Managing pollutant inputs from pastoral dairy farming to maintain water quality of a lake in a high-rainfall catchment

A study (2004-11) of a dairy catchment stream entering an oligotrophic lake in an area of very high rainfall (similar to 5m year(-1)) yielded median concentrations of total nitrogen (TN), total phosphorus (TP), suspended sediment (SS) and Escherichia coli (E. coli) of 0.584, 0.074 and 3.7 gm(-3), and 405/100mL (most probable number method), respectively. Trend analysis indicated significant (P < 0.01) decreases for TN (-0.08 +/- 0.02 gm(-3) year(-1)), TP (-0.01 +/- 0.005 gm(-3) year(-1)) and SS (-0.45 +/- 0.14 gm(-3) year(-1)) and were partly attributable to improved exclusion of cattle from the stream. Water balance calculations indicated that approximately one-half the rainfall left as deep drainage that by-passed catchment outlet flow recorders. Estimates of catchment yields for TN were improved by taking into account groundwater hydrology and concentrations from well samples. Storm-flow monitoring inflows exceeding the 97.5th percentile contributed similar to 40% of total loads leaving the catchment so that specific yields for SS, TN and TP augmented by groundwater inputs and storm flows were,960, 45 and 7 kg ha(-1) year(-1), respectively. These compared well with modelled results for losses from dairy farms in the catchment of 40-60 kgNha(-1) year(-1) and 5-6 kg P ha(-1) year(-1) and indicated that attenuation losses were relatively small.