Regional specification and calibration

For this research where the main focus is on groundwater bodies located in the Dutch Scheldt basin, the same model structure is used as described above. Additional information and more precise descriptions and parameter values are used in the model structure about the Scheldt geo-hydrological and chemical boundary conditions specifically in this study to simulate and evaluate the developed policy scenarios. The actual situation in the year 2000 of ground- and draining surface water in the Dutch Scheldt basin as reported in the WFD article 5 report is used as starting point, based on measured nitrate concentrations in the uppermost saturated groundwater. Hence, the model is calibrated using the available monitoring network data up to 2000 in such a way that the model results of 2000 runs approach the measured data values.

The existing national and provincial groundwater monitoring networks in the Dutch Scheldt basin are presented in Figure 17. The national monitoring network has 13 measurement points in Zeeland, while the Province Zeeland has an additional 56 monitoring points.

INPUT

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MODEL OUTPUT PER GRID CELL

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Explanatory note:

Provincial monitoring network groundwater Zeeland (PMGZ) National monitoring network groundwater (RIVM)

Figure 17: Groundwater monitoring networks in the Dutch Scheldt basin

The most important changes in the model include:

• the groundwater body classification is based on the delineation of the WFD groundwater bodies provided by the province of Zeeland;

• the aquifer depth of each groundwater body is based on the depth of the brackish-fresh water boundaries and the top of the clay layer of Boom;

• water ‘layers’ are added to the groundwater system simulating infiltration of precipitation surplus with changed characteristics due to the specifically applied groundwater protection measures;

• land use changes in the baseline scenario are based on predictions of the regional development of the agricultural sector in the area in the next 10 years as discussed in section 5, and includes the nature development plans on 4,400 ha arable land.

In view of the fact that measured nitrate concentrations in the uppermost saturated

groundwater are taken as the starting point of the model simulations, future situations with different nitrogen loads cannot simply be simulated through separate model runs as no direct

relationship with surface loads is given. To circumvent this problem, the actual situation is considered together with the following two variants. The first variant is one where

calculations are carried out assuming that all agricultural land (Figure 18) is converted into arable lands and the second variant that all agricultural land consists of grassland. Non-agricultural land use remains the same in these two variants. The three basic variants are then used in combination with the observation that a practically linear relationship exists between nitrogen loads at land surface and nitrate concentration in the uppermost groundwater for the considered range of nitrogen loads (150 to 400 kg N per ha) (Kolenbrander, 1981).

Figure 18: Agricultural land use in the Dutch Scheldt basin

Current agricultural activities in Zeeland are patchy and divers as shown in Figure 18. In Figure 18 green represents grassland, while main arable crops include corn (orange), potatoes (dark red), beets (purple) and mais (yellow). Table 5 presents actual land use in the six different groundwater body areas, distinguishing between arable land, grassland and a

‘residual’ category consisting of for example natural area and built environment (urban area).

Total

area Grassland Arable land Remaining area Groundwater

Table 5: Area size of the different groundwater body areas in the Dutch Scheldt basin

The two variants introduced above are used in cases where changes occur in land use,

including in the baseline scenario. Changes in groundwater and draining surface water quality follow from an adaptation of the share of arable and grassland to changes in fertilizer and manure use and a combination of weighted results from both variants. Land use can change in the policy scenarios and also the application of fertilizer (and/or atmospheric deposition) per type of land use. Land use changes imply changes in the values of evapotranspiration and hence net precipitation, but these second order variations are neglected here.

Values of groundwater and draining surface water quality vary among different soil types.

The changes in water quality for clay and peat areas can be determined in a relatively simple way. The values of shallow groundwater quality and drain water of the same quality follow from the linear relationship with nitrogen loads at land surface, assuming that land use remains the same as in the current situation with given values of nitrogen loads and water quality. NPKRUN calculations for future situations in areas with saturated groundwater in sandy layers are more complicated, because existing groundwater quality has to be taken into account.

Changing groundwater conditions are simulated by making calculations for sequential time steps of, for example, 5 years. Starting point again are the three variants for the year 2000. For each period of 5 years a new hydrological situation is simulated by adding a new water layer.

In this way, each groundwater body consists of a number of groundwater layers from specific time periods. These layers move vertically and horizontally. The thickness of these layers from a certain time period can be calculated as follows (Meinardi et al., 2005):

z/D= 1- exp(-I*t/(p*D)

where:

z= thickness new layer (m);

D= thickness aquifer (m);

I= groundwater recharge (m/year);

p= porosity (dimensionless, p=0.35).

So, for example, in 2005 a new groundwater layer is present over thickness z, where:

z/D= 1- exp(-I*5/(p*D). Porosity is defined here as the volume pore space per volume of soil, therefore porosity is a dimensionless parameter (as a result of dividing a metric volume measure by another metric volume measure). In the model the porosity is assumed to be 0.35 for all soil types. All parameter values are based on the actual situation in 2000, including average groundwater quality in 2000 (see Table 6).

Groundwater body

Table 6: Draining water characteristics of groundwater bodies in the Dutch Scheldt basin in 2000

Table 6 presents some of the general characteristics of each groundwater body area: total area size, net precipitation and aquifer thickness. Besides these specific characteristics, also data are needed regarding surface runoff and base flow and nitrate concentrations before the model can actually be used to simulate the impact of different policy scenarios on ground and surface water quality. The latter type of data depends among others on land use type, i.e.

surface runoff depends on whether agricultural land use is used for arable farming or as

grassland (as depicted in the actual situation in Figure 18) and forms the starting point for the further analysis and prediction of groundwater and draining water quality across the different groundwater body areas.

Groundwater and draining water quality can be simulated annually, but also for example for every five years (e.g. 2005, 2010, 2015). The average groundwater quality at any depth and for any year in a specific groundwater body area can be derived by determining the year of infiltration at the phreatic level and determining shallow groundwater quality (i.e. at one meter below surface) in that specific year. In this study we assume that 5-year periods are accurate enough in order to be able to simulate the environmental impacts of the proposed policy changes to the hydrological system and nutrient situation.

Upper groundwater quality in say 2005 then follows from the new simulated situation with regard to land use and nitrogen load (i.e. based on the proposed intervention compared to the baseline situation). Average groundwater quality in 2005 (=base flow quality) follows from the combination of z/D times the new quality situation plus (1-z)/D times the groundwater quality in 2000. Another five years later in 2010, a new layer is added with the same thickness, but with a quality level corresponding to the expected land use and nitrogen load situation in 2010. From this an average groundwater quality and draining surface water quality can be determined in 2010. The final step then is adding another layer in order to be able to simulate the situation in 2015. So, in this specific study we use time laps in NPKRUN of 5 years. Adding layers per year may seem more accurate, but one has to make in fact more assumptions in that case regarding for example corresponding nitrate loads each year.