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The relative effects of a Pinus plantation on the hydrology of an Atlantic dune system. (Newborough Warren case study)

Martin Hollingham (2008)


Water levels have risen at Newborough between 1995-1996 and 2007-2008 on average 0.7m in the forest and 0.4m in the warren. The lower water levels in 1989-1996 compared to 2007-2008 are a result of the below average rainfall.

The difference in storage and hence the hydrological balance between the Forest, Border and Warren is only slight, accounting for 1% of P and is not significant. This contrasts with the expected difference of 10% of P more for the Forest , caused by differences in actual evapotranspiration (Freeman, 2008). During the years analysed, average annual storage fell by 2-3% of P , while drainage (29% of P) was greater than effective precipitation (26.5% of P).

Drainage increases with effective precipitation, which is to be expected, but storage falls even in years with high rainfall. This highlights the effects of the timing and intensity of episodic rainfall compared to the slow constant continual losses of actual evapotranspiration and drainage. Water levels rise within minutes in response to rainfall and decline slowly at a constant rate afterwards. The largest change in storage, -83mm for the Warren in 1989-1990 is less than the average actual evapotranspiration (96mm) for May and slightly greater than the average monthly rainfall (70mm).

The response of Forest storage and drainage to effective precipitation appears to be damped and limited compared to the Warren . This can not be due to recharge from the rock ridge. If this were the case, the Forest storage should increase more than the Warren in response to above average rainfall, which does not occur. This is most likely due to:


•  Forest interception which has the effect of reducing the precipitation reaching the ground, but also reducing evaporative demand for soil moisture thus reducing the change in Forest storage.


•  extra evaporative stress caused by the Forest would cause a lowering of the water table and consequent lowering of the rate of drainage and hence reduce the fall in storage.


•  Hydrological errors caused by the assumption of similar aquifer properties. The specific yield from which the estimates of storage are derived could vary; and the depth of sand decreases towards the rock ridge under the Forest (Betson et al. 2002).

It appears that small additional evapotranspirational water losses from the forest are compensated for by a reduction in drainage from the Forest . Removal of the Forest will restore the hydrological balance of the felled areas to that of the Warren , but is unlikely to increase flooding within the Warren , as flooding is driven by the timing and intensity of rainfall.



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