**3.1. Calibration**

Water that enters the soil profile can move by several possible routes. Soil water can be removed from the soil by plant uptake or evaporation (evapotranspiration) or may percolate vertically through the soil horizons below the bottom of the soil profile, or laterally as surface runoff and interflow. The majority of the soil water is removed through evapotranspiration. Correct preparation of soil parameters is verified by soil water content and plant growth rate results.

PBIAS statistical test shows that simulated SWC at all three research locations is within a reasonable range and in good agreement with measured values (**Table 3** and **Figure 3**). The results fall within the very good category [23]. From **Figure 3**, SWC is well seen declining during prolonged periods of drought and the rise of SWC after precipitation events.


SAR, Sensitivity Analysis Rank; FCV, Final Calibrated Value; a , forest, permanent crops, grassland + arable; D\* , depends on soil type, land use and modeller set‐up; Cn2, SCS runoff curve number for moisture condition II; Esco, soil evaporation compensation factor; Gw\_Revap, groundwater "revap" coefficient; Revapmn, threshold depth of water in the shallow aquifer for "revap" to occur; Canmx, maximum canopy index; FFBC, Initial soil water storage expressed as a fraction of field capacity water content; Sol\_Bd, moist bulk density; Sol\_Awc, available water capacity of the soil layer; Sol\_K, saturated hydraulic conductivity; Sol\_Alb, moist soil albedo.

**Table 3.** Sensitivity analysis and daily time‐step soil water content (SW) calibration and validation performance statistics for the Ptuj, Maribor and Dobrovce research locations.

Modelling Impact of Adjusted Agricultural Practices on Nitrogen Leaching to Groundwater http://dx.doi.org/10.5772/66324 123

**Figure 3.** Visual comparison of soil water content calibration and validation at various periods for Ptuj (a and b), Mari‐ bor (c and d) and Dobrovce (e and f) research locations.

Based on the results of PBIAS test and visual comparison of the simulated and measured values of SWC, we can argue that the SWAT model is well enough calibrated to be suitable for carrying out simulations of SWC and nitrate leaching from the soil profile. It is necessary to be cautious in interpreting the results because the period of SWC measurement was short and calibration and validation periods do not cover all possible weather and land management events.

### **3.2. Nitrogen balance**

### *3.2.1. Base scenarios*

Wilcoxon rank‐sum non‐parametric test was used for the detection of significant differences between base and alternative scenarios. We compared the average annual values of two independent samples of equal size (*n*1 *= n*2 *=* 9). The results of alternative agricultural land management scenarios are statistically significantly different from base situation, if the

Water that enters the soil profile can move by several possible routes. Soil water can be removed from the soil by plant uptake or evaporation (evapotranspiration) or may percolate vertically through the soil horizons below the bottom of the soil profile, or laterally as surface runoff and interflow. The majority of the soil water is removed through evapotranspiration. Correct preparation of soil parameters is verified by soil water content and plant growth rate results.

PBIAS statistical test shows that simulated SWC at all three research locations is within a reasonable range and in good agreement with measured values (**Table 3** and **Figure 3**). The results fall within the very good category [23]. From **Figure 3**, SWC is well seen declining

during prolonged periods of drought and the rise of SWC after precipitation events.

**Parameter Default Range Ptuj Maribor Dobrovce**

Cn2 D\* ±25% 4 +5 3 +10 6 +25 Esco 0.95 0.5–1 3 0.87 5 0.85 4 0.80 Gw\_Revap 0.02 0.02–0.20 10 0.10 9 0.09 10 0.06 Revapmn 750 0–1000 8 760 10 634 9 530 Canmxa 0 0–20 7 2.3 4 2.5 5 2 FFBC 0 0–1 6 0.93 8 0.94 7 0.95 Sol\_Bd D\* ±25% 5 +2 6 +10 3 +21 Sol\_Awc D\* ±25% 1 +8 1 +12 1 +18 Sol\_K D\* ±25% 2 0 2 −5 2 −10 Sol\_Alb D\* ±25% 9 +1 7 −2 8 −3

Validation 4.76 −8.73 −7.64

on soil type, land use and modeller set‐up; Cn2, SCS runoff curve number for moisture condition II; Esco, soil evaporation compensation factor; Gw\_Revap, groundwater "revap" coefficient; Revapmn, threshold depth of water in the shallow aquifer for "revap" to occur; Canmx, maximum canopy index; FFBC, Initial soil water storage expressed as a fraction of field capacity water content; Sol\_Bd, moist bulk density; Sol\_Awc, available water capacity of the soil layer; Sol\_K,

**Table 3.** Sensitivity analysis and daily time‐step soil water content (SW) calibration and validation performance

**SAR FCV SAR FCV SAR FCV**

−7.59 4.33 9.61

, forest, permanent crops, grassland + arable; D\*

, depends

Wilcoxon test value exceeds 62 at *α* = 0.05 or 70 at *α* = 0.20.

