(-)Profile Type of

P1 Unclassified assemblage of pine forest and mixed

oak-pine forest (association Dicrano-Pinion) Pine (Pinus

silvestris) 40 1 3.2

II Meadow (Arrhenatheretum elatioris) - - 1 1.1

IV Alder wood (Ribo nigri-Alnetum) juvenile form Alder (Alnus glutinosa) 70%

20 0.89 3.07

This enables a quick estimation of these parameters based on the analysis of forest stand maps (Tab. 1).

In the case of non-forest communities, the data from literature was used (Soczyńska, 1997).

In order to describe the soil water flow process, it is necessary to estimate the physical parameters of the soil.

The set of parameters included in this model required field research. Bulk density and saturated water content were determined by laboratory analyses of soil samples (Tab. 2). The grain size distribution curves were used to calculate saturated hydraulic conductivity according to the so-called American formula (Pazdro and Kozerski, 1990):

K_s = 0.36 d₂₀^2.3 (cm¹ s^-1) (8)

where: d₂₀ - the effective diameter (in mm) of particles, such that 20 % of the soil mass is composed of particles smaller than d₂₀.

Table 2: Physical parameters of the infiltration process in selected plant-soil columns in the Pożary basin, estimated from field surveys.

Van Genuchten’s

P1 sand 404.67 1.528 0.369 0,0112 1,364 0.011

II sand 335.54 1.409 0.372 0,0117 1,363 0.011

IV peat 69.98 0.215 0.836 0,0133 1,261 0.255

The volumetric water content at pF 4.2 (residual water content, taken as equal to the wilting point) was estimated according to the formula proposed by Ślusarczyk (1979):

WTW = 0,709+0,386 X (9)

Where: WTW - residual water content (at pF 4.2), X - content of soil fraction below 0,002 mm (clay) in %.

In the case of peat soil, the residual water content was determined in the laboratory.

The Van Genuchten parameters α (α>0 (-)) and n (n>1 (-)) were estimated using relations given by Rawls and Brakensiek (1982), based on soil porosity and the content of sand and clay. The dispersion coefficient according to (7) is described by 3 parameters a, b and d. The values of a and b were taken after Maciejewski (1998) as: a = 0.2 cm, b = 0.1 cm. The value of d = 0.17 (cm² g^-1) was taken according to research by De Smedt et al. (1981). The parameter ks (cm³ g^-1) represents substance adsorption in the soil and is otherwise called the equilibrium constant. In the case of an ideal migrant, thus a non-adsorbed substance or ion, ks = 0. The ks coefficient for the chloride ion was estimated as 0.05 cm³ g^-1. The remaining model content and chloride concentrations are shown in Figs 3, 4, and 5 as the curves with time equal to zero.

The input variables to the infiltration and solute transport model were the net precipitation (Pg) intensity on the ground surface I (cm d^-1), the chloride ion concentration in precipitation water c (mg¹ dm^-3) and the daily totals of land evaporation E_t (cm d^-1). Precipitation data were obtained from the Granica meteorological station (Fig 1). Measurement results were corrected in order to obtain P_r according to formula (1) (Tab. 3).

A chemical analysis of precipitation water at the station in Granica was made once a month in a collective sample test (from the whole month). Therefore, it was assumed that the concentration of individual ions is constant within one month. In the modelling period, the chloride ion concentration in precipitation water was 0.2 mg Cl^- dm^-3 in June 1999 (Tab. 4). The daily totals of land evaporation as mentioned above were calculated using the Konstantinov method, based on meteorological data from the Granica station: daily mean air temperature and water vapour pressure (Tab. 3).

Table 3: Daily corrected precipitation (P_r) and land evaporation (E_t) totals at the Granica station and net precipitation in profiles IV, II and P1 in the period from 1 June to 18 June 1999.

P_g (mm/d) P_g (mm/d)

Date P_r (mm/d)

E_t

(mm/d) IV II P1

Date P_r (mm/d)

E_t

(mm/d) IV II P1

1.06 0.0 3.5 0.00 0.00 0.00 10.06 0.0 3.6 0.00 0.00 0.00

2.06 0.0 3.4 0.00 0.00 0.00 11.06 30.4 3.3 27.07 27.07 27.70

3.06 7.5 3.1 4.38 4.38 4.28 12.06 8.4 1.2 5.34 7.18 5.21

4.06 14.1 2.3 10.98 11.73 10.95 13.06 10.8 2.8 7.96 7.96 7.96

5.06 0.0 3.3 0.00 0.00 0.00 14.06 5.1 1.5 2.02 3.56 1.89

6.06 0.0 3.4 0.00 0.00 0.00 15.06 1.7 1.5 0.19 0.21 0.16

7.06 0.3 3.9 0.00 0.00 0.00 16.06 0.3 2.6 0.00 0.00 0.00

8.06 11.5 2.7 8.42 8.78 8.29 17.06 26.6 1.0 23.58 25.55 23.45

9.06 0.0 3.6 0.00 0.00 0.00 18.06 3.3 2.1 1.16 1.16 1.16

Table 4: Chloride ion concentration in precipitation water at the Granica station in 1999.

