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IZLAGANJE NA ZNANSTVENOM SKUPU - PRESENTATION AT THE INTERNATIONAL SYMPOSIUM Šumarski list - SUPLEMLNT (2005), 195-201 UDK 630* 116 UPORABA METODE CN-KRIVULJA ZA PROCJENU UTJECAJA ŠUMA NA POVRŠINSKO OTJECANJE APPLICATION OF THE METHOD OF RUNOFF CURVE NUMBERS TO EVALUATE THE EFFECT OF FOREST ON SURFACE RUNOFF Miloslav JANEČEK* SAŽETAK: Šuma općenito predstavlja vrlo djelotvoran biljni pokrov. Ona pojačava retencijski kapacitet vode nekog sliva i štiti tlo od erozije. Međutim, zbog ekstremne varijabilnosti hidroloških osobina dubljih slojeva tla, a posebice površinskog dijela (listinac), šuma ponekadprouzročuje znatne količine površinskog otjecanja, koje, između ostalog, dovode do erozije tla. Kao sredstvo za procjenu mogućeg otjecanja iz šumskog sliva do 10 km može se preporučiti CN-metođa, a posebice nedavno izrađen HydroCad model http://www.hydrocad.net. koji se temelji na ovoj metodi. Općenito govoreći, šumske se sastojine smatraju najboljim tlom koje pridonosi smanjenju površinskog otjecanja i erozije. Ovo se djelovanje općenito temelji na sljedećem: Zaštita tla od razornog djelovanja jakih kiša; Podrška vodne infiltracije u tlo; Poboljšanje konzistencije tla; Smanjenje prijenosne snage vode i akumulacije površinskog otjecanja; Zaustavljanje površinskog otjecanja i ispranog tla. Tablica 1. Vrednovanje površinskog sloja humusa Table 1 Evaluation of the surface layer of humus Površinski sloj humusa - Surface layer of humus Zbijeno -Compacted Srednje zbijeno - Moderately compacted Meko ili rahlo - Mellow or loose Primijene li se odgovarajući uzgojni zahvati u šumi, šumsko tlo postaje porozno, a njegov kapacitet infiltracije i akumulacije je velik. Za određivanje hidroloških uvjeta bilo koje šume, potrebno je poznavati hidrološku grupu tla, sloj listinca, humusni sloj i njegov tip. Miloslav Jancček, Istraživački institut za kultiviranje i zaštitu tla, Prag-Zbraslav, Žabovreska 250, 156 27 Praha 5 - Zbraslav, Czech Republic, telefon: 00420 257 921 497 e-mail: janecek@vumop.cz Treba napomenuti, da sve šume ne ispunjavaju potpuno ovu funkciju kako je iskazano vrednovanjem šumskih sastojina metodom krivuljnih brojeva (CN) (Kent-1971 etal.) Hidrološki uvjeti šumskih sastojina uglavnom se odnose na gustoću pokrova — vegetacije, listinca, itd. Naravno, lišće, iglice, grančice, kora i ostali otpad vegetacije na šumskom se tlu raspada i stvara listinac iz kojega nastaje sloj šumskog tla. Vrednovanje šumskog tla metodom CN izvodi se kako slijedi: Razred zbijenosti - Compaction class 1 2 3 Prema metodi CN, postoje četiri hidrološke grupe tala na temelju minimalne stope infiltracije u tlo bez pokrova nakon dugoročne zasićenosti. Ukoliko je sloj listinca niži od 1 cm, šumsko tlo se ne smatra zaštićenim, a razred hidroloških uvjeta je umanjen za koeficijent 0.5. Razlike u hidrološkim uvjetima općenito se izražavaju za šume prema metodi CN - vidi Tablicu 3. |
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M. Janeček: UPORABA METODE CN-KRIVULJA ZA PROCJliNU UTJECAJA ŠUMA Šumarski list - SUPLEMENT (2005), 195-201 Tablica 2. Hidrološka grupa tala Table 2 Hydrological groups of soils Grupe Opis hidroloških karakteristika Groups Description of hydrological characteristics A Tla s visokom stopom infiltracije (>0.12 mm . min"1) također pri punoj zasićenosti, sadržavaju uglavnom duboke pijeske ili šljunke s dobrom ili prekomjernom drenažom. A Soils with high infiltration rate (> 0.12 mm x min ) also at full saturation, comprising mostly deep sands or gravels with good or excessive drainage. B Tla srednje stope infiltracije (0.06-0.12 mm x min" ) također pri punoj zasićenosti, sastoje se uglavnom od srednje dubokih ili dubokih tala, sa srednjom ili dobrom drenažom, ilovasto-pjeskovitim ili glinasto-ilovastim tlima. B Soils with medium infiltration rate (0.06- 012 mm x min ) also at full saturation, comprising mostly medium-deep or deep soils, with medium or good drainage, loamy-sandy or clayey-loamy soils. C Tla s niskom stopom infiltracije (0.02 - 0.06 mm x min"1) i pri punoj zasićenosti, sastoji se uglavnom od tala s niskim propusnim slojem u profilu tla, te glinasto-ilovastih ili glinenih tala. C Soils with low infiltration rate (0.02 - 0.06 mm x min ) also at full saturation, comprising mostly soils with a low permeable laxer in the soil profile and clayey-loamy or clayey soils. D Tla s vrlo niskom stopom infiltracije (<0.02 mm x min"1) i pri punoj zasićenosti, sastoji se uglavnom od gline s visokim bubrenjem, tla s trajno visokom razinom podzemne vode, tla sa slojem gline na površini ili blizu površine, te plitka tla nad gotovo nepropusnim podslojem D Soils with very low infiltration rate (< 0.02 mm x min ) also at full saturation, comprising mostly clays with high swelling, soils with permanently high groundwater level, soils with a clay layer on the surface or closely under the surface and shallow soils above the almost impermeable subsoil. 