**3. Results and discussion**

**3.1. Calibration**

122 Water Quality

Calibration 0 = optimal

+ values (%) = underestimate – values (%) = overestimate

SAR, Sensitivity Analysis Rank; FCV, Final Calibrated Value; a

saturated hydraulic conductivity; Sol\_Alb, moist soil albedo.

statistics for the Ptuj, Maribor and Dobrovce research locations.

The base scenarios show a high average annual variability in nitrogen leaching from the soil profile (**Table 4**). Comparison of base rotations from practice between themselves showed that production technologies with higher N intake have negative impact on the balance of N causing higher leaching. Results of the model show that the same technology (rotation) is not suitable for all soil types (**Table 4**). As shown in the example from Maribor with rotation suitable for relatively deep soils, can this rotation cause from two to three times greater N leaching if used on shallow soils of Dobrovce. Measures for controlling nitrogen fertilisers' application are not defined on the basis of soil properties, according to the current regulation for the WPA of the Drava Plain. Areas of regimes I, II and III have been determined in order to prevent microbiological contamination of drinking water wells. The results show that for the purpose of preventing the negative N balance, the WPA zones and regime should be designed according to the soil properties. This is even more important because Water Frame‐ work Directive obliges member states to improve the water quality status of the entire aquifer and not only that part in the vicinity of wells.


avg, average; StDv, standard deviation; min, minimal; max, maximal; shaded cells, results of rotation management and soils type from the same research location.

**Table 4.** Average annual nitrogen leaching (kg N ha−1) from the soil profile (model SWAT) for all three base rotations of the three research locations Ptuj (P), Maribor (M) and Dobrovce (D) for the research period 2003–2011.

### *3.2.2. Agricultural land management scenarios*

Current base fertilisation rates at the Maribor research locations are higher than the rates for the average yield are (Scenario 2) (**Table 5** and **Figure 4**) [20]. On replacing part of the organic fertiliser with the mineral (Scenarios 4 and 5) and vice versa, organic animal fertilisers were shown to cause higher excess N in the balance (**Table 5** and **Figure 4**). It is necessary to invest in the education of producers and to strengthen the control of fertilisation plans. Analysis of soil properties is required to check how much fertiliser can soil hold and how much can be applied at given soil conditions to achieve optimum yields and to avoid excessive N leaching.

Comparison of Dobrovce base rotation similar to organic and conventional integrated horticultural rotation (Scenarios 11–13) with fertilising norms for optimal production of vegetables has shown that outdoor horticultural production in Dobrovce shallow and sandy soils with gravel parent material is probably not the optimal use of agricultural land from the water protection point of view (**Table 5** and **Figure 4**). Much better results for N leaching were archived by organic field crop rotation (Scenario 23) with N‐leaching yields lower from base rotations (**Figure 5**). Organic farming in WPZ is, beside water quality, also in pursuit of other goals, such as increased biodiversity, animal welfare and ban of synthetic plant protection products.


suitable for all soil types (**Table 4**). As shown in the example from Maribor with rotation suitable for relatively deep soils, can this rotation cause from two to three times greater N leaching if used on shallow soils of Dobrovce. Measures for controlling nitrogen fertilisers' application are not defined on the basis of soil properties, according to the current regulation for the WPA of the Drava Plain. Areas of regimes I, II and III have been determined in order to prevent microbiological contamination of drinking water wells. The results show that for the purpose of preventing the negative N balance, the WPA zones and regime should be designed according to the soil properties. This is even more important because Water Frame‐ work Directive obliges member states to improve the water quality status of the entire aquifer

and not only that part in the vicinity of wells.

**Rotation**

soils type from the same research location.

*3.2.2. Agricultural land management scenarios*

**Nitrogen leached from the soil profile(kg N ha−1 year)**

**Ptuj Maribor Dobrovce** ↓**Soil**↓ **avg StDv min max avg StDv min max avg StDv min max** Ptuj 51.3 43.4 1.2 109.1 32.4 17.5 0.7 56.2 71.4 50.1 13.2 152.8 **Maribor** 71.1 76.0 4.0 180.9 59.9 27.5 22.2 103.9 85.5 49.7 8.8 159.0 **Dobrovce** 91.5 105.5 6.0 267.6 97.5 62.3 12.3 208.4 91.1 56.1 4.9 188.3 avg, average; StDv, standard deviation; min, minimal; max, maximal; shaded cells, results of rotation management and

**Table 4.** Average annual nitrogen leaching (kg N ha−1) from the soil profile (model SWAT) for all three base rotations of

Current base fertilisation rates at the Maribor research locations are higher than the rates for the average yield are (Scenario 2) (**Table 5** and **Figure 4**) [20]. On replacing part of the organic fertiliser with the mineral (Scenarios 4 and 5) and vice versa, organic animal fertilisers were shown to cause higher excess N in the balance (**Table 5** and **Figure 4**). It is necessary to invest in the education of producers and to strengthen the control of fertilisation plans. Analysis of soil properties is required to check how much fertiliser can soil hold and how much can be applied at given soil conditions to achieve optimum yields and to avoid excessive N leaching.

