Appendix 2: Impact analysis of changes in weather and organic carbon on the FOCUS PELMO Hamburg scenario

The Predicted Environmental Concentrations in groundwater (PEC_gw) of nine dummy active substances under different climate change scenarios have been assessed using the FOCUS PELMO (version 6.6.4) model.

PELMO is a one-dimensional simulation model that analyses the vertical movement of pesticides in soil through chromatographic leaching. It is an official FOCUS model for pesticide registration in the European Union (FOCUS, 2000, 2023). The primary goal of FOCUS is to create a harmonized European Tier 1 approach. Consequently, nine EU scenarios are implemented in FOCUS PELMO, collectively representing agricultural practices across Europe. These scenarios do not specifically imitate individual fields or reflect the agriculture precisely at their named locations or within the respective Member States. 

The PEC_gw was assessed using the FOCUS Hamburg scenario (see Table 8).

Table 8: FOCUS Hamburg ground water scenario

PELMO utilizes a capacity water flow model (tipping bucket approach) with a variable time step for all hydrological processes. In the FOCUS version, run-off and preferential flow are not considered. Pesticide movement is simulated using the convection-dispersion equation, while pesticide degradation in soil follows a first-order model, with degradation rate constants adjusted for depth, soil moisture, and temperature. Sorption to soil is calculated using the Freundlich equation.

In FOCUS PELMO, crop data is also defined for each scenario, including five crops suitable for growth across the EU. The simulations were carried out using the FOCUS standard crop 'winter cereals', considering a single annual foliar application at a rate of 100 g a.s./ha. Both winter and spring applications were simulated, at BBCH stages 10 and 30, respectively. Input parameters for these applications are detailed in Table 9.

Table 9: Application patterns to winter cereals used in modelling

The current study considered hypothetical active substances defined by two key parameters that influence their environmental fate and transport in soil:

All possible combinations of these parameters yield nine unique substances, representing a spectrum of environmental behaviours from low adsorption and short half-life to high adsorption and long half-life. The input parameters for these dummy substances used in the modelling are presented in Table 10 and Table 11. All other input values were set at the default values unless otherwise stated.

Table 10:  Regular grid of K_FOC/DT₅₀ combinations considered for active substances for PEC_GW calculations (PELMO 6.6.4)

Table 11:  Input parameters related to active substances^a for PEC_GW calculations (PELMO 6.6.4)

To evaluate the impact of potential climate changes on Predicted Environmental Concentrations in groundwater (PEC_gw) for various active substances, projection outcomes from the UBA (2023) study were used. UBA (2023) provides modelling results for a 29-year period (2031–2060) under two climate scenarios: 'Climate Protection Scenario' (RCP 2.6) and 'High Emissions Scenario' (RCP 8.5). In the current study, two climate-related parameters—temperature and precipitation—and their potential impact on PEC_gw was the focus.

To align with the UBA report, the tested scenarios have the same names but abbreviated as Climate Protection (CP) and High Emissions (HE). For the CP scenario, a mean temperature increase of 1.5 °C was assumed, while for HE, an increase of 2.3 °C was applied. Precipitation adjustments included mean annual increases, with CP seeing an approximate rise of 3% and HE a rise of 4%, labelled as CPM and HEM, respectively.

In addition to annual changes, seasonal variability in precipitation was accounted for. For the Climate Protection Scenario with Seasonal Variation (CPS), maximum precipitation increase (7.5%) was set in spring, while minimum precipitation levels (-1.5%) were applied for summer. In the High Emissions Scenario with Seasonal Variation (HES), the maximum precipitation increase (12.5%) was applied to winter, while summer levels remained unchanged (minimum change).

These climate parameter adjustments are shown in Table 12. Daily temperature and precipitation data were adjusted within the FOCUS PELMO model’s climate files (*.cli), specifically modifying the “Temp 14.00” (temperature at 14:00), “Temp mean” (daily mean temperature), and “Rainfall” parameters. The PELMO model uses one climate file (*.cli) per modelling year, requiring a total of 26 years for annual applications, with the first 6 years designated as a 'warm-up' period. The remaining 20 years are used to assess leaching behaviour. Temperature and precipitation adjustments were applied uniformly across the assessment period.

