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

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==Nitrate input function==
 
==Nitrate input function==
 
===Model development===
 
===Model development===
The BGS NTB model previously used a single nitrate input function (NIF) (Wang et al, 2012a<ref name="Wang 2012a">WANG, L, BARKWITH, A, JACKSON, C, and ELLIS, M. 2012a. SLiM: an improved soil moisture balance method to simulate runoff and potential groundwater recharge processes using spatio-temporal weather and catchment characteristics. The 12th UK CARE Annual General Meeting. Bristol, UK.</ref>, 2012b<ref name="Wang 2012b"></ref>) providing a national average rather than a spatially distributed input based on agricultural activity. This reflected historical and future agricultural activity from 1925 to 2050 and was validated using mean pore-water nitrate concentrations from 300 cored boreholes across the UK in the BGS database (Figure&nbsp;2.2).
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The BGS NTB model previously used a single nitrate input function (NIF) (Wang et al, 2012a<ref name="Wang 2012a">WANG, L, BARKWITH, A, JACKSON, C, and ELLIS, M. 2012a. SLiM: an improved soil moisture balance method to simulate runoff and potential groundwater recharge processes using spatio-temporal weather and catchment characteristics. The 12th UK CARE Annual General Meeting. Bristol, UK.</ref>, 2012b<ref name="Wang 2012b">WANG, L, STUART, M E, BLOOMFIELD, J P, BUTCHER, A S, GOODDY, D C, MCKENZIE, A A, LEWIS, M A, and WILLIAMS, A T. 2012b. Prediction of the arrival of peak nitrate concentrations at the water table at the regional scale in Great Britain. ''Hydrological Processes'', Vol.&nbsp;26, 226–239.</ref>) providing a national average rather than a spatially distributed input based on agricultural activity. This reflected historical and future agricultural activity from 1925 to 2050 and was validated using mean pore-water nitrate concentrations from 300 cored boreholes across the UK in the BGS database (Figure&nbsp;2.2).
  
A low nitrogen loading rate between 1925 and 1940 reflects a pre-World War II low level with very limited use of non-manure-based fertilizers. The gradual intensification of agriculture during and just after the war resulted in a 1&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> year<sup>-1</sup> rise in nitrogen input to 40&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> by 1955. A more rapid rise of 1.5&nbsp;kg&nbsp;N ha<sup>-1</sup> year<sup>-1</sup> between 1955 and 1975 was due to increases in the use of chemical based fertilizers to meet the food needs of an expanding population. The nitrogen input declines from 70&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> in 1991 to 40&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> in 2020 with a rate of 1&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> year<sup>-1</sup> as a result of restrictions on fertilizer application under the Nitrate Directive. Finally, the model assumes a return to nitrogen input levels similar to those associated with early intensive farming in the mid-1950s, i.e., a constant 40&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> nitrogen loading rate from 2020 to 2050 (Wang et al., 2012<ref name="Wang 2012a">WANG, L, BARKWITH, A, JACKSON, C, and ELLIS, M. 2012a. SLiM: an improved soil moisture balance method to simulate runoff and potential groundwater recharge processes using spatio-temporal weather and catchment characteristics. The 12th UK CARE Annual General Meeting. Bristol, UK.</ref>, 2012b<ref name="Wang 2012b"></ref>). The model could readily be adapted to incorporate any agreed forward look (scenarios).
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A low nitrogen loading rate between 1925 and 1940 reflects a pre-World War II low level with very limited use of non-manure-based fertilizers. The gradual intensification of agriculture during and just after the war resulted in a 1&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> year<sup>-1</sup> rise in nitrogen input to 40&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> by 1955. A more rapid rise of 1.5&nbsp;kg&nbsp;N ha<sup>-1</sup> year<sup>-1</sup> between 1955 and 1975 was due to increases in the use of chemical based fertilizers to meet the food needs of an expanding population. The nitrogen input declines from 70&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> in 1991 to 40&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> in 2020 with a rate of 1&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> year<sup>-1</sup> as a result of restrictions on fertilizer application under the Nitrate Directive. Finally, the model assumes a return to nitrogen input levels similar to those associated with early intensive farming in the mid-1950s, i.e., a constant 40&nbsp;kg&nbsp;N&nbsp;ha<sup>-1</sup> nitrogen loading rate from 2020 to 2050 (Wang et al., 2012<ref name="Wang 2012a">WANG, L, BARKWITH, A, JACKSON, C, and ELLIS, M. 2012a. SLiM: an improved soil moisture balance method to simulate runoff and potential groundwater recharge processes using spatio-temporal weather and catchment characteristics. The 12th UK CARE Annual General Meeting. Bristol, UK.</ref>, 2012b<ref name="Wang 2012b">WANG, L, STUART, M E,
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BLOOMFIELD, J P, BUTCHER, A S, GOODDY, D C, MCKENZIE, A A, LEWIS, M A, and WILLIAMS, A T. 2012b.
 +
Prediction of the arrival of peak nitrate concentrations at the water table at the regional
 +
scale in Great Britain. ''Hydrological Processes'', Vol.&nbsp;26, 226–239.</ref>). The model could readily be adapted to incorporate any agreed forward look (scenarios).
  