100 15 10 2 3 ~a II 5 1 -1 & Cj 0 - Razred hidroloških uvjeta šume Class of forest hydrological conditions Slika 1. Nomogrami za određivanje broja krivulje otjecanja (CN) u odnosu na grupu tla i hidrološke uvjete šume Figure 1 Nomograms for determination of runoff curve number 2 3 4 5 (CN) in relation to the soil group and forest hydrological Razred hidroloških uvjeta šume conditions Class of forest hidrological conditions Tablica 3. Hidrološki uvjeti šume izraženi pomoću CN Table 3 Hydrological conditions of the forest expressed by CN Hidrološki uvjeti šume Hydrological conditions of forest Broj krivulje otjecanja CN za hidrološku grupu tla Runoff curve number CN for hydrological groups of soils A B C D Loši -Bad 45 66 77 83 Srednje-dobri Medium- good 36 60 73 79 Dobri Good 30 55 70 77 Gornje vrijednosti brojeva krivulje otjecanja opisuju potencijal otjecanja iz sliva — vidi grafikon i SI. 2. |
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M. Janeček: UPORABA METODE CN-KRIVULJA ZA PROCJENU UTJECAJA ŠUMA . Šumarski list - SUPLEMENT (2005). 195-201 100 120 100 90 80 Fl? E 80 H 60 s Y 70 a 40 60 20 50 40 20 40 60 80 100 120 Zbroj oborina (mm) Precipitation sum (mm) Slika 2. Odnos izravne dubine otjecanja (Hr) s količinom oborina (Hp) i brojevima krivulje otjecanja (CN) Figure 2 The relationship of direct runoff depth (Hr) with precipitation amount (HJ and runoff curve numbers (CN) Gornji odnosi temelje se na pretpostavci daje omjer volumena otjecanja i količine bujica izazvanih kišom jednak omjeru volumena intercepcije za vrijeme otjecanja s potencijalnim volumenom, kojega bi prihvatilo tlo i pokrov. Otjecanje obično dolazi nakon neke akumulacije ili kiše, što znači nakon početnog gubitka koji je zbroj intercepcije, infiltracije i površinske akumulacije procijenjene na količinu od 20 % potencijalne retencije (prema pokusnim mjerenjima). Ova jednadžba izvedena je iz gore navedenih odnosa: (Hp-0.2A)2 H [mm] Hp > 0.2A (H+0.SA) gdje su Hr = izravno otjecanje (mm) Hp = količina bujice od kiše, obično maksimalno tijekom 24 sata (mm) A = potencijalna retencija (mm) izražena brojem krivulje kao 1000 A = 25.4 C7V-10 Osim hidroloških osobina tla i sustava uporabe zemljišta, otjecanje ima posebnu vezu sa sadržajem vlage u tlu. Tri su situacije primijenjene u metodi CN: tlo je jako suho ili srednje natopljeno, ili prekomjerno natopljeno s vodom u odnosu na pojavu prethodne kiše. Kao što je dokumentirano grafikonom odnosa između brojeva krivulja otjecanja CN i hidroloških grupa tala, razred hidroloških uvjeta šume i stupanj zasićenosti tla, djelovanje šume na smanjenje površinskog otjecanja i na eroziju može biti značajno drukčije (Janeček -2002). |
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M. Jancček: UPORABA METODE CN-KRIVULJA ZA PROCJENU UTJECAJA SUMA . Šumarski list - SUPLEMENT (2005), 195-201 uv .´$ 7 7 / , / yu / 80 / / / / /U / / r / 60 03 / "S f .2, § / Y DU ´ / / / J2^ / / & 40 CQ / / JU / s ~7 / / s 20 / / / / 1U / / *r ^ A ^ 0 10 20 30 40 50 60 70 80 90 100 Broj krivulje otjecanja (CN) za IPS II Runoff curve number (CN) for IPS II Slika 3. Utjecaj sadržaja vode u tlu (IPS - suho tlo, IPS II - srednje zasićeno tolo, IPS III zasićeno tlo) na promjenu u broju krivulje otjecanja (CN) Figure 3 The influence of water content in the soil (IPS-city soil, IPS II -medium-satura-ted soil, IPS III -saturated soil) on a change in the runoff curve number (CN) Gornji grafikon ilustrira utjecaj zasićenosti na proputa više, toliko je puta propusnije tlo. Ova činjenica mjene vrijednosti brojeva krivulja otjecanja CN, npr. bila je iznenađenje tijekom zadnje poplave u Češkoj, CN = 30 će se smanjiti na CN = 15 na suhim tlima, a kada su