This thesis is concerned with phosphorus management in grassland agriculture in Ireland and the impact it has on catchment chemical water quality. It reports three inter-related studies which assess changes in phosphorus concentration in the soils and streams of two 5km2 grassland sub-catchments, Co.Tyrone (Northern Ireland) and Co. Monaghan (Republic of Ireland) of the cross-border River Blackwater following the implementation of mitigation measures. The first study assessed the changes in soil phosphorus status after five years of recommended zero phosphorus application to high phosphorus status soils. This study found that in the Co. Tyrone sub-catchment, where phosphorus had not been applied to soils with originally agronomically optimum or excessive phosphorus status, a significant mean reduction in Olsen-phosphorus concentration of 4.5 mg l-1 was achieved. The reduction was more pronounced (7.2 mg rl) when only excessive soil phosphorus status was considered. However, despite receiving the same advice in Co. Monaghan, the fields with optimum or excessive phosphorus status had a significant overall increase of 4.5 mg rl . Excessive fields had a mean increase of 4.9 mg l-1. In both catchments there were significant increases in low to moderate phosphorus status fields of 5.96 and 7.1mg rl (Co. Tyrone and Co. Monaghan, respectively). Application of a soil phosphorus decline model on the fields which demonstrated a decrease in the excessive soil phosphorus status estimated that the phosphorus deficit for these ranged between -0.1 and -30 kg ha-1, which is agronomically realistic for grassland management. Analysis of a five year high resolution phosphorus and discharge dataset showed that, although discharge from the Co. Tyrone and Co. Monaghan streams were not significantly different from 2006 to 2010, totalling 3170 mm and 3694 mm respectively, the phosphorus loads in Co. Tyrone were less than half of those in Co. Monaghan; 7.26 and 14.94 kg ha-1, respectively. Analysis of the extent of critical source areas in each catchment revealed that 3.75 % of the 19.3 8 km of stream network in Co. Tyrone had bankside fields of excessive phosphorus status while in Co. Monaghan the percentage was almost double that of Co. Tyrone with 7% of the 36.85 km of the stream network impacted by bankside excessive phosphorus fields. Analysis of change in phosphorus concentration at medium to high discharge percentiles showed that, in both catchments, there had been a significant increase in concentration between 2006-2010 at Q5-QIO , 0.152 to 0.280 mg l-1 in Co. Tyrone and in 0.228 to 0.391 mg l-1 in Co. Monaghan. When annual loads were calculated for each catchment, the largest load from both catchments was in 2009, 2.12 and 3.98 kg ha-1 , in Co. Tyrone and Co. Monaghan, respectively. This year, however, did not have the highest rainfall, which happened in 2006 (1299mm) compared with 2009 (l123mm). When analysis of the timing of rainfall events was conducted it was revealed that 2009 had continuous rainfall events throughout the year but 2006 had all but one storm event during the winter period. Analysis of extreme baseflows in both years (Q9S) revealed that 2009 had less than 20% of Q9S discharges compared with any of the other years which suggested than the continuous rainfall maintained the soil in a persistently wet state throughout the year and increased the potency of subsequent storm events for phosphorus transfer thereby increasing phosphorus load loss.