Infiltration equations and index

Infiltration equations and index

• 1.
  as obtained frominfiltrometer is essentially observed to be decaying curve (max to min)  Some mathematical expressions to describe the shape of curve, given by various investigators are :- a) b) c) Horton’s equation Phillips equation holtans equation INFILTRATION CAPACITY RATE CURVE
• 2.
  ft= Infiltration capacity(inches/hour) f0=Initial infiltration capacity. fc= Minimum infiltration capacity. t = Time since the start of rainfall. k = Constant depending upon soil type & vegetable cover. Note : fc is direct dependent upon hydraulic conductivity.
• 3.
  b) Phillips equation: Here a = Minimum infiltration capacity. s = Initial infiltration capacity. c) holtans equation :  Here in above methods a & n are constants depends on soil moisture & vegetable cover F = ( afn p + fc )
• 4.
   For consistencyin hydrological calculations, a constant value of infiltration rate for the entire storm duration is adopted. The average infiltration rate is called the INFILTRATION INDEX.  The two commonly used infiltration indices are the following: o φ – index o W – index There are extremely used for the analysis of major floods when the soil is wet and the infiltration rate becomes constant.
• 5.
  rainfall volume =runoff volume(saturation).  The assum that all lo only. Φ For determ is unshaded above that l to the of surface runoff.
• 6.
   Φ –INDEX for a catchment, during a storm depends on  Soil type  vegetation cover  Initial moisture condition  Application – Estimation of flood magnitudes due to critical storms.
• 7.
  soil conditions inIndiaFor the producing for flood has found relationship  R = Runoff in cm from a 24 hr rainfall of intensity I (cm/hr).  α = Coefficient depends upon soil type.  In estimating maximum flood for design purpose , in absence of any other data , a Φ- index value of 0.10 cm/hr can be assumed storms (C.W.C) Φ = (I - R)/24 , R = (α X I 1.2 )
• 8.
   Thisistheaverageinfiltrationrateduringthetimewhenthe rainfallintensity>infiltrationrate. W-index=(P–R–Ia)/tf=(F/tf) where P= Total storm precipitation(cm) R = Total surface runoff(cm) I a = Depression and interceptionlosses (cm) tf=Time periodofrunoff(inhours)  The w- index is more accurate than Φ – index becauseit excludestheDepression & interception.
• 9.
   W-index isthe refined version of Φ – INDEX.  Initial losses I a are separated from total abstractions.  W-index = Φ–index I a  The accurate estimation of W-index is rather difficult to obtain hence Φ – index is most commonly used.  Since retention rate is very low both index W & Φ are almost same.
• 10.
  a) Surface flow b)Stem flow c) Evapotranspiration For a given storm, the interception loss is estimated as Ii = Si + Ki Et
• 11.
  Where   Si = Interception storage varies from 0.25 to 1.25 mm depending on the nature of vegetation   K i =Ratio of vegetal surface area to its projected area.   E t = Evaporation rate in mm/h during the precipitation.   t = Duration of rainfall in hours.