Comparison of Dobrovce base rotation similar to organic and conventional integrated horticultural rotation (Scenarios 11–13) with fertilising norms for optimal production of vegetables has shown that outdoor horticultural production in Dobrovce shallow and sandy soils with gravel parent material is probably not the optimal use of agricultural land from the water protection point of view (**Table 5** and **Figure 4**). Much better results for N leaching were archived by organic field crop rotation (Scenario 23) with N‐leaching yields lower from base rotations (**Figure 5**). Organic farming in WPZ is, beside water quality, also in pursuit of other goals, such as increased biodiversity, animal welfare and ban of synthetic plant protection

the three research locations Ptuj (P), Maribor (M) and Dobrovce (D) for the research period 2003–2011.

**Research location**

124 Water Quality

products.

**Table 5.** Comparison of average annual applied fertiliser (mineral and organic) and nitrogen leaching from soil profile (kg N ha−1) between base and alternative scenarios for the research locations Ptuj (P), Maribor (M) and Dobrovce (D) in the research periods 2003–2011.

**Figure 4.** Comparison in simulated average monthly nitrogen leaching (kg ha‐1) between base and alternative agricul‐ tural management (Scenarios 1–13) for research locations Ptuj (P), Maribor (M) and Dobrovce (D) in the period be‐ tween 2003 and 2011 (Scenarios key in **Tables 2** and **5**).

Grassland land use and management proved to be an extremely beneficial for soil N balance (Scenarios 7–10). Interestingly, the four‐cut intensive grassland without excessive use of animal manure contributes to a drastic reduction in N leaching (**Table 5** and **Figure 4**). Through the process of modelling permanent pasture with average production technology, it was found that farmers on average spread slurry three times per year (in some cases even more) in addition to that they spread mineral fertilisers (Scenario 17). This practice causes on shallow soils such as in Dobrovce heavy losses of N, which are comparable to those in the arable fields. This shows that regulation on banning the organic fertilisers especially liquid animal manure (slurry) in the WPA I is appropriate and eligible measure. Awareness of this is even more important as currently a major part of slurry is applied on arable land as part of corn field fertilisation and not on the grassland areas.

**Figure 4.** Comparison in simulated average monthly nitrogen leaching (kg ha‐1) between base and alternative agricul‐ tural management (Scenarios 1–13) for research locations Ptuj (P), Maribor (M) and Dobrovce (D) in the period be‐

Grassland land use and management proved to be an extremely beneficial for soil N balance (Scenarios 7–10). Interestingly, the four‐cut intensive grassland without excessive use of animal manure contributes to a drastic reduction in N leaching (**Table 5** and **Figure 4**). Through the process of modelling permanent pasture with average production technology, it was found that farmers on average spread slurry three times per year (in some cases even more) in addition to that they spread mineral fertilisers (Scenario 17). This practice causes on shallow soils such as in Dobrovce heavy losses of N, which are comparable to those in the arable fields. This shows that regulation on banning the organic fertilisers especially liquid animal manure (slurry) in the WPA I is appropriate and eligible measure. Awareness of this is even more

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

126 Water Quality

**Figure 5.** Comparison in simulated average monthly nitrogen leaching (kg ha‐1) between base and alternative organic vegetable (22) and field crops (23) agricultural management scenarios for research locations Ptuj (P), Maribor (M) and Dobrovce (D) in the period between 2003 and 2011 (Scenarios key in **Tables 2** and **5**).

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 fixate 150–250 kg N per hectare [20].

**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**).

Rotations adapted to WPA zone I regime (Scenarios 24, 26, 28 and 30) reduce losses of N while the development of biomass and yield is not affected (**Table 5** and **Figure 6**). The main reason for this is the ban on the use of liquid animal manure and strictly controlled application of mineral N during the growing season. The effects of the measures are not equally effective in all areas. Efficiency is strongly related to the soil properties. This type of scenarios is a very attractive option for regulators (State), with few very relevant side effects on agriculture, for which the regulator will have to provide answers and solutions. The first effect is the surplus of livestock manure, the second is the cost for mineral fertil‐ isers and the third, the control of measures implementation if the zone I regime would be extended over greater area.

Measures of WPA zone II and III regime have minimal effects on arable land which means that farmers can practically farm without any serious limitations (Scenarios 25, 27, 29 and 31) (**Table 5** and **Figure 6**). It is also possible that farmers adapted production technologies according to the requirements of the regulations for water bodies in the area of the Drava Plain. Results show stable N balance, which is similar to the average situation outside of WPA. Given the fact that WPA zone I regime covers only a small part of the Drava Plain (2.3%), the effect of these measures on the quality of groundwater is minimal (**Figure 2**). In addition to that in the large central part of the Drava Plain with shallow soils and under the WPA zone II and zone III regimes, a normal agricultural practice is taking place. The results of the SWAT model show that it is possible to reduce the quantity of the applied and thus also leached N, without any important effect on biomass or yield production.