Table 12:  Temperature and precipitation variations due to climate change accounted for in the impact assessment

Figure 3: The monthly mean temperature (curves) and precipitation (columns) settings across various climate change scenarios, used to assess the impact of potential climate changes on Predicted Environmental Concentrations in groundwater (PEC_gw) for various active substances, modelled with FOCUS PELMO (version 6.6.4). The scenarios include: FOCUS (standard settings for the FOCUS Hamburg scenario), CP (Climate Protection scenario), CPM (CP with adjusted mean precipitation), CPS (CP considering seasonal precipitation variability), HE (High Emission scenario), HEM (HE with adjusted mean precipitation), and HES (HE considering seasonal precipitation variability).

Daily evapotranspiration data, along with rainfall and temperature, are entered directly into the climate data files, as these parameters are essential for modelling degradation processes. In PELMO, potential evapotranspiration for a grass reference crop (or alternatively pan evaporation) is read in as external input and then adjusted according to the crop type used in the simulation. This adjustment is made by applying a time varying crop Kc-factor, which modifies the standard reference evapotranspiration data.

For each tested scenario, potential evapotranspiration was recalculated to account for projected temperature changes, while assuming that radiation, wind speed, and relative humidity remained unchanged (Steffens et al., 2014). Calculations followed the FAO Penman-Monteith method (Allen, 1998), as detailed in Equation 1, using a grass reference crop. Potential evapotranspiration values were also recalculated for the FOCUS scenarios as a consistency check of this approach and to enable comparison between the tested scenarios and the FOCUS baseline.

Additionally, two rates of decrease in topsoil organic matter content due to global climate change were tested in the modelling: 10% and 25%. To implement these changes, the soil parameter files (*.soi) were updated accordingly.

Different combinations of climate change scenarios and topsoil organic matter content were tested for each dummy substance in the FOCUS PELMO model. The predicted environmental concentrations in groundwater were compared against those obtained using the FOCUS default settings for rainfall, temperature, and organic matter content. The various combinations are presented in the table below.

Table 13: Tested scenarios for each dummy substance

The daily potential evapotranspiration (PET) values, recalculated for the FOCUS scenarios using the FAO Penman-Monteith method, closely matched the original values, with a Root Mean Square Error (RMSE) of 0.023 cm day^-1, a bias of -0.001 cm day^-1, and a percent bias of -0.392%. The recalculated PET values produced PEC_gw values approximately 2% higher than those calculated with the original FOCUS PET values, considering both levels of reduced topsoil organic matter.

To maintain consistency, PEC values for different climate change scenarios were compared against the FOCUS scenarios using PET values recalculated with the FAO Penman-Monteith method. A summary of precipitation, PET, actual evapotranspiration (AET), and percolation data for both FOCUS scenarios and various climate change scenarios is presented in Table 14.

Table 14: Summary of temperature, precipitation, evapotranspiration and percolation for modelled scenarios.

AS4, AS7, and AS8 are characterized by low mobility, strong soil binding, or rapid degradation. Their specific DT₅₀ and K_FOC values contribute to their low leaching potential, as a result, they consistently show stable, near-zero PEC_gw values across varying seasons, climate conditions or OC levels.

AS1 and AS5 exhibit moderate mobility or moderate persistence in the soil, leading to moderate PEC_gw values. These substances show sensitivity to changes in organic carbon and climate conditions, with a tendency to leach more when organic carbon decreases or under climate scenarios involving increased temperatures and precipitation variability. 

AS2, AS3, and AS6 are distinguished by their greater mobility and slower degradation rates. Among all the substances, they show the highest PEC_gw values. These substances are also sensitive to variations in organic carbon and climate conditions, with an increased tendency to leach as organic carbon levels decrease or under climate scenarios with higher temperatures and/or changes in precipitation patterns.

AS9 demonstrates consistently low PEC_gw values across all OC levels and climate scenarios. This is due to its very low mobility, strong soil binding, and extremely slow degradation, which results in low risk of groundwater contamination. For these persistent and immobile substances, a warm-up period of 6 years might not be sufficient.

