Title: Methods for the prediction of the impact of groundwater abstraction on East Anglian wetlands (WD/95/SR)
Author: A. Williams
Author: K. Gilman
Author: J. Barker
Document Type: Monograph
Annotation: EA additional title info: methods for the prediction of the impact of groundwater abstraction on east anglian wetlands
Abstract:
This is the final report of the Anglian Regional Operational Investigation 558 The
Protection of East Anglian Wetlands. The objective of the project was the development of
methodologies for evaluation of the likely impact of groundwater abstraction on wetland sites.
The methodologies should be suitable for use at all wetland sites in East Anglia, regardless
of the amount of data available for the site. A primary requirement of the methodologies is
that they should indicate whether a wetland might be at risk from proposed abstractions.
In the Terms of Reference, the project objective was defined in relation to the groundwater
catchments of wetland sites. The project was run in two phases, and in the course of the first
phase the significance of groundwater catchments in the protection of wetlands from the
effects of groundwater abstraction was examined. It was concluded that identification of the
groundwater catchment was not required when assessing the possibility of the wetland being
adversely affected by a specific groundwater abstraction. The emphasis of the project thus
changed to a consideration of wetland behaviour in response to variations in water budget, and
to the effect of groundwater pumping on that water budget.
The maintenance of wetlands in a low rainfall region depends upon groundwater inputs, but
groundwater is not the only important component of the water budget. In this report emphasis
is placed on the construction of a simple but comprehensive water budget model, on a
monthly timescale, that will predict the impact of changes in the groundwater supply on water
levels in the wetland, most importantly during the summer months and in drought years. This
MIROS model, which can be used for sites with no local data but can also be refined to take
account of available data, has been developed in the form of a spreadsheet.
Prohibition of abstractions that would cause any fall whatsoever in water levels at wetland
sites is not a practical solution. For a given wetland site, predicted changes in water level and
flow, resulting from a proposed groundwater abstraction, should be evaluated by an objective
procedure which must recognise the water needs of plant communities. As an improvement
to the simple Theis method for predicting the propagation of impacts through the aquifer, a
new well function has been developed to quantify the effect of pumping on an idealised
wetland. The analytical model characterises a wetland by its size, expressed as an effective
radius, and a parameter which is a measure of the resistance to flow between the underlying
aquifer and the wetland.
Quantitative measures of the physical habitat needs of wetland plants are difficult to establish,
but one approach based on field evaluation has confirmed the value of the Sum Exceedance
Value as a measure of the length and intensity of summer drought in wetland soils.
The SEV can be calculated from measured or modelled water level data on an annual or long
term average basis. In this report, the combination of the MIROS model with the SEV
method has been used as one means of estimating the maximum acceptable drawdown in the
wetland.
The results lead to a suite of methodologies which allows an assessment of the susceptibility
of a wetland to a specific proposed abstraction to be made, regardless of the amount of data
available. These allow more data to be used, when available, to further quantify the wetland
water budget.
Publisher: National Rivers Authority
Publication Date: 1995
Publication Place: Peterborough
Subject Keywords: Wetlands; Groundwater; Water abstraction; Models; Methodology; Environmental impact assessment
Geographic Keywords: EA Anglian; Norfolk; East Anglia
Extent: 62; + appendices
Permalink: http://www.environmentdata.org/archive/ealit:4206
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