According to Mosier et al. [
16], the suggested Frac
LEACH value is 30%. Frac
LEACH represents the fraction of nitrogen losses in managed soils in regions where leaching occurs compared to total nitrogen inputs and sources [
17]. This was recommended for calculation of N
2O emissions through leaching in the 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines, which define that for areas with active irrigation and areas where total precipitation is higher than evaporation for a short time, the value of 30% of the proportion of nitrogen leached out of the utilized agricultural land (Frac
LEACH) should be used. For dryland regions, where precipitation and irrigation are lower than evapotranspiration throughout most of the year, leaching is unlikely to occur, and Frac
LEACH is equal to zero [
17].
Including irrigated and wet areas modifies the default nitrogen leached from arable land and grassland Frac
LEACH to a national value according to the following equation:
where Frac
IRR is the proportion of irrigated areas to total agricultural land area, Frac
WET is the share of the wet area to the total area of arable land and grassland (%), and Frac
LEACHN is the national value of the proportion of leached nitrogen from cultivated soil (%).
2.1. Analysis of Irrigated Areas in Slovakia
The share of irrigated areas in Slovakia was derived from official statistics published by Hydromelioration, a state enterprise. The data were compared with Eurostat datasets. Identified data gaps and inconsistencies are shown in
Table 1. The total area of utilized agricultural land was taken from the official statistics of the Statistical Office of the Slovak Republic. For the correct determination of the proportion of irrigated areas, distinguishing irrigation type was important. In the case of drip irrigation, water is gradually soaked into the soil and no nitrogen leaching occurs. Therefore, drip irrigation areas were excluded from the analysis [
17]. From the statistics, it can be seen that the proportion of irrigated areas in Slovakia is decreasing due to the obsolescence of the irrigation network, with a decrease of 79.9% from 2002 to 2017. Statistical data concerning irrigated areas could not be fully verified because only Hydromelioration publishes this type of data in its annual reports; the Statistical Office did not publish such data, and Eurostat published only an incomplete proportion of irrigated areas (proportions are available for 2006, 2008, 2011, and 2014) based on its own methodology or estimation, with a lack of transparency.
For 2017, the total irrigated area in Slovakia was 54,421 hectares, representing only 3.6% of agricultural land [
18]. According to Eurostat, the average in 28 European Union countries was 11.3%. Improving water efficiency and developing irrigation systems have been priorities of the Rural Development Program for 2014–2020. Farmers could apply for a nonrepayable financial contribution to restore their irrigation systems in 2017. An increase in the proportion of irrigated areas and the large year-on-year fluctuations in crop yield, which depend on climatic conditions, hence a lack of inadequate distribution of precipitation, can be expected in the future. The proportion of irrigated areas to total utilized agricultural areas is given in
Table 1.
2.2. Estimation of Wet Areas in Slovakia
The climatic parameters evapotranspiration and precipitation (
Figure 1) were used to estimate wet areas in Slovakia. Detailed data were obtained from 41 regular meteorological stations (
Figure 2) operated by the Slovak Hydrometeorological Institute (SHMI). Data were analyzed and aggregated to monthly and annual averages for this study.
Evaporation in agricultural areas occurs mainly through evapotranspiration (ET
0) and depends on meteorological conditions, soil characteristics, farming practices, and crop types. This means that evapotranspiration can vary within the country or over time and cannot be expressed by one single representative value. For the purposes of this study, we assumed the appearance of vegetation during the whole year, therefore we replaced evaporation with evapotranspiration [
19].
Evapotranspiration was estimated for all 41 regular meteorological stations with the Penman–Monteith combined method [
19]. The method combines the water–heat transport equation with Equation (3) as the energy conservation equation for the soil–plant–atmosphere system [
19]. The reference surface is the area on which the reference crop grows, i.e., grass with specific properties (0.12 m height, surface resistance to water vapor transmission r
s = 70 s·m
−1, with albedo a = 0.23) [
19]. The reference ET
0 concept was introduced to study water evaporation demand independent of crop type, crop development, and management practices. ET
0 values were calculated at different locations and seasons and are comparable because they refer to ET
0 from the same reference surface. The only factor influencing ET
0 is the climatic parameter. ET
0 expresses the evaporative power of the atmosphere at a particular location and time without taking into account crop characteristics and soil factors [
19]:
where ET
0 is evapotranspiration (mm·day
−1), R
n is net radiation at the crop surface (MJ·m
−2·day
−1), G is soil heat flux density (MJ·m
−2·day
−1), T is the daily air temperature at 2 m height (°C), u
2 is the wind speed at 2 m height (m·s
−1), e
s is saturation vapor pressure (kPa), e
a is actual vapor pressure (kPa), e
s−e
a is saturation vapor pressure deficit (kPa),
is the slope of the vapor pressure curve (kPa·°C
−1), and
is the psychrometric constant (kPa·°C
−1).
The equation uses standard climatological data of solar radiation (sunshine), air temperature, humidity, and wind speed. The measurements were made at 2 m (or converted to that height) above an extensive surface of green grass completely shading the ground and with adequate water [
20].
Aridity, a climatic indicator, is a climatological index used for regionalization of climate moisture conditions. It represents the relationship of the possible amount of water that can evaporate from the surface of weather-saturated soil and vegetation. The climatic index of aridity is calculated by the following equation [
21]:
where Aridity index is defined by variables ET
0, the sum of monthly values of potential evapotranspiration for the deficient months in mm, and P, the sum of monthly values of total precipitation in mm.
