2precipitation characteristic

7/23/2019 2precipitation Characteristic

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Hydrology I

Jozsef Szilagyi, Professor of HydrologyDepartment of Hydraulic and Water

Resources EngineeringBudapest Uniersity of !ec"nology andEconomics


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#ecture $%& Water 'alance and precipitation( Deriation of t"e 'asic )ater*'alance e+uation& t"e integral form

of t"e continuity e+uation states

d-./d.01dt 2 -3/d3 4 5 60

)"ere / is density not a constant0, 74u,,)08 is elocity9 !"e firstterm is a olume .0 integral, t"e second is a surface 30 one9 d3is t"e scalar product of and t"e surface unit normal ector9 60states t"at t"e mass )it"in t"e control olume c"anges in timeonly if t"ere is a difference in t"e in* and outflo) flu:es across t"e

surface of t"e control olume9 ;at/ 2=/08d. 4 5

)"ic" is alid for any olume, t"us

>t/ 2=/0 4 5 %0

)"ic" is t"e differential form of t"e continuity e+uation9 ?or an

incompressi'le fluid / is constant, t"us %0 'ecomes = 4 59


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( Returning to 60, for an incompressi'le fluid one can )rite

/d-.6d.01dt 2 /-3d3 4 5

i9e9, d.1dt 2 -3d3 4 5

!"e second term is positie for outflo)s, so it can 'e )ritten as @t0A It0, )"ere @ is out, I is t"e inflo) rate in time9 In "ydrology t"estored )ater olume, ., is often denoted 'y S, called t"e storage, soone can )rite t"e lumped ersion of t"e continuity e+uation as

dS1dt 4 I A @9

Precipitation c"aracteristics* Incremental and cumulatie rainfall "yetograp"s* Rainfall duration, dept", and intensity

;easurement of precipitation at a point* Simple essel& P 4 .13, )"ere 3 is t"e area of t"e opening orifice0* ylinder )it" a mm scale* Weig"ing*recording gauge

* !ipping*'uc


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( ;easurement of sno)& it is melted and )eig"ed( Ce)& optical precipitation gauges, 'ased on 'eam attenuation

emitter receier

orrupting factors in precipitation measurements( 60 Frifice size s"ould not 'e too small0( %0 Frifice "eig"t optimal is ground leel0

( G0 Wind s"ielding


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0 Distance to o'structions 0 Splas" and eaporation

*deep gauges )it" narro) inlet from collectorto )eig"ing essel*nonolatile immisci'le oil in


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Estimation of aerial aerage P0 precipitation& !"iessen polygons

P 4 QiPi3i1 Qi3i

P6

36

P%

3%

3G 3

3

3PG

P

P

P

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?or engineering design, t"e Ddf Dept"*duration*fre+uency0 andIdf Intensity*duration*fre+uency0 cures are ery useful

!"e fre+uency refers to t"e aerage return period RP0 of a precipitation eent )it" apredefined duration0 t"at "as a total dept" or larger t"an a certain alue9 E9g9, t"e

6*year return period rain eent means t"at suc" a dept" for a gien duration0 orlarger can 'e e:pected eery year, i9e9, 655 times out of a 655*year record of annualma:imum precipitations9 !"us, a %*year RP eent occurs eery ot"er year, or e+ually,5 times out of a 655*year record, t"e *year RP eent occurs eery t" year, ore+ually, %5 times, and so on9 !"e 655*year RP eent "appens only once in eery655 years on aerage9

Rain intensity is defined as dept" diided 'y duration9

W"at is t"e corresponding

Idf cure Ho) do youo'tain t"e t)o grap"s Uset"e alues in !a'le *N9

Home)or< or la'6&


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2precipitation characteristic

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