 
[[Image:OR16036fig2.2.jpg|thumb|center|400px|  '''Figure 2.2'''&nbsp;&nbsp;&nbsp;&nbsp;BGS NTB nitrogen input function.    ]]
 
[[Image:OR16036fig2.2.jpg|thumb|center|400px|  '''Figure 2.2'''&nbsp;&nbsp;&nbsp;&nbsp;BGS NTB nitrogen input function.    ]]
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</center>
 
</center>
  
Two sets of MC simulations were conducted to calibrate the model against: 1) the nitrate velocity values in USZs derived from measurements of porewaters from drill cores (Wang et al, 2012a<ref name="Wang 2012a">WANG, L, BARKWITH, A, JACKSON, C, and ELLIS, M. 2012a. SLiM: an improved soil moisture balance method to simulate runoff and potential groundwater recharge processes using spatio-temporal weather and catchment characteristics. The 12th UK CARE Annual General Meeting. Bristol, UK.</ref>, 2012b<ref name="Wang 2012b"></ref>), and; 2) the observed average nitrate concentrations for each aquifer zone calculated from monitoring data provided by the Environment Agency. In the former, the bias (absolute difference) between simulated and observed nitrate velocity in USZs was used to evaluate the model fit. In the latter, the ''NSE'' score was adopted to calculate the goodness-of-fit between observed and modelled nitrate concentrations.
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Two sets of MC simulations were conducted to calibrate the model against: 1) the nitrate velocity values in USZs derived from measurements of porewaters from drill cores (Wang et al, 2012a<ref name="Wang 2012a">WANG, L, BARKWITH, A, JACKSON, C, and ELLIS, M. 2012a. SLiM: an improved soil moisture balance method to simulate runoff and potential groundwater recharge processes using spatio-temporal weather and catchment characteristics. The 12th UK CARE Annual General Meeting. Bristol, UK.</ref>, 2012b<ref name="Wang 2012b">WANG, L, STUART, M E, BLOOMFIELD, J P, BUTCHER, A S, GOODDY, D C, MCKENZIE, A A, LEWIS, M A, and WILLIAMS, A T. 2012b. Prediction of the arrival of peak nitrate concentrations at the water table at the regional scale in Great Britain. ''Hydrological Processes'', Vol.&nbsp;26, 226–239.</ref>), and; 2) the observed average nitrate concentrations for each aquifer zone calculated from monitoring data provided by the Environment Agency. In the former, the bias (absolute difference) between simulated and observed nitrate velocity in USZs was used to evaluate the model fit. In the latter, the ''NSE'' score was adopted to calculate the goodness-of-fit between observed and modelled nitrate concentrations.
  
 
====Sensitivity analysis====
 
====Sensitivity analysis====

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