Intra- and inter-event characteristics and controlling factors of agricultural nonpoint source pollution under different types of rainfall-runoff events

Rainfall-runoff events can account for transferring a significant portion of agricultural nonpoint source (NPS) pollution loads to surface water bodies. Nevertheless, mechanistic understanding of intra- and inter-event characteristics and underlying controls are not sufficiently obtained especially under conditions of scarce monitoring data, which hampers the design of appropriate management strategies. This paper presented an approach for an increased understanding of NPS pollution characteristics and controls during events (intra-event) and between events (inter-event) by combining monitoring, modeling, and multivariate statistics. Event based NPS modeling by the Hydrological Simulation Program-Fortran (HSPF) model was improved of its predictive ability by incorporating intra-event variability based on grab samplings during representative rainfall runoff events. The validated model was then used to simulate the intra- and inter-event dynamics of flow, suspended sediment, total phosphorus (TP), and total nitrogen (TN) exports in 100 rainfall-runoff events with different event types. The simulation results suggested that the intra-event distribution (e.g., dispersion degree and maximum changing rate) varied between event types. A higher level of event types generally resulted in more sensitive and flashy catchment responses to rainfall-runoff events, while nutrient availability from fertilizer application, antecedent soil moisture, and precipitation intensity also drove the infra- and inter-event patterns. Load duration curves and flow-weighted concentrations showed that the TN pollution in the study area was extremely severe, especially for the high flow regime and V-E heavy (i.e. very heavy and extremely heavy) events which should be paid particular attention in designing vegetation- and storage-based conservation practices. The principal component analysis revealed that controlling factors varied among event types. Sediment and TP exports were transport-limited in medium and heavy events; TN export was transport-, pre-condition-, and source-limited in medium, heavy, and V-E heavy events, respectively. The combined framework offers insights into agricultural NPS pollution controls from an event-based prospective.