30

20

10

0

-10

Understanding DOC Mobilization Dynamics

through High Frequency Measurements in a Headwater Catchment Benedikt Werner , Andreas Musolff, Oliver Lechtenfeld, Gerrit de Rooij, Marieke Oosterwoud, Toralf Keller & Jan Fleckenstein Contact: benedikt.werner@ufz.de Helmholtz Centre for Environmental Research - UFZ, Leipzig

T h e R a p p b o d e F i e l d S i t e

Harz Mountains (540 m a.s.l.)

Temperate climate

Mean temperature: 6.9 °C

Annual precipitation: 1200 mm

Drainage area: 2.58 km²

Forest: 77 %

Grasslands: 11.2 %

Groundwater-influenced soils: 25 %

Drainage density: 2.91 km/km²

Processing of in situ UV-vis Absorption Spectra (220 nm - 720 nm)

DOC concentration (from PLS Regression)

Spectral Slope (275 nm - 295 nm)

Specific UV Absorption: SUVA254 =

Modeling DOC concentration, SUVA and Spectral Slope

Explaining time series variance

Linear Regression model selection and validation

Akaike‘s Information Criterion

Variance Inflation Factor

Nash Sutcliffe Eficiency (NSE)

Event-based patterns in DOC mobilization

Seasonal trends for the Slope, Intercept and R² of the Regression lines, Hysteresis Index and Spectral Slope (not shown) with maxima in winter and summer

Best explanation by 15 days mean temperature before event (r = > 0.52)

Indicate shift in mobilization mechanisms over the seasons

DOC quality variation in the stream

Linear Regression Modeling

C o n c l u s i o n s The variations in DOC concentration and

quality can be explained by four

variables: discharge (Q), temperature

(Temp) antecedent wetness (AW), and

the antecedent discharge-temperature

ratio ()

These variables can be linked to event-

based DOC source activation

(discharge) and more seasonal controls

(AW, ) of DOC production

O u t l o o k

Link stream-DOC concentration and composition with riparian groundwater flow and discharge generation

Fingerprint DOC compounds in the stream and riparian zone

Derive a mechanistic model of catchment-scale DOC export which includes the better understanding of DOC mobilization and transport

I n t r o d u c t i o n

Increasing DOC exports from catchments impact the ecology and quality of downstream waters

Riparian zones are the most dominant source zones for in-stream DOC in a temperate climate

High-flow events (snowmelt, heavy rain) drive DOC deliverance to streams

Hydrological and biogeochemical controls of mobilization and transport in riparian zones are complex and still elusive

High frequency, in situ sensing techniques provide new insights into dominant mobilization mechanisms and source zones of DOC

SAC254nm

DOC conc.

DO

C c

on

cen

tra

tio

n

Stream Discharge

Grab sampling

DO

C c

on

cen

tra

tio

n

Stream Discharge

High Frequency

Leipzig

Field Site

Berlin

G e

r m

a n

y

August 2013 January 2014 August 2014

August 2013 January 2014 August 2014

August 2013 January 2014 August 2014

Precipitation [mm/d]

1.5 Year High-Frequency Data Set (15 min)

Discharge derived from water level*

In-situ DOC concentration derived from UV-vis spectra

Weather Data

0.4

0.2

0

15

10

5

0

Air

tem

pera

ture

[C°]

0

40

80

[m³

s-1]

[mg

L-1]

0.75

0.5

0.25

0

1

0.9

0.8

0.7

5

4

3

2

0.4

0.2

0

-0.2

-0.4

Oct 13 Jan 14 Apr 14 Jul 14 Oct 14

Oct 13 Jan 14 Apr 14 Jul 14 Oct 14 Oct 13 Jan 14 Apr 14 Jul 14 Oct 14

Oct 13 Jan 14 Apr 14 Jul 14 Oct 14

Spectral Slope vs. Log Discharge SUVA vs. Log Discharge Comparison of the (scaled) DOC concentration -,

SUVA - and Spectral Slope Models

.0

.3

.6

Q

Q:AW

AW Q:

*calculated after Lloyd et al. 2016

O b j e c t i v e s

i) Investigate the hydrologic and biogeochemical controls for the mobilization and transport of DOC from riparian soils to the streams

ii) Evaluate how concentration and composition of DOC change during events and varying seasonal conditions

iii) Extract the most decisive factors which are needed to explain the variance of DOC concentration and composition in streams

5

4

3

2

1

-1

-2

-3

-6 -4 -2 Log(discharge)

-6 -4 -2 Log(discharge)

SUV

A [

L m

g-C

−1 m

−1 ]

Spe

ctra

l Slo

pe

DOC

wet

DOC

DOC

dry

All models use the same parameters

NSE of DOC: 0.62, SUVA: 0.52, Spectral Slope: 0.37

Different parameter loadings allow for mechanistic interpretations

* Each point indicates one discharge event. Slope, Intercept and R² were calculated from linear regressions of log DOC

vs. log discharge. Hysteresis analysis was conducted in non-log space.

DOC

SUVA

Spectral Slope

Slope of the Regression Lines*

Intercept of the Regression Lines*

R² *

15 days mean

temperature

before event

Hysteresis Indices** of Events

Contact: benedikt.werner@ufz.de

*Note: Discharge peaks cut above 0.38 m³ s-1 based on maximum observed discharge.

**HI calculated after Lloyd et al. 2016

In-stream DOC quality varies with discharge and antecedent temperature

Systematical change of DOC quality with increasing discharge:

Change in DOC-compound size distribution and increasing aromaticity

Less variability: Convergence of SUVA values and Spectral Slope

Indicates a shift in the activated source zones with rising discharge

Change of size distribution

Incr

easing aro

maticit

y