Vodni bilance a hydrologicke charakteristiky povodi Lysina ve Slavkovskem lese v letech 1990-2018

Hydrologic patterns of a small (0.273 km²) forested catchment (Lysina) situated in the Slavkov Forest, western Bohemia, Czech Republic were studied intensively using 29 measured water years (November and October) from the period of 1990 to 2018. The Lysina catchment is underlain by acid-sensitive leucogranite and is covered by acidic soils (e.g. Podzols). The catchment, which is primarily covered by Norway spruce (Picea abies), was heavily acidified by atmospheric deposition of anthropogenic pollutants (Kopáček et al. 2016, Oulehle et al. 2017). Stream water at Lysina experiences highly acidic events during high flow periods (Hruška et al. 2015) and contains high concentrations of potentially toxic dissolved Al mobilized mainly from soils (Krám et al. 2009). This paper focuses on the evaluation of precipitation and runoff depths at daily, monthly and annual time steps in the study catchment. Lysina representes mean hydrological conditions of Czech forests catchments studied in the frame of the GEOMON network (Lamačová et al. 2014). Annual mean precipitation depth at Lysina was 964 ± 163 mm.yr^-1 and annual mean surface runoff was 423 ± 117 mm.yr^-1 (Tab. 1). The annual mean runoff coefficient was 0.44 and oscilated between 0.21 and 0.59. The runoff coefficient declined towards the end of the study period. Similar hydrologic trends were also observed in other catchments of Central Europe (Fendeková et al. 2018, Pekárová et al. 2018). Annual mean air temperature at Lysina in the study period was 5.8 °C. Annul mean temperatures increased during the study period by 0.039 °C.yr^-1 (p < 0,01, R² = 0.21). Annual mean runoff depths at Lysina exhibited decline of 3.4 mm.yr^-1 but were not statistically significant (p > 0.05, R² = 0.06). No trend in annual mean precipitation depths (R² = 0.00) was detected. Cummulative daily precipitation and runoff depths for individual water years show variable patterns (Tab. 1). Cummulative daily precipitation depth of the whole study period of 29 water years showed a slope of 2.7 mm.d^-1 (R² = 0.999); however, cummulative daily runoff depth showed two distinctive slopes, the first one in the period of 1990-2012 (1.3 mm.d^-1, R² = 0.999) and the second one in the period of 2013-2018 (0.8 mm.d^-1, R² = 0.981). Surface runoff at Lysina in the last six years was negatively influenced by increased air temperatures and subsequent higher balanced evapotranspiration. The largest recorded precipitation depth in the study period was detected in July 2001 (92 mm.d^-1) and precipitation depths between 60 and 70 mm.d^-1 were detected six times (Fig. 3c). The largest daily mean runoff depth was observed on 12th August 2002 (27.7 mm.d^-1), during catastrophic floods in the Czech Republic during 2002 (Fig. 2a, 4c). This value corresponds to a discharge value of 322 l.s^-1.km^-2. This particular hydrologic event was generated by two waves of steady rain in eight days; the first one with precipitation depth of 80 mm.d^-1 and the second one with 94 mm.d^-1. The largest monthly precipitation depths (Fig. 5) were recorded in July (median (Md) 125 mm.mo^-1) and June (Md 109 mm.mo^-1), the smallest ones in April (Md 50 mm.mo^-1) and February (Md 55 mm.mo^-1). The largest montly runoff depth (Fig. 6) was generated by snow melt in March (arithmetic mean 75 mm.mo^-1, Md 50 mm.mo^-1). The smallest runoff depth was observed in August (mean 17 mm.mo^-1, Md just 7 mm.mo^-1).

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