Seasonal changes in phosphorus load flowing out of small agricultural catchments

In this article distribution of monthly phosphorus loads flowing out of two agricultural catchments which are located in different physiographic conditions of Lower Silesia was analysed. Loads of phosphorus runoff from the catchment located in the piedmont part of Lower Silesia in each month rarely exceed 0.10 kg P · ha^-1. The size of annual load is determined by loads obtained in two months of early spring. Much lower loads obtained for lowland catchment, located near Wroclaw. Values calculated for each month rarely exceed the value of 0.01 kg P · ha^-1. Culmination of loads bringing away is a bit more extended in a time compared to the catchment located on Sudety Mts. Foreland. Much higher loads are observed during the period from January to April – this period has a major impact on the size of phosphorus load that flows out from this catchment during whole hydrological year. The obtained results clearly indicate that the threat of watercourses and water reservoirs supply in phosphorus compounds from agricultural land is periodic and it is particularly high during early spring. Phosphorus load flowing out from the analyzed catchments is very diverse. From facility located on Sudety Foothill in hydrological year, during research period, flowed away average 0.81 kg P · ha^-1. Significantly lower values were obtained for second facility and it was average 0.15 kg P ha^-1 during a year. The size of load discharged during a year is largely determined by amount of phosphorus load flowing out during winter half of the year (from XI to IV). In case of foothill catchment in this period flowed out average 0.56 kg P · ha^-1, which presents 69% of annual load and in lowland catchment this percentage was even slightly higher and was 73%.