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

Klaudija Sapač,Andrej Vidmar,Simon Rusjan,Nejc Bezak

doi:10.3390/w12041092

Abstract

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 (LAGQ) and NO3-N flux lag time (LAGN) are highly correlated indicating similar temporal response to rainfall. Moreover, relatively high correlation between LAGN and soil moisture lag times was also detected. The MLR model showed that the most descriptive variable for both LAGN and LAGQ is amount of precipitation. For LAGN, 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.

Highlights

Human-induced land use changes and climate change have raised a number of questions related to their impacts on runoff generation, soil erosion, and nutrient cycle [1,2]
We argue that the lag times could indicate primarily the role of the forest soils as the main source of the NO3 -N flux, whereas the runoff formation through the forest soils could be the main controlling mechanism
The lag times were used to analyse the temporal dynamics of the rainfall runoff formation and the NO3 -N flux

Summary

Introduction

Human-induced land use changes and climate change have raised a number of questions related to their impacts on runoff generation, soil erosion, and nutrient cycle [1,2]. Nutrients, especially nitrogen, due to its essential role for all living things, and it’s most soluble and mobile form, nitrate (NO3 ), have been the subject of many hydrological and biogeochemical studies [3,4,5]. Rainfall events impact on the changes in the amount of nitrate-nitrogen (NO3 -N) in the stream water in two ways [6]. In headwater catchments, which respond relatively quickly to rainfall events (precipitation–discharge lag time ranges from hours to a few minutes in case of micro catchments), high-frequency measurements are needed to capture these changes [7,8,9], and to describe and quantify hydrological and biogeochemical processes [10].

Objectives

Methods

Results

Conclusion