Sediment and Stream Water Quality in a Changing Environment: Trends and Explanation

In the past few decades, reconstruction of agricultural areas, particularly vineyards was carried out in the Federal Republic of Germany to increase the productivity. The effects of the reconstruction on the dissolved solutes were studied from November 1982 to mid-September 1983 in an unaltered (small terraced) catchment and an altered (large terraced) catchment in the East Kaiserstuhl. Anion transport was highest from the altered catchment, which probably resulted from the increased impermeable area associated with the installation of a dense network of pipes and mole drains. The increased solute transport was pronounced during winter. Higher transport of ortho-P04 " occurred during runoff events throughout the entire year. In contrast, the transport of dissolved solutes, in particular N03", typically was highest during summer in the unaltered catchment. INTRODUCTION In the last 25 years, the landscape of the Eastern Kaiserstuhl, in the southwestern part of the Federal Republic of Germany, has undergone dramatic change. The many small vineyards characteristic of the area have been consolidated to improve the agricultural productivity. The larger terraces and associated changes have had not only visual impacts but a pedological, morphological, hydrological, and hydrochemical impact as well (Keller et al., 1973; Endlicher, 1979; Luft et al, 1981; Demuth & Mauser, 1983; Rieg, 1988; Demuth & Leibundgut, 1990). The Institute of Physical Geography and Hydrology of the University of Freiburg has been conducting experiments to identify, describe and quantify the hydrological impact of this landscape change, in cooperation with the West German Research Association (DFG). In this paper, the effects of field consolidation on the shortand long-term output pattern of dissolved solutes are examined using data from two neighboring catchments; one was relatively pristine and the other was altered. The study was conducted from November 1982 to 11 September 1983. THE RESEARCH BASIN IN EAST KAISERSTUHL The research area is situated about 15 km to the West of the Black Forest in the Eastern part of the Kaiserstuhl well known as a vine growing area and includes an unaltered catchment, Rippach, and an altered catchment, Lôchernbach (Fig. 1). The drainage area of the Rippach catchment is 1.2 km and of Lôchernbach catchment is 1.7 km. Both catchments are underlain by volcanic rock and contain deep loess soils (< 30 m). Apart from minor changes, the Rippach catchment has not been altered recently and contains a dense network of irreg239 240 A. Rieg et al. ular foot paths connected to deeply incised roads. FIG. 1 Location ofihe research basin within the Federal Republic of Germany. In contrast, the Lochernbach catchment was altered in 1969-1971 and again in 19751976 at which time large terraces and a regular network of small roads were constructed using heavy machinery. The consolidation resulted in the construction of very steep slopes at the edges of the new terraces and a dense drainage network. The characteristics of the two catchments are summarized in Table 1. The instrumentation of the research basins began in 1966 in the Rippach area and was extended to the Lochernbach catchment when consolidation was completed in 1977 Rainfall, runoff, and groundwater levels were continuously monitored at 10 minute intervals Further details about the instrumentation can be found in Luft (1980). HYDROLOGICAL BEHAVIOR The hydrological response of the research basins Rippach and Mittlingen-Lochernbach is shown m Fig. 3 for which monthly mean discharge are plotted against time for the longterm and for water year 1983, which had a wet winter and very dry summer. Since hydrologic monitoring began in the research basins (1972 at Rippach and 1977 at Lochernbach) the highest monthly means occurred in May 1983; the highest discharge was 26 1 s from the Rippach catchment and 721 s" from the Lochernbach catchment. Discharge from the Mittlingen-Lochernbach catchment was higher than from the Rippach catchment for the Human Impact on a Small Research Basin, FDR 241 entire year, except during the low flow period in September. Also, the discharge from the Rippach catchment is less variable than from the Lôchernbach catchment throughout the year. In fact, the monthly mean discharge from Lochernbach catchment was more than two times that from the Rippach catchment in the spring. TABLE ICatchment characteristics of Rippach and Mittlingen-Lochernbach. Catchment characteristics Rippach Lôchernbach Catchment area Length of small asphalt roads Length of subsoil pipes Mole drained areas Bank slope of 45° 1.2 km 1.4 km <1.0km 1.7 km 23.2 km 12.3 km 19.8% 12. % Land use (in percentage of total area): Forest Wine growing area Agricultural land Small asphalt roads 18. % 48. % 33. % 0.5% 5. % 65. % 12. % 6. % The hydrographs for water year 1983 reflect the differences in storage (Demuth & Leibundgut, 1990). In the Lochernbach catchment, low flow discharge is less than half that of the Rippach catchment. In this paper, the impact that these hydrological differences have on hydrochemistry will be evaluated. ANNUAL TRANSPORT Samples were collected weekly during the study. A comparison of flow duration curves for the hydrological year with discharge at the sampling times indicates that the discharge associated with the weekly samples covered 97% of the flow that occurred for the year. Consequently, variations in daily mean discharge are well represented by the samples. The mean concentration of each solute derived from the weekly samples in the Lôchernbach catchment was higher than those from the Rippach catchment and the individual concentrations were more variable. However, for samples collected during storms, the mean concentrations were higher in the Rippach catchment but the range of variation was less. Except for ortho-P04 \ solute concentrations are lower during runoff events than during base flow in either catchment. It is important to consider this when evaluating annual transport based on mean daily dissolved load. The cumulative transport of assayed anions including ortho-P04 " from each catchment is shown in Fig. 4 for water year 1983. The total dissolved anion transport was 134 and 128 t km" from the Rippach and the Lôchernbach catchments, respectively. The corresponding mean monthly transport was 13 and 12.8 t km" from November 1982 to August 1983.