Influence of seasonal and annual hydrological variations on erosion and phosphorus transport from arable areas in Finland

The bulk of the erosion and phosphorus (P) loading from agriculture to surface waters originates in Finland outside the growing season. The annual loading starts after the harvest in autumn and increases cumulatively during winter and spring, while loading during summer remains low. In terms of loading, the condition in which the fields remain after the growing season is crucial. When the (i) the tillage intensity is reduced, (ii) the tillage date is postponed till next spring, or (iii) when the cultivation practice is changed from tillage to no-tillage, the field surface begins to increasingly resemble a permanent crop cover and, consequently, both erosion and particulate P loading strongly decrease when compared with intensive autumnal tillage. On the other hand, the loading of dissolved P tends to increase in such a transition. The effect of annual hydrological variation on the loading is substantial. The suspended solids and particulate P concentrations may double in hydrologically disadvantageous (wet autumn followed by mild and rainy winter) years, thus greatly increasing the erosion and phosphorus loading when compared with “normal” years. A possible increase in runoff will further increase the difference. The levels of suspended solids and particulate P concentrations in fields under permanent crop cover are, particularly during hydrologically disadvantageous years, substantially lower than in fields under intensive tillage. The dissolved P concentrations showed minor variation and remained at their characteristic level regardless of hydrological year or tillage treatment. The annual distribution of loading was similar at the catchment and field scales. The differences among the flow-weighted annual mean concentrations of suspended solids and particulate P tended to strongly even out when the runoff waters from fields were mixed with those from the parts of the catchment with other land-uses. At catchment-scale, it seems that the increase in total runoff rather than in concentrations govern the increases in total loading. The catchment-scale phenomena may, however, cause misconceptions about the character of the loading from fields and the possibilities of mitigation. While it is possible to efficiently reduce the loading from fields, the gained benefit is hard to prove on the base of catchment-scale measurements and, moreover, it often disappears in the variation caused by hydrological conditions. This will pose major challenges for the mitigation measures in a warmer future climate, particularly if the autumn and winter precipitation will increase and the winters will turn milder.