Editing OR/16/036 Development of the BGS unsaturated zone model

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where ''Vobs<sub>i</sub>'' is the observed surface flow at the ''i<sup>th</sup>'' time step; ''Vsim<sub>i</sub>'' the simulated flow at the ''i<sup>th</sup>'' time step; ''N ''is the total number of simulation time steps; and ''Vobs'' is the average value of observed flow in  ''N ''simulation times.
 
where ''Vobs<sub>i</sub>'' is the observed surface flow at the ''i<sup>th</sup>'' time step; ''Vsim<sub>i</sub>'' the simulated flow at the ''i<sup>th</sup>'' time step; ''N ''is the total number of simulation time steps; and ''Vobs'' is the average value of observed flow in  ''N ''simulation times.
  
In general, a negative ''NSE ''indicates that the observed mean is a better predictor than the modelled results. Where ''NSE ''is zero modelled data are considered as accurate as the mean of the observed data, and ''NSE ''between zero and one can be treated as acceptable levels of model performance. The closer to 1, the more accurate the model is and an ''NSE ''of one corresponds to a perfect match of modelled to observed data (Nash and Sutcliffe, 1970<ref name="Nash 1970"></ref>).
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In general, a negative ''NSE ''indicates that the observed mean is a better predictor than the modelled results. Where ''NSE ''is zero modelled data are considered as accurate as the mean of the observed data, and ''NSE ''between zero and one can be treated as acceptable levels of model performance. The closer to 1, the more accurate the model is and an ''NSE ''of one corresponds to a perfect match of modelled to observed data (Nash and Sutcliffe, 1970<ref name="Nash 1970">NASH, J E, and SUTCLIFFE, J V. 1970. River flow forecasting through conceptual models part I—A discussion of principles. ''Journal of Hydrology'', Vol.&nbsp;10, 282–290.</ref>).
  
 
Field experiments (e.g. Butcher et al., 2009<ref name="Butcher 2009">BUTCHER, A, GRIFFITHS, K, LAPWORTH, D, HUMPAGE, A, and BURKE, S. 2009. Investigation of rising nitrate concentrations in groundwater in the Eden Valley, Cumbria. 4, estimating recharge rates through glacial till using an applied tracer technique. ''British Geological Survey  Open Report'' OR/09/059.</ref>) showed that the thickness of low permeability superficial deposit affects the amount of water and soluble pollutants (such as nitrate) entering the groundwater system. Traditionally the thickness of superficial deposits has seldom been considered in simulating groundwater recharge. Other factors such as changes in composition within the deposits and fracturing are also important but were not addressed here as it is difficult to model these at a national scale. Enhanced recharge at the margins of low permeability deposits was also not included.
 
Field experiments (e.g. Butcher et al., 2009<ref name="Butcher 2009">BUTCHER, A, GRIFFITHS, K, LAPWORTH, D, HUMPAGE, A, and BURKE, S. 2009. Investigation of rising nitrate concentrations in groundwater in the Eden Valley, Cumbria. 4, estimating recharge rates through glacial till using an applied tracer technique. ''British Geological Survey  Open Report'' OR/09/059.</ref>) showed that the thickness of low permeability superficial deposit affects the amount of water and soluble pollutants (such as nitrate) entering the groundwater system. Traditionally the thickness of superficial deposits has seldom been considered in simulating groundwater recharge. Other factors such as changes in composition within the deposits and fracturing are also important but were not addressed here as it is difficult to model these at a national scale. Enhanced recharge at the margins of low permeability deposits was also not included.

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