The view from a small corner of Anglesey...
|Contact||Harvest Your Rain||Home|
The relative effects of a Pinus plantation on the hydrology of an Atlantic dune system. (Newborough Warren case study)
Martin Hollingham (2008)
Prior to 1947 the sand dune system of Newborough, Anglesey, Wales, UK was a mixture of mobile and fixed dunes, wet and dry slacks. In 1947 Forestry Commission (FC) planted Newborough Forest , with the aim of stabilizing the sand dunes. The remaining unforested warren became a national nature reserve (NNR) and a site of special scientific interest (SSSI), and is now managed by the Countryside Council for Wales (CCW).
During the late seventies and eighties there were concerns that the water levels across the warren were falling due to increased forest evapotranspiration, with the result that winter flooding was not to the same depth and that dune slack pools were drying out sooner, especially within the forest. It was feared that the drop in water levels would soon affect some plant species in the warren. To investigate this a series of piezometers were placed in the forest and warren in 1989 and monitored until 1996. This information was used to create a groundwater flow model (Betson et al, 2002). In 2004, CCW commissioned ADAS to investigate evaporation and interception losses for broad vegetation classes within the forest and warren (Betson and Scholefield, 2004). The conclusions of this research were that the forest was having an adverse affect on the water table of the warren, but that further research was needed to quantify it. Recommendations were to remove parts of the Forest , especially upon the rock ridge.
In late 2004 CCW put forward proposals for the removal of large parts of the forest adjacent to the warren. The local community objected to the proposals and a consultation committee was formed and recommended a review of the research so far. The main findings of the review were that research was deficient and the magnitude of the forests impact on the water table could not be properly assessed (Stratford et al., 2006). During the consultation in March 2005, water level monitoring recommenced, and from May 2006 a new series of 20 piezometers were installed, the last in October 2008.
The analysis assumes that the sand aquifer is identical under the Forest, Border and Warren , the only difference is the vegetation above. The Warren vegetation is the control. Rainfall is added periodically, some water is evaporated from the surface, and some water drains away slowly through the aquifer. When drainage and evapotranspiration are greater than rainfall, the water levels fall and so does the amount of water stored in the aquifer. When rainfall is greater, water levels rise as does the amount of water stored. If rainfall, evapotranspiration and storage are known, the drainage can be calculated from the water balance equation, and the difference is the net effect of the vegetation.
The water balance equation is
Q = P - Et - S
P = rainfall
Et = evapotranspiration
Q = water leaving the catchment which also incorporates net groundwater flow,
S = change in water Storage in the aquifer
The amount of rainfall less the evapotranspiration is know as Effective Precipitation (EP)
EP = P- Et.
And substituting EP for P-Et the water balance equation:
Q = EP- S
Water level records from piezometers located within the topological catchment affected by the Forest are analysed and are used to estimate storage. The storage estimates for three groupings Forest , Border and Warren are compared to the effective precipitation to estimate drainage and storage for each grouping.
Copyright © Martin Hollingham