Monitoring the ecohydrological impact of restoring degraded
peatland within Dartmoor National Park
Pre-restoration monitoring update
May 2014
Summary
In order to improve understanding of the impact of restoration on the ecohydrological function of peatlands, preand post-restoration monitoring is being carried out on Dartmoor by a team based at the University of Exeter. The
research informs three areas of work relating to water quantity, water quality and gaseous fluxes. The monitoring
site is located on Flat Tor Pan, where hydrological monitoring commenced in spring 2012. Pre-restoration data are
currently being collected and work on the analysis of these data is in process.
Over 100 rainfall/runoff events between 2nd June 2012 and 31st March 2014 were captured by the meteorological
monitoring, groundwater level sensors and monitoring within the catchment outlet channel. Across this period the
rainfall recorded on Flat Tor Pan totalled 4127 mm and the flow depth in the flume ranged from 0 cm to 22.3 cm. A
total of 506 water samples were collected and analysed from the outlet channel across 16 rainfall/runoff events
using flow-based and time-based sampling. Initial analysis of these samples shows dissolved organic carbon
concentrations ranged between 2.6 mgl-1 and 41.6 mgl-1 at the upper site within the catchment, and 5.4 mgl-1and
29.9 mgl-1 at the catchment outlet. Monitoring of the gaseous flux of CO2 at Flat Tor Pan in 2013 shows
photosynthesis, ecosystem respiration and total, heterotrophic and autotrophic below-ground respiration all varied
seasonally with photosynthetically absorbed radiation, a proxy for vegetation phenology, soil temperature and
water level. Respiration of stored carbon increased during warm and dry weather, suggesting that drier bogs will
lose more carbon to the atmosphere than wetter bogs.
The site is due to be restored in August 2014, after which monitoring will continue to provide comparisons with the
data summarised herein. Results reported are preliminary and should be treated as such until the final project
report and science papers are published.
i
Contents
1
Introduction .......................................................................................................................................... 1
2
Water quantity ...................................................................................................................................... 1
2.1
Experimental design & methods ..................................................................................................................... 1
2.2
Monitoring update .......................................................................................................................................... 1
3
Water quality ........................................................................................................................................ 1
3.1
Experimental design & methods ..................................................................................................................... 3
3.2
Sampling and analysis update ......................................................................................................................... 3
3.3
Comparison between sites in the South West ................................................................................................ 5
3.4
Summary ......................................................................................................................................................... 5
4
Gaseous flux.......................................................................................................................................... 5
4.1
Experimental design & methods ..................................................................................................................... 6
4.2
Monitoring update .......................................................................................................................................... 6
4.2.1
4.2.2
Photosynthesis and respiration ...................................................................................................................... 6
Below-ground respiration ............................................................................................................................... 7
4.3
Summary ......................................................................................................................................................... 7
Table 1.
Table 2.
Seasonal rainfall and the number of rainfall/runoff events . .......................................................................... 1
Collection dates and number of samples for the each Dartmoor sites. ......................................................... 3
Figure 1. Rainfall at Flat Tor Pan during the monitoring period compared to monthly mean rainfall at
Princetown, Dartmoor – demonstrating the dry summer and wet winter. .................................................... 2
Figure 2. Rainfall, stage and ground water level for the period July 2013 – October 2013........................................... 2
Figure 3. Sample times with stage for monitored hydrological events at the flume. .................................................... 4
ii
1
Introduction
The Dartmoor Mires Project is a peatland restoration initiative which aims to reinstate peat-forming mires across
upland catchments through the use of blocks across erosion gullies. In order to improve understanding of the
impact of restoration on the ecohydrological function of these peatlands, extensive pre- and post-restoration
monitoring is being carried out. Hydrological monitoring is being undertaken by the University of Exeter under the
lead of Professor Richard Brazier and in collaboration with the Dartmoor Mires Project (DNPA), the Environment
Agency, South West Water, and a wide group of stakeholders.
Monitoring at Flat Tor Pan commenced in April 2012 and pre-restoration data is currently being collected. Work on
the analysis of these data is in process. This report provides a summary of the monitoring divided into three study
areas:
•
Water quantity
•
•
Water quality
Gaseous fluxes
In addition to these three areas of work meteorological data is being collected, including: air temperature, wind
direction, relative humidity, soil temperature, precipitation, incoming solar radiation, wind speed and barometric
pressure.