Month I II III IV V VI VII VIII IX X XI XII

Concentration

(mg Cl^- dm^-3) 2.3 1.8 1.5 0.8 0.5 0.2 0.6 0.8 0.8 1.0 2.0 2.2

RESULTS

The calculated soil moisture distribution in the swamp profile (Fig 3a) shows a strong sensitivity to rainfall totals. There was a large rainfall event (10.98 mm) on the second day after the beginning of the simulation.

The soil immediately reached full saturation (θ = 0.836) and water started to collect on the ground surface.

This can be seen in the calculation results. A decrease of soil moisture over the first day was caused by the absence of precipitation. In the case of the chloride ion concentration, a decrease in the lower part of the profile is related to a small content of chloride in the precipitation water. The measured concentration in the groundwater, which initially was equal to 10 mg Cl^- dm^-3, decreased to 7 mg Cl^- dm^-3. The modelling result was 8 mg Cl^- dm^-3 (Fig 3b), which agrees quite well with the measurement, taking into consideration a very small input of chloride with precipitation. The increase of the chloride concentration in the upper soil layers must be a numerical modelling artefact because the boundary conditions adopted did not make any upward flow possible.

In the transitional soil profile II (Fig 4), the measured groundwater level during the period of 14 days raised from 72 to 59 cm b.t.l. (below terrain level). The daily precipitation total once even exceeded 20 mm.

The initial soil moisture content above the capillary fringe zone was about θ = 0.275. The measured chloride ion concentration in the groundwater was quite high at the beginning (27 mg Cl^- dm^-3) but it drastically fell during the 14 days (to 8 mg Cl^- dm^-3). This was due to little chloride supply with the precipitation water and due to the effect of the previous period, when the chloride ion concentration both in the soil and in the precipitation water was higher (Tab. 4). The calculation results show quite a strong reaction to precipitation

of overdried soil in terms of the soil moisture content (Fig 4a). High daily precipitation totals have in the unsaturated zone is 0.5 mg Cl^-dm^-3 higher than the measured value.

0,22 0,24 0,26 0,28 0,3 0,32 0,34 0,36

Fig 3: Results of soil moisture distribution (a) and chloride ion concentration (b) modelling in profile IV during the period 3-4 June 1999 (time in hours).

Fig 4: Results of soil moisture distribution (a) and chloride ion concentration (b) modelling in profile II during the period 3-18 June 1999 (time in days).

Fig 5: Results of soil moisture distribution (a) and chloride ion concentration (b) modelling in profile P1 during the period 3-18 June 1999 (time in days).

The dune profile (P1), 280 cm thick, was the deepest of the analysed soil profiles. Initially the whole profile was strongly overdried and water content was close to the permanent wilting point (θ^r = 0.011), whereas the groundwater table was measured at 258 cm b.t.l. Quite a large precipitation event in the period from 3 June to 18 June 1999, exceeding 27 mm/d, caused the water table to rise to 250 cm b.t.l. The measured chloride ion concentration in the groundwater did not vary in time, staying constant at 14 mg Cl^- dm^-3. In the top soil layer, due to abundant precipitation, a strong variability in water content was observed (Fig 5a). The results show that water migration in this profile, composed of loose sands, is very fast and after intense precipitation a state close to saturation can be observed in the top layers. Below 100 cm, the percolating water is only partially able to replenish the soil moisture deficiency. The calculated groundwater level at the end of simulation is about 7 cm higher than the measured level. Changes in the calculated chloride ion concentration are only visible in the top soil layer, which is clearly a result of the ion’s small concentration in precipitation water (Fig 5b). The calculation results show that the chloride ion concentration in the middle part of the profile nearly always approach zero. The highest calculated chloride concentration occurred in the lower part of the profile and slightly changed over the modelling period (it decreased from 14 mg Cl^- dm^-3 to about 13 mg Cl^- dm^-3). The difference between the calculated and measured concentration in groundwater at the end of simulation is about 1 mg Cl^- dm^-3.

CONCLUSIONS

The modelling results confirm the presumptions concerning the basic rules governing water circulation in the Pożary basin:

• Because of favourable infiltration conditions in the dune area, and less favourable conditions on the wetlands, the dune zones are the main place of groundwater recharge in the basin.

• The greatest role in the summer recharge of groundwater is played by long lasting intense precipitation, which causes an immediate reaction of the swamp area groundwater. Precipitation events with intensities less than 4 mm/d cause no reaction of the groundwater, because during the summer period evaporation is very high.

• The main source of chloride ions in the basin is precipitation water. The chloride concentration in groundwater decreases as soon as a reduced chloride load in precipitation water is observed.

• The influence of chloride concentration variability in precipitation water on chloride content in groundwater decreases with depth. The most visible reaction takes place in relatively shallow sandy profiles (II).

The presented mathematical model of water and solute transport in the vertical plant-soil column may be applied in various other catchments where groundwater is shallow and the vertical components of water flow and solute transport in the saturated zone are negligible. An attempt to apply this model in a wetland area shows that it is also possible to use it there, even if the unsaturated zone is not very thick. The only limitation for the application of the presented mathematical model seems to be the availability of data and the proper identification of model physical parameters.

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