Soil erosion in areas of intensive cultivation in southwestern Finland

Abstract

The amounts of solid material and dissolved substances washed out from the soil of an intensively cultivated clay area in southwestern Finland were studied. The drainage basins examined were those of Paimionjoki in its entirety (area 1080 km²), two of its tributaries (145 km² and 63 km²), and six smaller streams (1.7–4.2 km²). These basins vary in topography from flat agricultural plains to hilly country with relative heights of 30–50 m. The proportions of arable land vary in range 32–68 %. Water samples were taken at mean intervals of one week throughout the open‑water season, i.e. from the beginning of April to the end of November, over three years. Rainfall figures were virtually identical in all the basins, but varied from one year to another in the range 466–524 mm for the above season.

The mean transport of material from the areas with a variable top­ography and high incidence of arable land was 220–360 t/km², increasing to 300–560 t/km² in rainier years, whereas transport from the flatter areas was only 70‑100 t/km². The mean value for the whole Paimionjoki basin was 50t/km² during the season studied. Transport during the winter season. December–March, accounted for about 20 % of the annual total. The amounts of dissolved substances transported from the basins close to the coast were extremely high, over 500 mg/1 on average, and these accounted for over 60 % of the total transport. This proportion declined rapidly further up the river, however, being only 20–30 % some 5 km from the mouth, after which it remained more or less constant. These substances constituted 49 % of total transport in the Paimionjoki system as a whole. The solid material washed out of the basins proved to be almost entirely derived from the agricultural land. Thus 500–600 t/km² of solid material alone were lost from the sloping fields beside the lower course of the river in the rainiest years, although the mean values were only around 200–300 t/km² of arable land.

Of the variation in the amounts of solid material transported 98 % could be explained in terms of the proportion of arable land in the drainage basin, the gradient of this land, the proportion of fields with open ditches and the proportions of ploughed land and grassland. Similarly 98 % of the variation in dissolved substances could be explained by proximity to the sea and the proportion of arable land.