2
2.1
Water quantity
Experimental design & methods
Within the monitoring site on Flat Tor Pan there are 68 sensors that record water level data at a high temporal
resolution (15 minute time-step), and feed it back to the University of Exeter via a remote telemetry system.
Sensors are arranged in a large nested geo-statistical grid and a smaller high density array transecting the main
channel. The depth of water table sensors below ground ranges between 90 cm and 200 cm below ground level;
five level sensors are also situated in stilling wells at ground level. Catchment discharge through the main gullybased channel is being monitored via a flume-mounted ISCO 2150 Flow Meter - recording level and velocity.
2.2
Monitoring update
Monitoring started during the summer of 2012, and covers the 2012/13 hydrological year and the winter 2013/14.
Initial event separation processing of the data indicates there have been over 100 rainfall/runoff events during the
period 2nd June 2012 to 31st March 2014.
Table 1. Seasonal rainfall and the number of rainfall/runoff events (between 2 June 2012 and 31 March 2014).
Hydrological season
Flat Tor Pan total rainfall * (mm)
N° of rainfall-runoff events
Summer 2012 (Jun-Sept)**
741
20
Winter 2012/13 (Oct-Mar)
1266
33
Summer 2013 (Apr-Sept)
Winter 2013/14 (Oct-Mar)
554
9
1566
>45 (TBC)
*Recorded rainfall at Flat Tor Pan for period monitored
**Monitoring period covers only part of the season
Monitoring the ecohydrological impact of restoring degraded peatland within Dartmoor National Park
Pre-restoration monitoring update
May 2014
1
450
Total monthly rainfall (mm)
400
350
300
250
200
150
100
50
Monthly mean (1971 to 2000) rainfall for Princetown
Mar-2014
Feb-2014
Jan-2014
Dec-2013
Nov-2013
Oct-2013
Sept-2013
Aug-2013
Jul-2013
Jun-2013
May-2013
Apr-2013
Mar-2013
Feb-2013
Jan-2013
Dec-2012
Nov-2012
Oct-2012
*Sep-2012
Aug-2012
Jul-2012
*Jun-2012
0
Rainfall recorded at Flat Tor Pan
Figure 1. Rainfall at Flat Tor Pan during the monitoring period compared to monthly mean rainfall at Princetown, Dartmoor – demonstrating the
wet summer of 2012, dry summer of 2013 and wet winter of 2013/14 . Months with incomplete rainfall data from monitoring are marked (*).
Figure 2. Rainfall, stage and ground water level (sensor R1.2) at the flume for the period July 2013 – October 2013.
Across this period the rainfall recorded totals 4127 mm and the flow depth in the flume ranges from 0 cm to
22.3 cm. Initial analysis of data up to the end of January 2014 indicates that during the monitoring period, 40% of all
discharge from the catchment and 65% of all rainfall occurred during rainfall-runoff events.
Further exploration of these data, alongside peak event rainfall and discharge, illustrate that for the 2012
hydrological year there is a seasonal difference in total event rainfall, total event discharge and average event
duration; with greater variation in the observed discharge during runoff events in the winter months. Data collected
during the first part of winter 2013/14 suggest that there is less variation than winter 2012/13, although data
outliers indicate that with further data collection a similar pattern may emerge.
Monitoring of the rainfall/runoff relationship within the catchment provides an important benchmark of flow
production, which we hypothesise, will shift following restoration interventions. The variation in the relationship
between rainfall and runoff across the monitoring period to date, demonstrates the effect of seasonal and interannual changes in flow production prior to restoration.
Monitoring the ecohydrological impact of restoring degraded peatland within Dartmoor National Park
Pre-restoration monitoring update
May 2014
2
3
Water quality
3.1
Experimental design & methods
Sampling for water quality was conducted on a runoff event basis using ISCO 3700 pump-samplers. Samplers were
either set to trigger at regular time intervals during the event, or were set to trigger based on the changing flow
within the channel. Water samples were analysed in the laboratory for DOC, pH, colour, and humic to fulvic acid
ratio (E4/E6) using a mixture of wet-chemistry and UV/Vis spectroscopy techniques. In addition to laboratory analysis
an in-stream sensor measures pH, temperature and conductivity in 15 minute intervals.
3.2
Sampling and analysis update
To date, between 11 and 16 events were analysed at the Flume (DF) and Upper (DU) sites respectively, as shown in
Table 2. An additional two events occurred where the samplers were triggered but no samples were collected due
low flow during events (the water level remained below the level of the trigger for the sampler). Fewer events were
captured at the flume either due to low flow during events, or due to technical problems with the pump sampler.
