Analysis of the hydrology of raised bogs in the Irish Midlands : a case study of Raheenmore Bog and Clara Bog

Abstract

In the framework of the Irish-Dutch Raised Bog Project the hydrology of two raised bogs in the Irish Midlands, Raheenmore Bog and Clara Bog, was studied.The work focuses on relationships in the bog system and how they are affected by drain-age and turf cutting along margins. The concept of diplotelmy, a differentiation of the bog body into a shallow highly permeable top layer called acrotelm and a subjacent deep poorly permeable peat layer called catotelm, is followed. Some measuring methods yielded wrong results in bogs, so were modified to produce acceptable results. A novel field method to measure the transmissivity of the acrotelm was developed.The acrotelm behaves as an aquifer with a constant hydraulic gradient -the surface slope-and a transmissivity that is controlled by the phreatic level. The transmissivity increases by an order of magnitude when the phreatic level rises less than 10 cm and decreases as the level falls. Discharge varies accordingly. This mechanism and the large storage coefficient of the acrotelm ensure small seasonal fluctuations of the phreatic level (20 cm or less). Thus the acrotelm has a regulating effect on the hydrological conditions in a raised bog. Acrotelm transmissivity and -depth depend on surface slope. Well-developed acrotelms occur almost exclusively at surface slopes below 1%.The catotelm acts as an aquitard Downward seepage from Clara bog amounts to 5-10 mm a-1. The seepage from Raheenmore Bog is 10-15 mm a-1, in spite of differences of up to 3 m between phreatic levels in the acrotelm and piezometric levels in the mineral sub-soil.Water loss by horizontal flow in the catotelm is 1 mm a-1or less in both bogs. Thus the hydrological system of the bogs depends little on its surroundings. Turf cutting and drainage of bog margins directly cause surface subsidence over dis-tances of only a few metres from the margin. This results in a local increase of the surface slope. A steeper surface slope causes a reduction of the regulating properties of the acrotelm and thus a spreading of subsidence into the bogs. Because of the difference in composition of the peat in the centre and along natural margins, subsidence in the centre caused by internal drainage may be larger than along margins, causing watershed positions to shift to the margin. A prominent example is the convergent flow on Clara Bog towards the soak system of Shanley's Lough. Both are the result of subsidence caused by the road that bisects Clara Bog. Drainage on the bog destroys the acrotelm within a few years. Natural recovery of the bog ecosystem from such damage may take more than a century. Evapotranspiration from both bogs was 0.9 to 1.2 times Penman open water evaporation, depending on pre-cipitation in spring and summer.</p