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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)
Annual rainfall and actual evapotranspiration totals ending on the end of August every year from 1961 to 2008 were calculated, and from these the annual effective precipitation calculated and plotted. Water level records for both monitoring periods were plotted and examined. To assess difference in the average height of the water table in both periods of record, only CCW piezometers which have been monitored since 1997 were examined.
Water level records from piezometers within or lying close to the same topological watershed as the forest were selected. For this reason piezometer 1F was excluded from both data sets and the majority of the CEH piezometers. The remaining piezometer records were split into 3 classes, Forest , Border and Warren, according to the position of the piezometer in relation to the surrounding vegetation at the time of monitoring. Piezometer 2B is the only piezometer which changes from one position to another. In 1989 it was surrounded by forest which was clear felled around the piezometer in 1995, and hence switches from a being Forest piezometer in 1989-1990 to a Border piezometer from 2006.
The 1989-1996 CCW records were examined. The records for the exceptionally dry year 1991-1992 were excluded because the water level in piezometers 1A, 1B, 2A, 2B, 2C and 2D were below the base of the piezometer most of that year. The whole record for piezometer 1B during 1989-1996 was excluded for the same reason.
The 2006-2008 data includes piezometers 1E, 2B, 2D, 2E, 2F. Piezometers 1D, 2A, 2C, and 2G have not been found and no records exist for them since 1996. Wells 1A-1C were excluded from the 2006-2008 data, as records only exist from July 2007. The records for the CEH wells which lie with the forest affected catchment and had complete record from 2006-2008 were included, as were the two additional piezometers NW2 and NW7. The division of the piezometers into each class is presented in table 2 below and the locations and groupings are shown in figure 2.
Table 2: Piezometer data used in the analysis
The water level movements of the piezometers were averaged according to grouping and the annual change in water level measured from the previous year on the hydrological year ending 31 st August. The annual averages were converted into estimates of annual change in storage assuming a specific yield of 37%, based on the porosity (Betson et. al. 2002). Estimates of Q where derived by subtracting the annual change in storage from the effective precipitation.
Copyright © Martin Hollingham