Phosphorus (P) export from the Foron River watershed was intensively monitored. Water was analysed for total P, soluble total P, soluble orthophosphate and suspended solids. Watershed soils and river sediments were sampled and the size fraction 50Á2 mm analysed for total P, water extractable P, bioavailable P, 1 minute exchangeable P and P ®xation capacity. Interstitial waters were analysed for soluble total P.
Four hydrological conditions recurred, two during low river ¯ows and two during increased ¯ow. The ®rst occurs in dry weather with a constant or decreasing ¯ow over at least seven days and when there is no surface runo. Exported phosphorus, predominately soluble and bioavailable, is from point sources. Phosphorus inputs exceed P export so P accumulates in the river. The second condition occurs when a small storm ¯ow increases the average seven-day ¯ow to exceed the preceding weekly average. Phosphorus export exceeds P inputs and originates from urban runo, point sources and release of P stored in the river. Exported P is largely particulate but highly bioavailable. The third condition is when substantial runo follows at least a seven-day period of constant or decreasing ¯ow. Phosphorus export is from diuse urban runo. All the P stored is exported. Exported P is highly bioavailable. High concentrations and ¯uxes of P export are often seen. The fourth condition happens when the soils are wet and increased ¯ow is from both urban and agricultural runo. Phosphorus export from diuse agricultural runo predominates and is largely not bioavailable. Phosphorus concentrations are low but export ¯uxes are high when ¯ows are high. These hydrological conditions, when integrated with concepts of mass balance de®ne a phosphorus export typology comprising four regimes. These regimes explain total phosphorus (TP) storage, transport and export patterns, changes in P speciation and allow identi®cation of probable sources of TP in the Foron river watershed.