Peatland Management: Blanket peats and the upland hydrology of the UK

Hey guys, I thought in we'd take a look at the topic area of peatlands and their place in the landscapes of the UK into the future. I've had to limit the content, but even doing that its still been necessary to split the topic into two posts. In the first we’ll take a look at specifically Upland Blanket Bogs which constitute 87% of the UK peat resource (Holden 2009). I'll discuss their impact on the hydrology of drainage catchments and the potential impact of direct human alteration to them. In the second post we will then take a gander at the MASSIVE issue of climate change and the future of peatlands in the UK.

As can be seen from the distribution of relief, precipitation and peat soil in the UK (Figures 1, 2, 3), Peatlands (primarily upland blanket bogs) cover large areas of high relief which constitute significant sources of water to catchments due to their high amounts of precipitation. It is important to understand how peatlands impact the hydrology of upland regions and the rivers sourced from them, in order to assess the impact of anthropigenic activity on the hydrology of the bog and whole catchments.

Figure 1: UK relief map

Figure 3: Peat and Peaty soil area coverage

Figure 2: Rainfall amount annual average 1981-2010

 

                                            

Rivers whose sources are located in upland blanket peat have been shown to express “quickflow”, meaning that discharge rises and falls sharply with precipitation events. An interesting study conducted by Bullock and Acreman (2003) claims to have disproved the often used metaphor that blanket peat acts as a large “sponge” depressing the flashiness of discharge events, (at least that’s what my Geography teacher told me). Although peat does retain large volumes of water the storage capability for single precipitation events is low as the peat complex is already saturated. On average the water table is no more than 40cm from the surface and even throughout periods of drought water retention is high. Thus the storage capacity including surface pooling is low, leading to fast reaction times in the discharge of peatlands

Undisturbed blanket peat soils express primarily saturation overland flow and through-flow in response to rainfall events, which in turn are processes that cause rapid movement of water from the slope to the channel. A key cause of this is the characteristics of the peat that result in almost no lateral flow of water between pores below a few centimetres of the surface. In addition to these processes are soil pipes which form large networks under the peat surface and provide relatively high discharge rates of up to 14 l/s, with studies suggesting they can contribute 10-50% of streamflow (Holden 2009). So although Blanket Upland Bogs do provide some base flow component to catchment streams/rivers, as they are large stores within the catchment throughout the year, they do not provide significant flood relief.

An example of this that is close to my heart (as it flooded my local) was the 8^th-9^th June 2012 flooding of the Aberystwyth area. The region is typified by shallow peats with soil piping that have a low storage capacity for rainfall events. Within the River Leri Catchment; which is small and steep spanning from the Pumlumon massif within the Cambrian mountains to the sea, the short lag and peak lag times associated with the heavy rainfall event resulted in rapid flooding that caught people unawares in the Dol-y-Bont Caravan site and flooded The White Lion (Y Llew Gwyn) in Tal-Y-Bont!!!!!!!!

 An Aerial view of the flood extent within the River Leri catchment over Dol y Bont and Borth.

An important consideration with peatlands within the UK is that, almost all altered in some way by human activity generally through cutting, burning or drainage. In the case of drainage, observations of drained upland blanket peats have displayed both increases and decreases in peak flood discharge within different locations (Holden 2009). By lowering the water table through the installation of drainage ditches, there is an increase in the soil storage capacity for rainfall however, this can be counteracted by ditches ability to speed up the transfer of water from the peat to the stream/river channel.

Gilman (2002) in a study of Cors Llyn in Mid Wales identified ditch network design, slope and local vegetative cover as important variables in the rate and volume of discharge produced by a drained region. This topic is of particular importance in that rehabilitation of active peat surfaces as it is noted under the EU habitat Directive as a priority and thus across the UK drainage ditches are being blocked in order to re-establish the water table in disturbed sites. This policy, which has driven protection designations of SSSI’s and SAC’s to upland blanket bogs means that the emphasis remains on the ecology and not perhaps runoff and flood risk. However, will that remain so? It seems to me that it depends on the future of active peat surfaces as carbon sinks into the future.

So there we have stay tuned for the next blog where we'll get to grips with climate change and it's impications of Peatlands and the plocies that shape these landscapes.

Catch you later
Matt

References:

Bullock, A. and Acreman, M., 2003. The role of wetlands in the hydrological cycle. Hydrology and Earth System Sciences, 7, 358-389.

Gilman, K., 2002. A review of evapotranspiration rates from wetland and wetland catchment plant communities, with particular reference to Cors y Llyn NNR, Powys, Wales, CCW Science Report; 504, Countryside Council for Wales, Bangor

Holden, J., 2009. A grip-blocking overview.[pdf] Edale:Moors for the Future Partnership.

Available at: http://www.moorsforthefuture.org.uk/sites/default/files/Holden%20(2009)%20grip%20block%20review.pdf [Accessed 19 November 2012].