Dynamika transportu fluwialnego górnej Parsêty jako odbicie funkcjonowania systemu zlewni

Dynamics of fluvial transport of the upper Parsêta River as a response of the catchment system

Andrzej Kostrzewski, Malgorzata Mazurek, Zbigniew Zwoliñski

7. Spatial and temporal variations in the sources of dissolved and suspended load

Knowledge of spatio-temporal variations in the supply of sediment to the river channel is crucial for determining the activity of the catchment's denudation system, which ultimately decides the contemporary evolution of the relief. The hydrochemical mappings of the upper ParsÍta catchment were carried out in two extreme seasons, namely the summer base flow periods (September 1992 and August 1993), and spring periods of high soil moisture content and high discharges, when a variety of supply sources were active (April 1992 and March/April 1993). In their ionic composition, the upper ParsÍta waters belong to the calcium-bicarbonate type (Figs. 7.1.1, 7.3.1). It is characteristic of young-glacial lakeland areas built of forms rich in calcium carbonate. Worth noting are the waters of basins without outlets that can be found throughout the catchment, which are largely of the calcium sulphate type (Fig. 7.1.1). The range of variation in physical-chemical parameters of those waters reveals considerable differences in the properties of surface water in the catchment (Tables 7.1.1, 7.3.1). A grouping procedure carried out on the basis of 11 physical-chemical parameters characterising the measurement sites during the four mappings produced five hydrochemical groups (Fig. 7.1.2). Their distribution in the catchment area is shown in Fig. 7.1.3. The groups mainly reflect the influence of the lithology and soil cover of the upper ParsÍta catchment; they can be treated as the dominant natural factors in the young-glacial area.

Out of the many factors controlling the temporal variation in the ionic composition of water, hydrological conditions can be regarded as the most significant ones. The relations between ion concentration Cj and water discharge Q in the upper ParsÍta in the years 1992 and 1993 show considerable differences among particular ions. There is a significant, inversely proportional dependence between calcium cations, bicarbonate anions and ionised silica, and the discharge. In turn, concentrations of sulphate and potassium ions tend to increase with the discharge. The nature and magnitude of these dependences result from the mechanism underlying water circulation in the catchment and from the location of the sources supplying particular chemical components (Figs. 7.3.2, 7.3.3, Tables 7.3.2, 7.3.3). The upper ParsÍta flow combines the flow of substances from geochemically diverse sources. The physical-chemical properties of water change with variations in the quantitative and temporal relations among the river flow components. It was found a discharge of 1,5 m3s-1 was a threshold value in the supply of particular ions for fluvial transport (Fig. 7.3.4). With such flood discharges, most ionic components were observed to be greatly diluted and river water enriched with potassium and sulphate ions. The nature of relationships between the discharge and concentrations of chemical components, as well as their seasonal and annual variations in the upper ParsÍta waters (Figs. 7.4.1, 7.4.2) were undoubtedly determined by highly diversified hydro-meteorological conditions obtaining in the two hydrological years under study. An analysis of variation tendencies has shown climatic conditions, their seasonality and impact of biological activity to have effect on water chemistry revealed in a long-term measurement programme. The relief, permeability of the substratum and high storage capacity of the upper ParsÍta catchment are responsible for the lack of direct links between the magnitude of precipitation and water chemistry, except for intensive rains.

The described patterns of suspended material concentration down the upper ParsÍta's long profile were confirmed by mappings along streams of subcatchments. Only a small part of the catchment was found to take part in the supply of suspended material to river channels. The limitation of farming and turning arable land into fallow in 1990 have greatly reduced the effects of overland flow in the slope areas of the catchment.

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