iznimno velike količine vode istekle iz šumpovećati će se na CN = 50 na natopljenim tlima. skih kompleksa, jer je tlo bilo natopljeno prethodnim Slika 4 prikazuje utjecaj promjena u krivuljama velikim kišama. otjecanja CN, te opisuje hidrološke karakteristike šumskih zemljišta na početku površinskog otjecanja. Grafikon prikazuje količine oborina na početku površinskog otjecanja. Ako je, npr. CN = 30, količina oborina mora prelaziti 120 mm (da bi uzrokovala površinsko otjecnaje), dok je ono samo 10 mm pri CN = 80. On također određuje kapacitet tla i pokrova za zadržavanje vode pri danim hidrološkim karakteristikama opisanim brojevima krivulja otjecanja CN. Iako šuma može obično prihvatiti 4-5 puta veću količinu vode nego ista površina bez vegetacije, potrebno je uzeti u obzir da nakon prethodnih kiša pri punoj zasićenosti tla izravno se otjecanje iz šume povećava više puta nego s površine bez vegetacije: koliko |
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M. Janeček: UPORABA METODE CN-KR1VULJA ZA PROCJENU UTJECAJA ŠUMA . Šumarski list SUPLEMENT (2005), 195-201 [mm] ´ 1-KD / / / / SO. -po Qci 35. 4a SO aa o B>G Slika 4. Količina oborine kao početak izravnog otjecanja u uvjetima opisanim brojem krivulje otjecanja CN Figure 4 The precipitation amount that is the incipience of direct runoff in conditions described by a runoff curve number CN ZAKLJUČAK - Conclusion Šuma općenito predstavlja vrlo djelotvoran biljni Za procjenu mogućeg otjecanja iz šumskog sliva do pokrov. Ona pojačava retencijski kapacitet vode nekog 10 km2, preporučuje se metoda CN, a posebice nedavno sliva i štiti tlo od erozije. Zbog ekstremne varijabilnosti izrađen model HydroCad http://www.hydrocad.net.. hidroloških osobina dubljih slojeva tla, posebice listin-koji se temelji na ovoj metodi. ca, iz šume ponekad otječu velike količine površinske vode, koje dovode do erozije tla. LITERATURA References K e n t, K. M. - et al, 1971: Hydrology Section 4, Na-Janeček, M., 2002: Ochrana zemedelske pudy pred tional Engineering Handbook SCS. Washington erozi. ISV Praha, s. 201. D. C, USDA, s. 350. |
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PRESENTATION AT THE INTERNATIONAL SYMPOSIUM Šumarski list - SUPLEMENT (2005), 195-201 APPLICATION OF THE METHOD OF RUNOFF CURVE NUMBERS TO EVALUATE THE EFFECT OF FOREST ON SURFACE RUNOFF Miloslav JANEČEK* SUMMARY: The forest, in general, can be regarded as a very efficient vegetation cover. It enhances the water retention capacity of a catchment and protects the soil against erosion. However, because of the extreme variability of hydrological properties of underlying soils, especially their uppermost layers (the forest litter), the forest may sometimes generate a considerable amount of surface runoff causing, among other impacts, soil erosion. The CN´-method and, in particular, the recently developed HydroCAD model (http://www.hydrocad.net), which is based on this method, can be recommended as tools for estimation of probable runoff from the forest catchments up to 10 km . Key words: forest, retention capacity, surface runoff, hydrological con ditions In general, forest stands are considered as the best soil cover markedly contributing to a reduction of surface runoff and erosion. These effects are generally based on: protection of soil from destructive effects of incident raindrops support of water infiltration into the soil improvement of soil consistence reduction of the transporting power of water and surface runoff accumulation retardation of surface runoff and washed soil. It is to note that not all forest stands fully fulfil this function as indicated by the evaluation of forest stands by the method of curve numbers (CN) (Kent - 1971 et al.). Hydrological conditions of forest stands are mainly related with the density of the cover - vegetation, litter, etc. Naturally, leaves, needles, twigs, bark and other residues of vegetation on the forest soil are decomposed to form litter from which the layer of forest floor originates. Evaluation of the forest floor by the CN method is as follows: * Miloslav Janeček, Research Institute for Soil and Water Conservation Praha, Žabovreska 250, 156 27 Praha 5 - Zbraslav, Czech Republic, phone: 00420 257 921 497 e-mail: janecek@vumop.cz If appropriate cultural practices are applied in the forest, the forest