In climate scenarios with increased temperatures (CP and HE), most substances show a clear reduction in PEC_gw values compared to the FOCUS baseline (Figure 4 and Figure 5). The HE scenario, which represents the highest temperature increase, shows the lowest PEC_gw values, and this reduction trend is observed for both winter and spring applications. For example, for AS1, PEC_gw decreases in winter from 2.928 µg/L (FOCUS) to 2.288 µg/L (HES) and in spring from 0.013 µg/L (FOCUS) to 0.004 µg/L (HES). This decrease is likely due to faster degradation rates at higher temperatures, which reduce the amount of active substance available for leaching. The impact of increased temperatures is slightly greater for spring application, possibly because warmer baseline conditions further accelerate degradation in soil.

Seasonal precipitation variability, with increased precipitation in winter and spring, produced a PEC_gw change of around 1% in CP and 3% in HE. Notably, precipitation levels were slightly higher in HES than HEM, while CPM and CPS scenarios showed equivalent rainfall amounts. The effect on PEC_gw was higher in winter than in spring applications. Comparing CPM to CPS in winter applications shows that increased winter precipitation leads to higher PEC_gw values, particularly for more mobile substances. However, in HEM and HES with higher temperatures and precipitation, no similar trend was observed.

Figure 4: Predicted Environmental Concentrations in groundwater (PECgw) for various active substances (AS1-AS9) and scenarios, following winter application to winter cereals, modelled using FOCUS PELMO (version 6.6.4). The scenarios include: FOCUS (standard settings for the FOCUS Hamburg scenario), CPM (Climate Protection scenario with adjusted mean precipitation), CPS (Climate Protection scenario considering seasonal precipitation variability), HEM (High Emission scenario with adjusted mean precipitation), and HES (High Emission scenario considering seasonal precipitation variability). Two rates of decrease in topsoil organic carbon content (OC) due to global climate change were tested: 10% and 25%.

Figure 5: Predicted Environmental Concentrations in groundwater (PECgw) for various active substances (AS1-AS9) and scenarios, following spring application to winter cereals, modelled using FOCUS PELMO (version 6.6.4). The scenarios include: FOCUS (standard settings for the FOCUS Hamburg scenario), CPM (Climate Protection scenario with adjusted mean precipitation), CPS (Climate Protection scenario considering seasonal precipitation variability), HEM (High Emission scenario with adjusted mean precipitation), and HES (High Emission scenario considering seasonal precipitation variability). Two rates of decrease in topsoil organic carbon content (OC) due to global climate change were tested: 10% and 25%.

Most active substances show increased PEC_gw values as organic carbon (OC) decreases (Figure 4–Figure 6). On average, a 10% and 25% reduction in OC led to an approximate increase in PEC_gw of 3% and 9%, respectively. Lower OC reduces the soil’s adsorption capacity, allowing more of each substance to remain in the soil solution and potentially leach into groundwater. This trend is consistent across most substances and observed in both applications. The only exception is AS3, which exhibits an unusual pattern in the winter application, where the PEC_gw at -10% OC is higher than at -25% OC (Figure 4).

Figure 6: The effect of decreased topsoil organic matter content on Predicted Environmental Concentrations in groundwater (PEC_gw) for various active substances (AS1-AS9) and scenarios, following application to winter cereals, modelled using FOCUS PELMO (version 6.6.4). The scenarios include: FOCUS (standard settings for the FOCUS Hamburg scenario), CPM (Climate Protection scenario with adjusted mean precipitation), CPS (Climate Protection scenario considering seasonal precipitation variability), HEM (High Emission scenario with adjusted mean precipitation), and HES (High Emission scenario considering seasonal precipitation variability). Two rates of decrease in topsoil organic matter content due to global climate change were tested: 10% and 25%. The error bars represent the Standard Error of the Mean, introduced by the different substance properties.

In this modelling study, the effects of different climate change scenarios on the Predicted Environmental Concentrations in groundwater (PEC_gw) for nine dummy active substances using the FOCUS PELMO model (version 6.6.4) were assessed. The modelling study evaluated the impacts of increased temperature and precipitation (including seasonal variability) and reduced soil organic matter content. Our results indicate a general decrease in PEC_gw values with rising temperatures and precipitation, while a reduction in organic matter content leads to an increase in PEC_gw values.