Lag Times as Indicators of Hydrological Mechanisms Responsible for NO<sub>3</sub>-N Flushing in a Forested Headwater Catchment: Fid-Bau Portal

Lag Times as Indicators of Hydrological Mechanisms Responsible for NO₃-N Flushing in a Forested Headwater Catchment

Klaudija Sapač / Andrej Vidmar / Nejc Bezak / Simon Rusjan

Understanding the temporal variability of the nutrient transport from catchments is essential for planning nutrient loss reduction measures related to land use and climate change. Moreover, observations and analysis of nutrient dynamics in streams draining undisturbed catchments are known to represent a reference point by which human-influenced catchments can be compared. In this paper, temporal dynamics of nitrate-nitrogen (NO₃-N) flux are investigated on an event basis by analysing observed lag times between data series. More specifically, we studied lag times between the centres of mass of six hydrological and biogeochemical variables, namely discharge, soil moisture at three depths, NO₃-N flux, and the precipitation hyetograph centre of mass. Data obtained by high-frequency measurements (20 min time step) from 29 events were analysed. Linear regression and multiple linear regression (MLR) were used to identify relationships between lag times of the above-mentioned processes. We found that discharge lag time (LAG_Q) and NO₃-N flux lag time (LAG_N) are highly correlated indicating similar temporal response to rainfall. Moreover, relatively high correlation between LAG_N and soil moisture lag times was also detected. The MLR model showed that the most descriptive variable for both LAG_N and LAG_Q is amount of precipitation. For LAG_N, the change of the soil moisture in the upper two layers was also significant, suggesting that the lag times indicate the primarily role of the forest soils as the main source of the NO₃-N flux, whereas the precipitation amount and the runoff formation through the forest soils are the main controlling mechanisms.