Table 2. Collection dates and number of samples for the each Dartmoor sites.
Collection date
Overall event #
DU
Samples
analysed
DF
Samples
analysed
Type of sampling
18/03/2013
1
24
24
Time based
25/03/2013
2
24
24
Time based
12/04/2013
3
24
24
Time based
19/04/2013
4
24
24
Time based
20/05/2013
5
24
24
Time based
16/06/2013
6
15
\
Flow based
09/08/2013
7
16
\
Flow based
29/08/2013
8
24
\
Flow based
25/09/2013
9
24
\
Flow based
08/10/2013
10
24
24
Time based
05/11/2013
11
12
16
Flow based
17/12/2013
12
3
3
Flow based
29/01/2014
13
18
14
Flow based
07/02/2014
14
5
13
Flow based
09/05/2014
15
7
\
Flow based
14/05/2014
16
24
24
Time based
Summary
DU
DF
Total
Total number of samples analysed
292
214
506
Total number of events sampled
16
11
11 to 16
2014
2013
Year
Monitoring the ecohydrological impact of restoring degraded peatland within Dartmoor National Park
Pre-restoration monitoring update
May 2014
3
Figure 3. Sample times with stage for monitored hydrological events at the flume (DF).
Monitoring the ecohydrological impact of restoring degraded peatland within Dartmoor National Park
Pre-restoration monitoring update
May 2014
4
Figure 3 illustrates the water quality sampling times in relation to the stage at the outlet for the events sampled at
DF. Overall, the comparison between the two experimental pools shows a significant difference for both DOC and
colour concentrations (Mann Whitney, P<0.05), with higher values being observed for the DU site. DOC ranged
between 2.6 mgl-1 and 41.6 mgl-1 at DU, and 5.4 mgl-1 and 29.9 mgl-1 at DF. Seasonal variations show a rise in the
DOC losses and colour during the hotter, summer months (July to September) for DU. This is due to higher
decomposition rates and DOC production during dry times, and subsequent increased losses during rain events. The
humic/fulvic acid ratios (E4/E6) present a similar seasonal pattern.
3.3
Comparison between sites in the South West
Comparison between Exmoor and Dartmoor shows generally higher DOC concentrations on the Dartmoor sites.
Differences between sites is statistically significant (Kruskall Wallis, P<0.05). Colour results are generally in the same
range, but get episodically higher on Exmoor (Spooners). E4/E6 ratios remain in the same order of magnitude for all
sites in the SW, with a predominance of humic acid being lost for all sites and throughout the year.
3.4
Summary
Results so far show higher concentrations at DU compared to DF. Local environmental conditions, contributing areas
and dryness of the site are generally used could explain such differences. However, differences between sites could
be an effect of the higher number of samples collected for one site during the hotter period of the year (summer
months), when concentrations tend to be higher. The continuation of the sampling should correct this problem.
Overall, DOC concentrations on Dartmoor tend to be more variable, and can be higher than on Exmoor. The
calculation of DOC loads in the near future, will give us a better understanding of C losses on these degraded
peatlands, and allow a better comparison with other sites in the SW, but also with measurements further north in
the UK.
Humic to Fulvic ratios on Dartmoor are similar to what has been measured on Exmoor, and also to what has
generally been measured in Wales immediately after restoration (e.g. Wilson et al., 2011). They are, however,
significantly lower than measurements in drained peatlands elsewhere in the north of England. The seasonal pattern
E4/E6 also indicates that the DOC lost at drier times of year might contain slightly more fulvic acid, which is less
degraded, compared to winter times. However, these variations remain very subtle throughout and never reach
levels usually assigned to DOC with a dominance of fulvic acid (i.e. 8 to 10).
Monitoring the ecohydrological impact of restoring degraded peatland within Dartmoor National Park
Pre-restoration monitoring update
May 2014
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4
4.1
Gaseous flux
Experimental design & methods
Three pairs of locations were set up in spring 2013 within the enclosure. One of each pair will be fully restored the
other will be “partially restored” and act as a control to enable the effect of restoration to be separated from
climatic effects.
At each site light response curves were collected approximately monthly from the vegetated areas and fortnightly
from a floating collar in a nearby ephemeral pool. This will enable the estimation of ecosystem respiration and
photosynthesis under any light levels.
