Potassium delivery to rivers : a case study of the Gara catchment, Devon
Stott, Rachel E.
PublisherUniversity of Reading
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Potassium is one of the three most important elements in fertiliser, but in comparison to the mobile anions nitrate and phosphate, there has been little study of its transport within drainage basins. Potassium is a monovalent cation that sorbs strongly to soil constituents; leaching losses are expected to be minimal. However, data from a variety of catchments indicate that concentrations of dissolved potassium increase up to ten-fold during storm events, suggesting that leaching does occur. Possible sources of potassium are vegetation, the soil, mobilised sediment on the hillslope and sediment within the river channel. Previous work has only considered vegetation as a source. Field data at three scales: the hillslope, first-order subcatchment and fourth-order catchment, are used to investigate the sources of potassium and the mechanisms involved during transport from the hillslope to the drainage basin outlet. The drainage basin studied is that of the River Gara, near Slapton in south west Devon, UK. A temporally and spatially intensive sampling programme of mobile and immobile soil waters at the hillslope scale was conducted during storm events in the winter of 1995-6. At this scale, old water that has been resident in the soil for some time is of higher potassium concentration than new water (precipitation inputs for that storm). Discharge of such water to the surface as return flow is particularly significant in potassium transport. The subcatchment response during storm events is consistent with these observations: maximum potassium concentrations occur at the same time as old water contributes to the storm hydrograph. Vegetation and mobilised sediment on the hillslope are demonstrated to increase the potassium concentration of overland flow, but the relative significance of these two mechanisms could not be assessed. Fertiliser applications of potassium are efficiently retained within the soil; no changes in the subcatchment response are apparent after a fertiliser application. At the catchment scale, the maximum potassium concentrations during storm events occurred at the same time as new water contributed to the storm hydrograph. This response is not consistent with the hillslope and subcatchment response. It is interpreted to indicate that the link between potassium transport from the hillslope to the basin outlet is complex, with storages and sinks of potassium reducing the significance of hillslope-scale processes in the catchment response. Suspended sediment within the river channel is demonstrated to contribute to the increase in potassium concentrations during storm events, although they do not account for all of the increase. An alternative explanation is that the land use of the subcatchment differs from that of the whole catchment. The percentage of land put to arable crops in the subcatchment was higher than in the whole catchment; this is likely to reduce the potassium concentration of overland flow (composed of new water), since organic material is more evenly distributed throughout the soil profile in this case. An autumn increase in potassium concentrations has previously been observed; this has been attributed to autumn leaf fall. Experimental data indicate that potassium lost from fallen leaves is efficiently retained within the soil. Weekly data from two gauging stations of Slapton Ley Field Centre (1987-1996) display a highly variable potassium response, and demonstrate no seasonal trend.