Nutrient and sediment movements from soil to surface water in a forested watershed and two agricultural fields

In North America, the acceleration of the eutrophication of surface waters due to nutrient pollution is still present. Soil studies have not entirely succeeded in linking nutrient and sediment losses to field hydrology because relationships between discharge and dissolved ions/sediments are complicated by a hysteresis effect which has been only described qualitatively. The objective of this thesis was to better understand the effects of hydrology on N, P, and sediment transfer from agricultural and forest soils to surface waters. This was done by developing a technique, called the H index, to quantify the hysteretic behaviour of ion and sediment transport in stream/overland water. The chemical and sediment concentrations in a stream of a forested watershed in the Sierra Nevada during snowmelt and in overland runoff of two agricultural fields during rain events in the Montreal area were examined. In the stream of the forested watershed, H indices for suspended sediment increased (looser hysteresis loop) with the availability of sediments and the lag between peaks in suspended sediment concentrations and discharge. In agricultural fields, nutrient concentrations increased with time during each event with presence of counterclockwise and clockwise hysteresis. The hysteretic behaviour of suspended sediments was not significantly related with either prior soil moisture content or rainfall characteristics. In order to simultaneously monitor P and N in the stream and soils of the forested watershed, a laboratory study was conducted to investigate the efficacy of various mixedbed exchange resins in absorbing dissolved organic and inorganic N and P. Results showed that mixed-bed resin was adequate for characterizing P on a short-time scale but longer exposure periods were required for N. Results from the resin exchange reveal the possibility that the spring time pulse of NOs'-N in stream water was due to the melting of the snowpack