Additionally 4 collars were installed at each site to measure below-ground respiration. All collars had above-ground
vegetation removed by regular clipping so they measured below-ground fluxes only. In addition half the collars had
a 56 cm diameter circle trenched to 20 cm depth around the collar and the area enclosed cleared of vegetation by
regular clipping, this excluded both root and above-ground respiration allowing the measurement of the belowground heterotrophic (soil respiration) component. The collars with only above-ground vegetation removed were
used to measure total below-ground respiration. Autotrophic (root) respiration was assumed to be the difference
between total and heterotrophic respiration.
Water level and soil temperature at 5 cm to 30 cm were measure concomitantly with flux measurements. Above
ground net primary productivity and vegetation composition were estimated in August 2013.
4.2
Monitoring update
4.2.1
Photosynthesis and respiration
A total of 30 light response curves were collected between 5/6/2013 and 20/9/2013 from the vegetated areas.
Both ecosystem respiration and photosynthesis showed strong seasonal variation. Photosynthesis did not show
any significant relationships with water level or soil temperature. Ecosystem respiration showed the strongest
relationship with MODIS FPAR although it also showed a weak but significant increase with increasing temperature.
Photosynthesis was modelled using MODIS FPAR data and solar radiation data from the on-site weather station with
gaps filled by correlation with North Wyke Met Station (UK Meteorological Office 2012 1). These two variables
explained 60 % of the variability observed.
Ecosystem respiration was modelled using FPAR and soil temperature at 5 cm depth. These variables explained 58%
of the observed variability. Including water level increased the proportion of variability explained to 64% and
indicated maximum ecosystem respiration occurred when the water level was around 8.5 cm below the ground
surface. More work is required to add a water level term into the model.
Estimated net ecosystem exchange indicated the site was a sink of CO2 from June-September. The photosynthesis
model indicates photosynthesis continues all year, further sampling is required to test this before an annual balance
can be attempted.
1
UK Meteorological Office. Met Office Integrated Data Archive System (MIDAS) Land and Marine Surface Stations Data (1853-current), [Internet].NCAS British
Atmospheric Data Centre, 2012, 28/01/2014. Available from http://badc.nerc.ac.uk/view/badc.nerc.ac.uk__ATOM__dataent_ukmo-midas
Monitoring the ecohydrological impact of restoring degraded peatland within Dartmoor National Park
Pre-restoration monitoring update
May 2014
6
It should be noted that these models are based on very limited data (n=30) across 6 locations within the vegetated
blocks. The methods used are designed to show the effect of restoration not estimate CO2 budgets. A large
proportion of variability remains unexplained and the uncertainty is high. It is anticipated the bare peat pans will
behave differently. A total of 48 light response curves were collected from 6 floating collars located in the bare peat
pans between 29/05/2013 and 07/102013. It is hoped these will enable a fuller understanding of the gaseous fluxes
from this heterogeneous landscape.
4.2.2
Below-ground respiration
Total, heterotrophic and autotrophic respiration all showed a similar seasonal pattern rising from a minimum in
early May to a maximum in late-July, remaining high through July and August then decreasing to a minimum by late
October. Heterotrophic respiration was greater than autotrophic respiration except in early October.
Total, heterotrophic and autotrophic respiration all showed a strong quadratic relationship with both soil
temperature at 5 cm and water level. Heterotrophic respiration shows a stronger relationship than autotrophic
respiration indicating respiration of carbon stored in the soil is potentially more sensitive to temperature and water
level than respiration from roots.
Soil temperature at 5 cm and water level strongly co-varied with warm dry conditions increasing soil temperature
and reducing water levels. More work is required to separate the effects of temperature and water level on belowground respiration rates.
4.3
Summary
•
Photosynthesis, ecosystem respiration and total, heterotrophic and autotrophic below-ground respiration all
varied seasonally.
•
Photosynthesis exhibited strong relationship with photosynthetically absorbed radiation (PAR) and a proxy
for vegetation phenology (MODIS FPAR).
•
Ecosystem respiration exhibited strong relationship with soil temperature, water level and a proxy for
vegetation phenology (MODIS FPAR).
•
Modelled photosynthesis (r2=0.64) and ecosystem respiration (r2=0.58) indicated the vegetated blocks were
CO2 sinks from June to September although the uncertainty in these simplistic models was high.
•
Work is ongoing to understand CO2 fluxes from the bare peat pools.
•
Respiration of stored carbon (heterotrophic respiration) increased during warm and dry weather.
Monitoring the ecohydrological impact of restoring degraded peatland within Dartmoor National Park
Pre-restoration monitoring update
May 2014
7