Linking Dissolved and Particulate Phosphorus Export in Rivers Draining California's Central Valley with Anthropogenic Sources at the Regional Scale

Pollution of water resources by phosphorus (P) is a critical issue in regions with agricultural and urban development. In this study, we estimated P inputs from agricultural and urban sources in 24 catchments draining to the Central Valley in California and compared them with measured river P export to investigate hydrologic and anthropogenic factors affecting regional P retention and export. Using spatially explicit information on fertilizer use, livestock population, agricultural production, and human population, we calculated that net surface balances for anthropogenic P ranged from −12 to 648 kg P km −2 yr −1 in the early 2000s. Inorganic P fertilizer and manure P comprised the largest fraction of total input for all but two catchments. From 2000 to 2003, a median of 7% (range, −287 to 88%) of net annual anthropogenic P input was exported as total P (TP). Yields (kg P km −2 yr −1 ) of dissolved inorganic P (DIP), dissolved organic P, particulate P, and TP were not significantly related to catchment‐level, per area anthropogenic P input. However, there were significant relationships between mean annual P concentrations and P input from inorganic fertilizers and manure due to the concentration of agricultural land near catchment mouths and regional variation in runoff. Catchment‐level P fertilizer and manure inputs explained 4 to 23% more variance in mean annual DIP and TP concentrations than percent of catchment area in agriculture. This study suggests that spatially explicit estimates of anthropogenic P input can help identify sources of multiple forms of P exported in rivers at management‐relevant spatial scales.