Stochastic structures between quantity and quality responses of rainfall-runoff at an upland agricultural watershed

The causality between hydrologic parameters and water quality variations ha s been studied for a small upland agricultural watershed using a time-serie s analysis approach. The stage at the outlet of the watershed, which is a p roxy for the outflow, and the average saturation deficit, which is a surrog ate for the groundwater level, were used as hydrologic variables. The stage s were measured at a triangular weir at the watershed outlet. The saturatio n deficits were obtained by means of a rainfall-runoff simulation model for agricultural upland watersheds (an extended version of TOPMODEL). Sequenti al water quality data were also collected at the watershed outlet. A system atic procedure for testing the causality between the hydrologic and water q uality time-series is presented and used to investigate possible relationsh ips between the two systems. The stochastic structure of a causality betwee n hydrologic and water quality systems is shown. This stochastic structure is associated not only with the transport pathways (surface or subsurface) of each water quality parameter but also with the characteristics of the ra infall events.