Claire S. Goulsbra and Martin G. Evans

1. UPLAND ENVIRONMENTS RESEARCH UNIT Evidence for the occurrence of infiltration excess overland flow in an eroded peatland catchment: implications for connectivity Claire S. Goulsbra andMartin G. Evans Upland Environments Research Unit, School of Environment and Development, University of Manchester, UK

2. Introduction Overland flow and Connectivity • OLF generation is a crucial processes in catchment hydrology • Different flow pathways and flow processes influence the magnitude and timing of the delivery of water, sediment and solutes. • Need to understand the spatial and temporal distribution of different flow processes so models can be adequately developed. OLF in peatlands • OLF is the most important runoff pathway in peatlands • OLF in peatlands is produced almost exclusively by saturation excess overland flow as opposed to infiltration excess overland flow. • high stream flows always occur at times of high water table • fluctuations in water table are swift with recoveries occurring more rapidly than recessions.

3. Holden and Burt (2003) WWR North Pennines (in tact) OLF occurs most frequently on footslopes from return flow and least frequently on steep mid-slopes Daniels et al. (2008) JoH South Pennines (eroded) Water table drawdown at the gully edge, esp within 2 m (‘erosional acrotelm’ effect) Models of peatland OLF generation What are the key controls on OLF generation in peatlands in space and time?

4. No flow – low conductivity Flow – high conductivity Electrodes Electrodes ER sensor ER sensor Monitoring OLF using ER sensors • Traditionally overland flow production in peatlands has been examined by the use of crest-stage tubes. • limited temporal resolution of measurements • Temperature sensors can be converted to ER sensors • Inexpensive – high spatial density; user-selectable sampling intervals – high temporal density • Laboratory testing of the converted sensors revealed that they can consistently differentiate between the presence and absence of water > Binary flow/no-flow distinction(Blasch et al., 2002 Vadose Zone Journal)

5. Electrodes are housed in electrical conduit with a lid. Drainage holes and a small gap at the bottom of the lid allow the entry of surface flow. Minimise chances of false positives. Installed at the ground surface. 40 mm 16 mm Sensor ‘lid’ with plastic at either end to prevent entry of rain/sediment Small gap to allow surface flow to enter sensor Holes in bottom of sensor for free drainage Ø 3mm 40 mm Sensor electrodes ~3 mm long Electrical conduit Holes through which nails are driven to secure sensor to the ground Ø 6mm Insulated wire connecting electrodes to data logger Sensor base-plate OLF sensor design

6. Upper North Grain research catchment • South Pennines, Peak District National Park, UK • 0.38 km2 • Elevation 480 – 540 m • 1,500mm rainfall • Blanket peat cover (ombrotrophic) • Heavily eroded (Bower type II gullies) > implications for carbon flux • Previous and on-going Monitoring • Heavily instrumented • Met station • Discharge • Dipwells • LiDAR data (gully maps, water table models) UNG Experimental Catchment

7. OLFsensor Dipwell D0.5 D1.5 D3 D8 0 1 2 Meters Data Collection In UNG catchment, the average distance to a gully is just 10.3 m; 13.7% of the intact peat mass lies within 2 m of a gully May to July 2008 • 43 sensors was located at the head of an erosional gully (2 m grid) September to November 2008 • 40 sensors was located at a gully edge site. Readings at 1 minute intervals > 36 days of continuous logging

8. OLF at the gully head • The sites which experience OLF the most regularly are those at the eastern side of the plot, the furthest away from the gully. • Positive relationship between distance from the gully edge and % flow (not found to be statistically significant). • At sites within 2 m of the gully flow was recorded an average 5.2% of the time (n=19) compared with 11.2% at sites which are 2 m or more away from the gully (n=24) (not statistically significant). • No flow at one site out of 43 • OLF <1% of the study period at 9 sites • Max 34.1% • Average 8.6%

9. 0 0.5 1 2 Meters OLF at the gully edge • OLF is produced more frequently at sites closer to the gully edge than those further away. • Weak negative relationship between distance from the gully edge and overland flow generation at each site (not statistically significant). • At sites within 2 m of the gully flow was recorded an average 8.7% of the time (n=20) compared with 2.3% at sites which are 2 m or more away from the gully (n=20) (statistically significant at the 0.1 level). • No flow at one 15 fifteen out of 40 sites • OLF <1% of the study period at 7 sites • Max 28.0% • Average 5.5%

10. A A B B C C D D E F E F G H C D B E A D C F E G H B F A Temporal pattern of OLF • Prolongation of OLF after rainfall • OLF ceases after rainfall • Low WT at the gully edge!

11. IEOLF Limited OLF SEOLF Acrotelm Erosional acrotelm Hydrophobic ‘crust’ Erosional acrotelm Catotelm Differences in OLF • Different patterns of OLF generation at the two sites • Gully head site shows aspects of Holden and Burt and Daniels et al. models of OLF generation • Gully side shows the opposite pattern • WT never at the surface at gully edge • Enhanced erosional acrotelm?

12. OLF following drought • Four day period from 27 to 31 May 2008. • Low water table is low following a prolonged period with little rainfall. • Rainfall produces a response in 28% of the OF sensors. • The water table rises once overland flow has subsided. • No discharge is produced at the catchment outlet • Rainfall on the morning of 28 May produces a larger overland flow response • water table levels are much closer to the surface, • A discharge response is produced 1 2 3 • At 17:00 on 28 May, high intensity rain leads to a sharp increase in both discharge and overland flow.

13. Implications • Importance of water table variation in time and space • Climate change > summer conditions in the UK may become warmer and ‘stormier’ • More frequent water table drawdowns may lead to an increase in hydrophobic conditions in time and space. • Shift in the dominant OLF process from saturation excess to infiltration excess as expanses of the peat surface become hydrophobic. • This has implications for floodwater delivery • IEOLF can be produced rapidly following rainfall • IEOLF will result in a lower proportion of incident rainfall will enter the peat mass, resulting in higher runoff totals

14. Summary • ER sensors are a viable alternative to crest stage tubes for monitoring OLF generation. • OLF is widespread at both the gully head and the gully side • Water table variation in time and space is key in controlling connectivity. • Both saturation and infiltration excess overland flow are observed in this study. • IEOLF occurs at the dry gully edge site - enhanced erosional acrotelm effect • IEOLF is also observed at the ‘wetter’ gully head site following drought. • IEOLF may become more widespread under future climate change scenarios • This has implications for the timing and magnitude of floodwater delivery • The apparent importance of infiltration excess overland flow has hitherto not been widely acknowledged and as such this represents a major advancement in our current knowledge of the dominance of various runoff mechanisms in peatlands.

15. UPLAND ENVIRONMENTS RESEARCH UNIT Thank you