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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)

Results

The annual rainfall, actual evapotranspiration and effective precipitation from 1961 to 2008 based on the hydrological year starting 1 st Sept are presented in figure 3. The years of piezometer record are highlighted in green. The effective precipitation over the period 1989-1996 is below the average 200mm, the period 2006-2008 above average and cover 90% of the range of annual precipitation.

Figure 3: The annual rainfall, actual evapotranspiration and effective precipitation from 1961 to 2008 based on the hydrological year starting 1 st Sept.

The records for piezometers measured in both 1989-1996 and 2006-2008 are plotted in figure 4 along with the monthly effective precipitation. Compared to the 1989-1996 records the water levels from 2005-2008 are much higher. Compared to Aug 1996, the Aug 2008 water levels:

 

•  are on average 0.56m higher, equivalent to an increase of 200mm in storage over the missing 9 year period.

•  in piezometer 1B have risen by 0.67m, where previously it was below the bottom well, highlighting the effect of clearfelling nearby in the mid 1990's.

•  have risen by 0.7m in the Forest, and the 0.4m in the Warren an increase in storage of 260mm and 150mm respectively.

Although water levels appear to have risen between 1996 and 2008, predominantly they have fallen during the periods for which records exist.

 

Figure 4: The piezometric head in m AOD for all CCW wells that were also monitored in 2008 and effective precipitation.

 

 

Rainfall, actual evapotranspiration, effective precipitation, estimated drainage and storage for the periods 1989-1995 and 2006-2008 are presented in table 3 and figure 5. Rainfall varies from 711mm in 1995-1996 to 969mm in 2007-2008, and actual evapotranspiration ranges from -531 mm in 1989-1990 to -679mm in 2006-2007, and on average is 73.5% of P. As a result the effective precipitation is 26.5% of P and ranges from 177mm (23 of P) in 1995-1996 to 303mm (31% of P) in 2007-2008.

 

Table 3: Precipitation, Actual Evapotranspiration, Effective Rainfall, change in storage and estimated drainage and hydrological error (all in mm), for selected hydrological years.

 

 

P
AET
EP
Forest S
Border S
Warren S
Forest Q
Border Q
Warren Q
1989-1990
711.8
-530.5
181.3
-44.4
-81.4
-82.9
-225.7
-262.7
-264.2
1990-1991
736.3
-538.4
197.9
-2.5
-13.0
-20.0
-200.4
-210.9
-217.9
1992-1993
840.4
-659.9
180.5
7.4
8.3
-8.9
-173.1
-172.2
-189.4
1994-1995
860.0
-610.8
249.2
3.7
-2.8
20.0
-245.5
-252.0
-229.2
1995-1996
774.1
-597.6
176.5
4.9
25.9
8.9
-171.6
-150.6
-167.6
2006-2007
935.3
-678.7
256.6
-42.6
-50.0
-69.1
-299.2
-306.6
-325.7
2007-2008
968.9
-665.6
303.3
-43.5
-32.7
-24.7
-346.8
-336.0
-328.0
Average
832.4
-611.6
220.8
-16.7
-20.8
-25.2
-237.5
-241.5
-246.0
% P
100
-73.5
26.5
-2.0
-2.5
-3.0
-28.5
-29.0
-29.6
St Dev
97.7
60.4
49.3
25.2
36.7
38.2
65.6
68.0
63.0
SE
36.9
22.8
18.6
9.5
13.9
14.4
24.8
25.7
23.8

 

On average there is a fall in storage equivalent to 2 - 3 % of P. The fall in Forest storage (-16.7mm) is 4.1mm less than the Border (-20.8mm), which is 4.4 mm less than the Warren (-25.2mm). There is no significant difference in mean storage values of the Forest, Border and Warren , The annual change in storage ranges from -6 to 1% of P for the Forest, -11 to 3 % of P for the Border and -12 to 2 % of P for the Warren .

 

The estimated drainage, ranges from -150mm for the Border in 1995-1996 to -346mm for the Forest in 2007-2008. As drainage is derived from effective precipitation less storage, the Forest drainage (-237.5mm) is also 4.1mm less than the Border (241.5mm) which is 4.4mm less than the Warren (-246.0mm). The estimated drainage (28.5 to 29.6% of P on average) generally exceeds the effective precipitation except for the Forest in 1989-1990, 1990-1991, and 1995-1996. The range of annual drainage is 36 to 22% P Forest, 37 to 19 % P, Border, 37 to 22 % of P

 

 

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Copyright © Martin Hollingham