How does afforestation affect the hydrology of a blanket peatland? A modelling study

Over the last century, afforestation in Ireland has increased from 1% of the land area to 10%, with most plantations on upland drained blanket peatlands. This land use change is considered to have altered the hydrological response and water balance of upland catchments with implications for water resources. Due to the difficulty of observing these long-term changes in the field, the aim of this study was to: utilise a hydrological model to simulate the rainfallrunoff processes of an existing pristine blanket peatland; and then to simulate the hydrology of the peatland if it were drained and afforested. The hydrological rainfall-runoff model (GEOtop) was calibrated and validated for an existing small (76 ha) pristine blanket peatland in the southwest of Ireland for the two year period 2007-2008. The meteorological data required to drive the hydrological model (precipitation, atmospheric pressure, temperature, global shortwave radiation, relative humidity and, wind speed and direction) were available from an onsite meteorological tower. As well as metrological inputs, the model also required data on the spatial variability of the hydrological properties of the soil in the catchment. For mineral soils, there is a wealth of information on soil hydrological properties and their spatial variability. However, the same cannot be said for peatlands as there is limited knowledge of both hydrological properties and of elementary properties (such as bulk density) and particularly spatial variability. Therefore in order to provide the model with the necessary soil hydrological properties, a field investigation of saturated hydraulic conductivity and bulk density was undertaken. The results of this study (presented elsewhere) found that saturated hydraulic conductivity ( Ksat ) was higher (~10 -5 m s -1 ) in the bog interior than the riparian zone (~10 -6 m s -1 ) while the converse applied to bulk density. These results support the idea that the lower Ksat at the margins control the hydrology and stability of blanket peatlands and provided the necessary inputs for model. Vegetation details (Leaf Area Index (LAI), height, root depth) were adopted from previous studies. The current hydrological response of the pristine blanket peatland catchment with regard to streamflow and water table (WT) levels was captured well in the simulations. Two land-use change scenarios of afforestation were also examined; (A) a young 10-year-old and (B) a semi mature 15-year-old Sitka Spruce forest. Scenario A produced similar streamflow dynamics to