short-term temporal variations in solute levels. IN a recent paper, Imeson (1973) has pointed to the increasing interest in the solute loads of rivers shown by geomorphologists and ecologists and to the particular need to focus attention upon temporal variations. However, if it is suggested that this particular field has received little attention, then this ignores a considerable volume of literature representing studies by workers in various disciplines and which can be only briefly sampled in this communication. The variation in solute levels associated with fluctuations in discharge has been documented by many workers, particularly in the United States. In 1948, Hem (1948) pointed to interesting temporal variations in specific conductance and chloride concentration in rivers of the southwestern United States and these were attributed primarily to the dilution of baseflow constituents during periods of high flow, although diurnal fluctuations were also described. This dilution effect was subsequently substantiated, elaborated and quantified by many other studies (e.g. Durum, I953; Korven and Wilcox, 1964; Ward, I963). Goto (1961) described detailed stormperiod variations in several chemical components within a Japanese stream and Hendrickson and Krieger (I960) and Toler (1965) reported clockwise and anticlockwise hysteretic effects in the relationship between solute concentrations and discharge during individual storm events in American rivers. Examples have also been cited where certain solute concentrations exhibit either a positive relationship with discharge or no significant relationship at all (e.g. Edwards, I973). The frequent use of the dilution effect to explain temporal variations in solute levels has prompted the application of simple mixing models based on the mass balance equation and involving individual run-off components (e.g. Hart et al., 1964; Hem, I970; Steele, 1968; Walling, I974) or theoretical storage reservoirs (Hall, I970, I97I; Johnson et al., 1969). Other studies have highlighted control by additional factors. Seasonal effects on the relationship between solute concentrations and discharge have been described by Glancy et al. (1972), Gunnerson (1967), Ledbetter and Gloyna (1964) and Steele (1968). Work in Russia (e.g. Skakalskiy, 1966) has focused attention on the flow paths of individual run-off components. Temporarily increased concentrations during the initial periods of storm run-off events have been related to the 'flushing' of accumulated salts by Edwards (i973), Steele (1968) and Walling (1974), and Gburek and Heald (1970) have attempted to distinguish between those ions which show smooth changes during storm events and those which fluctuate more rapidly as a result of control