Summary:

Estimation of influence of land use changes to flood outflows from affor-estation areas of the Roztocze Środkowe was worked out basing on SCS-CN meth-od. This method permits calculation from total rainfall a direct runoff as equivalent to effective rainfall. The original SCS-CN method in principle was worked out for catchments of cultivated areas [National Engineering Handbook 1956, 1985]. Present paper is focused on the idea of adaptation original method to forest con-ditions [Okoński 2006; Okoński, Miler 2010]. The main parameter of this method - CN is a function e.g. of land use. The empirical values of parameter CNemp were calculated basing on hydro-meteorological data of flood outflows at representative catchment of the Krynica River in the Roztocze Środkowe, in the area of the Tomaszów Lubelski Forest District, the Bełżec Forest Range. The dependence be-tween empirical value of parameter CNemp and his equivalent's value from original method is a basis idea of adaptation method SCS-CN to characteristics of investi-gated catchment. In consequence it gives then possibility to prognoses how the change of land use, e.g. forest stand reconstruction, afforestation, deforestation, change of field culture, significant change in infrastructure etc., will influence to flood outflows (change to water retention) at a representative areas (a quasi-homogeneous in relation to investigated catchment). The analyzed catchment of the Krynica River (area 19.1km2, afforestation 49.3%) is representative for the Tomaszów Lubelski Forest Division and quasi-representative to the Roztocze Środkowe (Poland). The Tomaszów Lubelski Forest Division is located in south-east part of lubelskie province and north-east part of podkarpackie province. The area of investigated catchment is situated within the following units: macroregion - Roztocze, mesoregion - Roztocze Środkowe; this area belong to IV Mazowiecko-Podlaska Land. On area of the Tomaszów Lubelski Forest Division predominate the rusty soils (42.65%) as well as the brown soils (26.51%). Associations of beech forests and oak-hornbeam forests predominate. Pine is the dominant species, cov-ering 55.37% forested area, beech is found at 19.70%, oak at 11.07%, alder at 3.49%, fir at 3.24%, hornbeam at 1.73%, birch at 2.14% and larch at 1.19%, while other species constitute a highly valuable admixture in the biocenosis. On area of investigated catchment granulometric composition is following: loose sand 70%, loamy sand 15% and loamy sand on loamy gravel 15%. Average daily specific discharge (in investigation period 2009-2010) was on level 3.4 [l•s-1•km-2], and maximum to minimum suitably 17.2 to 1.0 [l•s-1•km-2]. The calculation was conducted using data of measuring rainy flood waves. The values of CNemp ware a base for introducing scenarios how changes of land use at investigation catchment are influencing to flood outflows (changing water retention).