THE WATER CIRCULATION IN A PEATLAND
WATER BALANCE MEASUREMENTS OF WIELKIE TORFOWISKO BATOROWSKIE The investigation of the hydrological cycle was conducted for the period from 1 November 1998
to 31 October 1999 using daily observations. This period was chosen because the end of October is in most catchments the time of the biggest groundwater deficit and all water balance calculations and observations in Poland are mostly made from November to October. The measured parameters were: precipitation, discharge, encharge, groundwater level, and meteorological parameters needed to calculate evapotranspiration. The calculation of the water balance was based on these observations using the following formula modified from Bavina (1975), Dooge (1975) and Lensen (1991):
P + Qin + Gin = I + E + Qout Gout + ∆s,
where: P – precipitation, Qin – surface inflow to the bog, Gin – groundwater inflow to the bog, I – interception, E – evapotranspiration, Qout – surface outflow from the bog, Gout – groundwater outflow from the bog and ∆s – changes of water storage in the bog.
Precipitation and other meteorological parameters were measured at a meteorological station situated 3000 m to the North-West of the peatland at the same altitude and in similar relief conditions. The current meter discharge measurements, along with daily surface water level observations, allowed the establishment of a rating curve, which was the basis for inflow and outflow calculations. Detailed investigations of vegetation cover (Potocka, 1999) allowed for the calculation of interception and evapotranspiration.
Additionally all the meteorological parameters needed for these calculations were measured at the meteorological station. Interception of precipitation was established using the general formula for canopy interception (Von Hoyningen-Hüne, 1983; Braden, 1985):
precipitation and LAI – leaf area index. Evapotranspiration calculations were based on the Penman-Monteith formula with respect to the soil and vegetation cover characteristics and measured meteorological parameters. Groundwater level measurements allowed for the calculation of changes in the water storage.
During the whole hydrological year the groundwater table was no deeper than 30 cm below the surface Groundwater (subsurface) inflow and outflow was assumed as the difference in the equation.
RESULTS
The hydrological year 1999 (1.11.1998-31.11.1999) in the region of the Stolowe Mountains may be described in terms of precipitation as an average year in the decade of 1990-1999. Measured precipitation during the investigated period (757 mm) is almost the same as the average annual precipitation for 1990-1999 (745 mm). Precipitation was measured using Hellmann’s raingauge, and correction of the precipitation was performed. This caused an increase in precipitation by 12 % (Table 1).
Every parameter (except groundwater inflow) of the water balance was calculated separately. The difference in the equation is very small (about 1 %) and proves that the groundwater inflow to the peatland has a small influence on the water circulation.
Table 1: Water balance parameters of Wielkie Torfowisko Batorowskie.
Parameter of the water balance Qin P ∆s I E Qout Gin
[mm] 378.1 857.5 -6.7 85.1 433.0 735.3 11.1
[1000 m3] 110.4 254.3 2.0 25.5 129.9 220.6 9.3
The main parameter in the water balance inputs (Fig 4a) is precipitation (69 %). The large contribution of surface inflow (30 %) is caused by artificial influence where hand-made ditches accumulate water in the channels and transport it to the bog from the upper parts of the catchment, thereby decreasing the subsurface flow to the bog. The water storage changes should be included into the water balance, however the changes are so small that they seem not to be important in the water cycle.
a) b)
Fig 4: Main parameters of the Wielkie Torfowisko Batorowskie water balance; a – input, b – output.
The output part of the water balance (Fig 4b) is characterised by a large contribution of the surface outflow (59 %). This is a result of the artificial drainage system in the peatbog (Fig 2). The second important contributor is evapotranspiration (34 %). Interestingly, the evapotranspiration contribution is much smaller than the amount estimated on the Irish raised bogs which can be up to 80 % of the water balance output (Lensen, 1991). The interception of precipitation by plant cover (7 % of the output) amounts to about 10 % of the total annual precipitation, which is much smaller than the maximum interception capacity of Sudety spruce forests (Wozniak, 1977). This is the result of the bad condition of the spruce forest on the peatland (Potocka, 1999).
CONCLUSIONS
Wielkie Torfowisko Batorowskie is not a typical highmoor bog. Despite having dominating plant species characteristic for raised bogs, this bog does not lay on the catchment border. The vegetation community is the result of water supply by very poor water coming from precipitation and shallow groundwater flow accumulating in the ditches. This water is characterised by low values of pH and mineralisation.
The hydrogeological situation causes an isolated groundwater level to occur in the peatland, without any hydraulic connections with deeper aquifers of the Stolowe Mountains. Apart from precipitation, surface inflow plays an important role in the bog water supply, which is a result of a very dense artificial drainage system. This is why the outflow from the bog is the most important parameter of the water balance output.
Relatively small evapotranspiration may be connected with the influence of altitude. In the Stolowe Mountains, with increasing altitude the moisture deficit decreases and relative humidity increases (Kicinska et al., 1999) reducing the ability of the atmosphere to receive water from the soil and plant cover. The results of the water balance differ from those obtained on the raised bogs of the Atlantic type in Ireland (Table 2).
Nevertheless, the results are similar to the results of investigations on forested peatlands in the highlands of European parts of Russia (Bavina, 1975). These results indicate a similar scheme of hydrological processes as seen on Wielkie Torfowisko Batorowskie. The significant difference is only in the outflow, which may be explained by the artificial drainage system on Wielkie Torfowisko Batorowskie.
Table 2: Comparison of water balance components in different peatlands.
Parameters of the water
One of the first Polish scientists who began hydrological investigations on wetlands was Ostromecki, who studied peatlands on Polesie (1938). The water balance was calculated for the Czemerne wetland in the years 1933-1937, resulting in the following average annual values: precipitation 551.2 mm, surface outflow 133.0 mm, evapotranspiration 421.9 mm, and storage changes –2.9 mm. The main phase of the water cycle is the vertical part of the water balance and the value of evapotranspiration may be compared to that of Wielkie Torfowisko Batorowskie. After World War II, the Institute of Meteorology and Water Management established a wetland station in Grodek (the North-East part of Poland) where investigations were conducted on the upper boggy Suprasl catchment (Bortkiewicz, 1959).
The measurements were made in the period 1958-1963. The results of hydrological observations described by Mikulski and Lesniak (1975) indicate the big influence of the artificial drainage system, which caused the lowering of the groundwater level in the peatland. The same situation occurs in Wielkie Torfowisko Batorowskie, where a dense ditch network influences the functionality of the peatland resulting in decreasing active peat formation area. The role of Wielkie Torfowisko Batorowskie should rather be considered on a local scale, mostly because of its small dimensions. Nevertheless it is a very important element in the mentioned local scale. The conditions for peat accumulation and wetland formation in the Stolowe
Mountains are rather poor (Woronko, 1988). An object of surface water storage may equalise the hydrological processes, but only after land reclamation focussed on the problem of the ditch network.
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