The wet season must be identified for estimating wet areas. The rainy season is defined as the period when precipitation is higher than evapotranspiration. If the aridity index of the soil is greater than 1, the Equation (4) becomes [
22]:
According to the definition of Frac
LEACH in the 2006 IPCC Guidelines, the determination of rainy seasons is based on precipitation and pan evaporation (E
PAN) data. Rainy seasons are defined as periods when rainfall > 0.5 × pan evaporation, then P/E
PAN > 0.5, where P is monthly precipitation [
17]. In the case of this study, we used evapotranspiration ∑P/∑ET
0 ≥ 1 [
23]. The P/ET
0 share was analyzed for the 41 regular meteorological stations, and leaching was identified at 17 of them in 2017 (bold values in
Table 2).
To cover the whole area of Slovakia, the meteorological data presented were interpolated and processed in a geographic information system (QGIS software) using the distance weighting interpolation function. Interpolation parameters were as follows: distance coefficient = 2, number of columns = 3000, and number of rows = 1500. The resulting map is shown in
Figure 3. The red and orange parts of the map indicate places with no nitrogen leaching in 2017. The driest areas were in the lowlands (Danubian and Záhorská lowlands). In contrast, the yellow, green, and blue parts of the map show areas where nitrogen was leached (northern and central parts).
The biggest deficit of moisture occurred mostly in the summer in the Danubian and Zahorská lowlands, where evaporation exceeded total precipitation by 60 mm on average. The deficit of precipitation is significant in lowlands in the yearly balance (in the Danubian lowland, it is 200 mm) according to moisture characteristics from the climate atlas of Slovakia for the period 1995–2010 [
22].
In the raster image (
Figure 3), areas with ∑P/∑ET
0 ≥ 1 were extracted by using the contour function and used to trim the underlying layers by available geoprocessing tools. A highly accurate database called the Land Parcel Identification System (LPIS) was used as an underlying layer. LPIS is a part of the control mechanism under the Common Agricultural Policy [
24]. It plays a significant role in verifying eligibility for area-based subsidies, monitoring farmers’ cross-compliance with selected environmental rules, and managing the Rural Development Programmes [
25].
2.3. Estimation of N2O Emissions from Leached Nitrogen
Agricultural soil, a significant source of nitrous oxide (N
2O) emissions in Slovakia, accounted for 72% of emissions in the country in 2017. N
2O emissions from agricultural soil consist of direct emissions from the application of animal manure and fertilizer and indirect emissions from nitrogen leaching and runoff from ammonia and nitrogen oxides (NH
3 and NO
x) [
26].
The accurate way to calculate N
2O emissions in agriculture is based on nitrogen flow. Nitrogen is an essential element for livestock and crop growth. The main pathways of nitrogen from the soil are demonstrated in
Figure 4. The agricultural sector has strongly altered nitrogen cycles. Nitrogen exceeding plant and animal needs may have a greater chance of being transferred to the atmosphere and aquatic ecosystems, thus the addition of N can result in increased nitrogen saturation in the environment [
27].
Part of the inorganic nitrogen in or on the soil, mainly in the form of NO
3−, can bypass biological retention mechanisms in soil/vegetation systems by being transported in overland water flow or flow through soil macropores or pipe drains. Where more NO
3− is present in the soil than required by biological demand (e.g., under cattle urine patches), the excess leaches through the soil profile. Denitrification and nitrification transform some of the NH
4− and NO
3− to N
2O. This can take place in the groundwater where the N was applied (synthetic N fertilizer; organic N applied as fertilizer, e.g., applied animal manure, compost, sewage sludge; urine and dung N deposited on pasture; N in crop residues, including N-fixing crops and forage) [
17].
The equation to calculate N
2O emissions according to the 2006 IPCC methodology follows:
where N
2O
(L) is the annual amount of gaseous N
2O emissions in Gg per year during erosion and leaching; F
SN is the annual amount of inorganic fertilizer applied to agricultural land in kg N per year; F
ON is the amount of manure and slurry produced during livestock breeding, including compost and sludge from sewage treatment plants, in kg N per year; F
PRP is the amount of manure and slurry produced during pasture grazing; F
CR is the amount of nitrogen in crop residues (above and below ground), including N-fixing crops from forage and pasture renewal, returned to soil annually in regions where leaching occurs, in kg N per year; EF
5 is the factor for N
2O emissions from leaching; and 44/28 is the stoichiometric conversion factor for N to N
2O [
17].
N
2O emissions from soil were calculated using Equation (6). A default emission factor, EF
5 = 0.0075 kg N
2O–N/kg N [
17], and the national value for nitrogen loss in 2017 (7.86%) were used. Other inputs of nitrogen to agricultural land and grassland by activities in 2017 are shown in
Table 3. The amount of nitrogen from synthetic fertilizers (F
SN) was taken from the Statistical Office of the Slovak Republic. The amount of organic nitrogen from compost was estimated from the total consumption of compost applied to agricultural land provided by the Central Control and Testing Institute in Agriculture [
30]. Applied manure in soils is included in organic fertilizers (F
ON). Nitrogen left on agricultural land in the form of postharvest residues (F
CR) was estimated by using the 2006 IPCC methodology; nitrogen from grazing animals (F
PRP) was estimated in the same way. A detailed description of the calculation can be found in the 2019 National Inventory Report of the Slovak Republic 2019 [
26].