Catchment Characterisation Tool: Prioritising Critical Source Areas for managing diffuse nitrate pollution

Abstract

Where diffuse losses of nutrients from agriculture is a major challenge for integrated catchment management and the achievement of Water Framework Directive objectives, modelling tools can be used to target the high-risk areas and focus the limited resources available for mitigation measures. The Catchment Characterisation Tool (CCT) is a GIS-based model developed to assess the potential risk posed by nitrate and phosphate from diffuse agricultural sources to surface water and groundwater by delineating critical source areas in Irish sub-catchments. The CCT model results have been generated to support pressure-impact assessments following the source-pathway-receptor concept to target local catchment stream walks in areas where the potential impact may be higher. These risk maps can be used at a maximum scale of 1:25,000 (e.g. water body scale) to target areas for Local Catchment Assessments and are not designed or suitable to be used on their own as a basis for decisions at local or field scale. Consequently, these maps act as signposts for where further characterisation and engagement actions should be prioritised. This paper details the model structure and data requirements for the CCT for nitrate followed by validation of the results by comparing a national dataset of measured nitrate concentrations in Irish water bodies with values predicted by the CCT. The model performed well at predicting the annual average nitrate concentrations, with surface waters showing better correlation with CCT predictions than for groundwater. More detailed comparisons with intensively monitored test catchments showed satisfactory correlation between the predictions and measured concentrations. The outputs are displayed in pollution impact potential (PIP) maps that rank the modelled values so that prioritisation can be given to the higher ranked areas or critical source areas.

Appendices

Appendix 1. Parameters and Equations

Table 6 Parameters in the CCT, their relationship to the layers in Table 1 and data sources

Nitrate input equations:

$$ {N}_{\mathrm{A}}={A}_{\mathrm{A}}\hbox{--} O+{A}_{\mathrm{D}}, $$

(A1)

$$ {N}_{\mathrm{P}}=\left\{\begin{array}{c}{S}_{\mathrm{R}}\times {S}_{\mathrm{F}}\kern0.5em \mathrm{if}\ \mathrm{LPIS}\ \mathrm{data}\ \mathrm{used}\\ {}{L}_{\mathrm{U}}\times {L}_{\mathrm{S}}\times {S}_{\mathrm{F}}\ \mathrm{if}\ CSO/ DED\end{array}\right\}. $$

(A2)

Nitrate soil process equations:

$$ {R}_{\mathrm{A}}={N}_{\mathrm{A}}\times {T}_{\mathrm{S}}, $$

(A3)

$$ {L}_{\mathrm{A}}={R}_{\mathrm{A}}\times \left(1-\exp \left(-{L}_{\mathrm{C}}\times \left(\frac{F_{\mathrm{C}}}{E_{\mathrm{R}}}\right)\right)\right), $$

(A4)

$$ {L}_{\mathrm{P}}=N\mathrm{Cycl}{\mathrm{e}}_{IRL\left({N}_P,{S}_D,{A}_D,{C}_T\right)}, $$

(A5)

$$ L=\left\{\begin{array}{c}{L}_{\mathrm{A}}\ \mathrm{if}\ \mathrm{landuse}\ \mathrm{is}\ \mathrm{Arabl}e\\ {}{L}_{\mathrm{P}}\ \mathrm{if}\ \mathrm{landuse}\ \mathrm{is}\ \mathrm{Pasture}\end{array}\right\}. $$

(A6)

Nitrate subsurface pathway equations:

$$ {P}_{\mathrm{S}}=L\times \frac{R_{\mathrm{C}}}{E_{\mathrm{R}}}, $$

(A7)

$$ {P}_{\mathrm{U}}={P}_{\mathrm{s}}\times {\beta}_{\mathrm{N}}, $$

(A8)

$$ {P}_{\mathrm{B}}={P}_{\mathrm{U}}\times {\varphi}_{\mathrm{N}}. $$

(A9)

Nitrate near-surface pathway equations:

$$ {P}_{\mathrm{N}}=L\times \left(\frac{E_{\mathrm{R}}-{R}_{\mathrm{C}}}{E_{\mathrm{R}}}\right), $$

(A10)

$$ {P}_{\mathrm{D}}={P}_{\mathrm{N}}\times {\alpha}_{\mathrm{N}}. $$

(A11)

Nitrate surface receptor equation:

$$ P={P}_{\mathrm{D}}+{P}_{\mathrm{B}}\times {T}_{\mathrm{D}}. $$

(A12)

Appendix 2. Input Coefficients and Transport Factors for Ireland

Table 7 Arable input and offtake amounts

Table 8 Soil transport factors

Table 9 Subsoil transport factors

Table 10 Bedrock transport factors

Table 11 Near-surface delivery factor