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Pool and drainage ditch in Newborough forest Malltraeth side Feb 2005

 

 

 

 

Newborough Warren, Feb 2005


Concerns over the hydrological impact of the Vision of CCW and FC for Newborough Forest.

One of the main objectives of the management of Newborough forest is the restoration of extensive flooding of the Dune slacks in the Warren. CCW have commissioned several reports (Betson, and Scholefield, (2004); Bristow (2002), the conclusion of these reports is that the forest is using more water than the warren.

Although the results from forest research have shown that trees evaporate more water than grass or bare soil, there is a danger of applying this generalization to specific cases where the same conditions do not apply as has been done by Bristow (2002).

Betson and Scholefield (2004), and Bristow (2002) calculated that evaporation and interception from the forest are only marginally above that of the warren.

  Potential Et including Soil (mm/day) Net Recharge (mm/day)
Warren 1.97 0.68
Forest 2.18 0.55

And Betson and Scholefield (2004), also quote Et figures for various pine species presented by other researchers, whose highest Actual Et estimate is 650mm, whilst their own estimate, calculated using the computer model SWAP for actual Et is 800mm.

For comparison Betson and Scholefield (2004) estimate for fixed dune is 600mm (also calculated using SWAP).

So how were the evaporation estimates for the forest derived? Bristow (2002) used the Penman equation and crop factors derived from research at Plynlimion and Betson, and Scholefield, (2004) also quote the same Plynlimon research.

Plynlimon is an exposed upland site with rainfall twice that of Newborough, and the predominant tree species is Sitka Spruce. There is evidence that Scots Pine has a far higher Et and interception losses than Sitka spruce, while Pinus Pinaster has a much lower Et and Interception rate, even lower than most Broadleaf species. Amongst the broad leafed species Willow is well known for its large use of water. Planting willow would exacerbate the problem of lower water tables during the summer.


Evaporation and Interception losses (results from various temperate research) Arnell (2002)

Tree type

Evap loss %
of gross P

Interception % of gross P
Total Loss % of P
Scots Pine

31-42
42-33
64-75
Sitka Spruce
27-32
27-31
54-63
Maritime pine

12-18
13-27
25-45
Oak
26
26
52
Mixed hardwood
19
23
42

 

Interception is the process by which trees catch rain on their leaves andevaporate it before it reaches the soil. Interception rates used by Bristow (2002) were derived from a maturing Sitka spruce crop in Plynlimon, Later research from Plynlimon shows that a mature crop intercepts less water (18% of Precipitation (P)) than a maturing one (60% of P) (Hudson et al 1997).

Water tables in the dune slacks show little variation from 1989-1996 (0.8m on average) (Bristow, 2002)) despite seasonal variations. Boreholes nearest the sea show the greatest variation. This indicates that forest evaporation might not be the controlling factor for the water table.

Water levels in BH in Newborough (1989-1996) plotted as an average for each month. There are two transects, transect 1 is near to the sea than transect 2 and boreholes in each transect are labelled A-G, A being nearest to the forest and G nearest to the sea (Bristow, 2002).

 

 

Other factors maybe responsible for the drop in the water table such climate change. Climate records for the UK indicate that summers are becoming drier and winters wetter, winter rainfall minima are increasing and summer rainfallminima is decreasing, relative to the time of planting. Work by Burden (1998) on 3 Devon sand dune systems shows that they have also suffering declining water tables. Also I have been able to model the watertable based on the water balance with some accuracy, and this shows that indeed that flooding is becoming less frequent because of changes in the rainfall pattern.

 

Long term 5 yr average rainfall record for Wallingford UK.

 

Other factors that may be influencing the water table of the warren are the drainage ditches of adjacent agricultural land and drainage from the sewage treatment works at Newborough.

Early accounts of the planting of the forest reported that there was extensive flooding in the forested areas at planting, to the extent that drainage was required (and this might be in part responsible for the drop in the watertable under the forest). Detailed long term records of water levels have not been kept nor has water table height been compared to rainfall records over the time since the establishment of the forest (or this data is not presented in the CCW reports).

Investigations of the water table at the RSPB reserve at Malltraeth has shown that the water table responds very quickly to rainfall sometimes within minutes, and that there are relic channels where water flow concentrates (Etheridge et.al., 2000). A similar situation probably exists in Newborough.

Most of the above points may seem irrelevant to the overall goal of preserving the flora and fauna of the warren and I freely admit that I have not had time to fully research my arguments, however as Newborough is a scientific research site, the results of which are applied to other dune systems it is important that the mechanisms used at Newborough for water table restoration are understood and the hydrological system is properly monitored, otherwise the results might be misapplied else where, just as the Plynlimon research has been misapplied to Newborough.


References


Arnell, N. (2002) Hydrology and global change. Prentice Hall.

Betson, M. and Scholefield P (2004) Review of the Evapotranspiration of Herbaceous and Afforested Duneland Ecosystems: The implications for the Water Balance in Newborough Warren National Nature Reserve, Anglesey, Wales. Report for the Countryside Council for Wales. Environment Systems and Information Systems. ADAS Consulting Ltd. Wolverhampton.

Bristow C. (2002) The Impact of Forestry on Coastal Geomorphology at Newborough Warren, Ynys Llanddwyn NNR, SSSI, pSAC, Hydrogeology Volume 5 Contract number: FC 73-05-18. Birkbeck University of London,

Burden R.J. May (1998) A hydrological investigation of three Devon sand dune systems; Brauton Burrows, Northam Burrows and Dawlish Warren. PhD Thesis, Plymouth University.

Etheridge, Z., Cratchley, C.R., Hollingham, M., Rees, D. (2000). Modelling groundwater flow at the RSPB wetland reserve, Malltraeth Marsh, Anglesey. Proceedings of the Inter-Celtic Hydrology Conference 2000, 3-7 July, University of Wales, Aberystwyth, pp 227-234.

Hudson, J., Crane, S., and Blackie, J. (1997) The Plynlimon water balance 1969-1995: the impact of forest and moorland vegetation on evaporation and streamflow in upland catchments. Hydrology and Earth System Sciences. p. 409. European Geophysical Society.

 

 

 


 

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