**4. Conclusions**

One of the options for the reduction in N leaching could be expanding ban on organic fertilisers with exclusion of cattle and pig slurry from the practice also on WPZ II and III. This could lead in farmers' revolt and dramatic socio‐economic changes on short term and restructuring the farm production on long term. This was investigated in Scenarios 18 (cat‐ tle farms) and 20 (pig farms) (**Table 5** and **Figure 6**). However, results did not show dra‐ matic changes in the reduction of N leaching as organic fertilisers were substituted with mineral ones. In addition to that, a new problem would emerge as surplus N would need to be properly treated. The same was simulated when we replaced corn with soya beans (Scenarios 19 and 21) (**Table 5** and **Figure 6**). Although applied amount of fertilisers (or‐ ganic and mineral) were reduced in the rotation, the nitrogen from symbiotic fixation was still released in the environmental and subject of mineralisation. On annual level, legumes

**Figure 6.** Comparison in simulated average monthly nitrogen leaching (kg ha‐1) between base and alternative agricul‐ tural management (Scenarios 14–21 and 24–27) for research locations Ptuj (P), Maribor (M) and Dobrovce (D) in the

period between 2003 and 2011 (Scenarios key in **Tables 2** and **5**).

fixate 150–250 kg N per hectare [20].

128 Water Quality

The results show that the soil‐type properties have the greatest impact on the nitrogen balance, with the same technology of production and weather conditions. Comparison of base‐ and adapted‐farming practices with each other showed that the same agricultural practice is not suitable for all soil types. According to current regulation of WPA measures, restricting the intake of nitrogen fertilisers is not defined in terms of soil type. The results show that for the purpose of preventing the negative nitrogen balance, it is necessary to design WPZ regimes according to the soil types.

It is also important to increase control over the implementation of the measures prescribed by the regulation for the aquifer water bodies in the Drava Plain and the Rural Development Programme of the Republic of Slovenia, especially through cross‐compliance under EU Common Agricultural Policy.

The comparison also showed that in locations Ptuj and Maribor fertilise more than is recom‐ mended for the average yield, according to national guidelines. Replacing part of organic fertilisers for the mineral and vice versa showed that organic fertilisers cause excess nitrogen that is available for leaching. For each type of soil, it is necessary to check the nutrient's holding capacity. It is necessary to ensure that we know what quantity of nutrients is required at a given soil properties to achieve optimal or even maximal yields for preventing leaching of excessive nitrogen. We recommend more efforts in introducing crops that require less inputs of nitrogen for growth (e.g. soya beans), and also in raising awareness of the need to reduce the fertilisation rates by their number and quantity.

In the organic farming, it is necessary to introduce fertilisation management based on the soil properties. Results show that outdoor vegetables production on shallow and sandy soil is not optimal agricultural practice from the drinking water protection perspective and is in certain situations even comparable to conventional production.

Grassland use is a good alternative to arable. Different methods of farming practices on grassland land use, also intensive ones, have according to national guidelines proved to be extremely beneficial from the nitrogen balance perspective. However, it is necessary, with the help of professional services, to clearly specify the amount and type of N fertiliser rate, which has to be dependent on the soil type and properties.

Agricultural practices adjusted to the stricter WPA I regime considerably reduce the loss of nitrogen from the soil and do not impact the yield which remains stable. The current state represents a good balance between the benefits for good‐quality status of drinking water and economic situation of agricultural holdings.

On the other side, we have less strict WPA II and III regime minimal effects on N leaching from arable land which could mean that producers cultivate land without any serious re‐ striction or they adjusted agricultural practices according to the requirements of the WPA regulations.

Measures of WPZ II and III regimes are virtually no different than the average convention‐ al production practices outside the WPZ. Since these two areas occupy the vast majority of WPZ and because we have, in regard to the commitments adopted from Water Framework Directive, achieved good‐quality status of groundwater throughout the aquifer, it is neces‐ sary to change the current approach of forming WPZ regimes. The current system for de‐ termining the WPZ regimes is positioned so that it fully ignores the characteristics of the soil.

Assessing the impact of the scenarios was done with the knowledge of the uncertainties of the model. Uncertainties were associated with the establishment of production technologies, rotations, dates of harvest, grass‐cut dates, dates of mechanical tasks and dates of fertilisers' application. All these data are just an average estimate, as each farmer has their own time schedule and technology of production, which varies according to the type of crops, crop and livestock species, intensity of agricultural production and changing weather conditions. Additional uncertainty originates in soil maps, firstly because of spatial resolution, and secondly because the model requires information on wilting point, field capacity and hydraulic conductivity which are not part of standard soil map.

According to the results, we suggest that future measures or WPA regimes are formed according to the type and properties of the soil and not only on the groundwater flow direction and proximity of drinking water wells. Constant communication with the land owners and cultivators in the area and their regular education is vital for successful trans‐ formation of the area. We recommend more efforts of professional agricultural services in the introduction of agricultural crops that require less inputs of nitrogen for growth such as soya and in raising awareness on the need for reducing fertiliser norms by its number and quantity. Grassland is a good alternative to the arable use, but help of professional services is needed to specify the type of fertiliser N and norms depending on the soil type and properties.

The results of this study are a product of a one‐computer model for catchment modelling (SWAT) and understanding of one catchment modeller. Therefore, the final assessment of the scenarios should never be regarded as definitive, but only as a possible response of the system to changes. Model results and their interpretation by the modeller must lead to constructive discussions with the aim of achieving and maintaining good water quality in the research area of the Drava Plain, which is also the aim of the Water Framework Directive.