floor is porous and its infiltration and accumulation capacity is high. To determine hydrological conditions of any forest it is necessary to know the hydrological group of soil, litter layer, humus layer and its type. According to the CN method there exist four hydrological groups of soils on the basis of minimum rate of infiltration into the soil without cover after long-term saturation. The class of hydrological conditions is determined according to the layer of litter or forest floor and its compaction. If the litter layer is lower than 1 cm, the forest floor is not considered as protected and the class of hydrological conditions is reduced by the coefficient 0.5. Differences in hydrological conditions are generally expressed for forests by the CN method - see Table 3. The above values of runoff curve numbers describe the potential of runoff from the watershed - see the graph in Fig. 2. The above relationships are based on an assumption that the ratio of runoff volume to the amount of torrential rain equals the ratio of the volume of water intercepted during runoff to potential volume that may be intercepted by the soil and cover. |
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M. Janeček: APPLICATION OF THE METHOD OF RUNOFF CURVE NUMBERS TO EVALUATE Šumarski list SUPLEMENT (2005), 195-201 Runoff usually originates after some accumulation of rainfall, that means after an initial loss that is the sum of interception, infiltration and surface accumulation that was estimated to amount to 20 % of potential retention (by experimental measurements). This equation was derived from the above-mentioned relationships: (//„-0.2A)2 Hr = - [mm] Hp > 0.2A (#,,+0.8/1) where Hr= direct runoff (mm) H = amount of design torrential rain, usually maximum over 24 hours (mm) A = potential retention (mm) expressed by the curve number as (CN-10) Besides the hydrological properties of soils and systems of land use runoff is particularly related with soil moisture content. Three situations are applied in the CN method: the soil is either quite dry or medium saturated or oversaturated with water in relation with the occurrence of preceding rainfall. As documented by the graph of the relationship between runoff curve numbers CN and hydrological groups of soils, class of forest hydrological conditions and saturation degree of soil the effects of the forest on a reduction of surface runoff, and on erosion, may be markedly different (Janeček - 2002). The above graph illustrates the influence of saturation on changes in the values of runoff curve numbers CN, e.g. CN = 30 will decrease to CN = 15 on dry soils and it will increase to CN = 50 on saturated soils. Fig. 4 shows the influence of changes in runoff curve numbers CN, describing hydrological characteristics of forest lands, on the incipience of surface runoff. The graph shows the precipitation amounts resulting in the incipience of surface runoff. If e.g. CN = 30, the precipitation amount must exceed 120 mm (to cause surface runoff) while it is only 10 mm at CN = 80. It also determines the water-retaining capacity of soil and its cover for the given hydrological characteristics described by runoff curve numbers CN. Even though the forest can usually intercept a 4-5 times higher amount of water than the same area without vegetation, it is necessary to take into account that after preceding rains at full saturation of soil direct runoff from the forest increases more times than from the area without vegetation: the more times the more permeable is the soil. This fact was surprising during the last flood in Bohemia when unusually large amounts of water ran off forest complexes because the soil was saturated with preceding heavy rains. CONCLUSION In general, forests arc a very efficient soil cover increasing the watershed retention and protecting the soil from erosion. As the range of hydrological characteristics of soils, and especially of their cover- litter is large, it is necessary to be aware of the fact that large amounts of water may leave the forest in the form of surface runoff, causing also soil erosion. To estimate potential runoffs from the forest in a watershed maximally 10 km´ in size we can recommend to use the CN method or a recently derived model on the basis of this method HydroCAD http://www.hydrocad.net. |