Lecture 13 Precipitation Interception (2)

Interception Estimation• General Comments• General Models• Horton’s Model• Merrian’s Model• Jackson’s Model• Gash’s Model

General Comments 

• Models are generally simpler than measurements

• Many models are developed with different assumptions and for different applications

 

General Model

I = P – TF – SF 

I = InterceptionP = Precipitation above vegetation canopyTF = ThroughfallSF = Stemflow

Interception water loss equals precipitation less throughfall (TF) and stemflow (SF)

Empirical Models

I = aP + b  

I = Interception lossP = Gross rainfalla = Slope (empirical coefficient)b = Intercept (empirical coefficient) 

Interception loss can also be modelled as a linear function of precipitation:

I

P

b

Relationship between rainfall and interception

Empirical Models (Jackson, 1975)

I = Interception lossP = Average rate of rainfall during eventT = Duration of eventa,b,c = Empirical coefficients

 

It is a semi-empirical logarithmic model:

I

P

aTcPbaI lnln

_

I

lnP

a

Interception model of Horton (1919)

I = Interception losst = Duration of rainfallS = Interception storage capacityE = Rate of evaporation of intercepted water 

Interception loss equals the combined losses from:       Intercepted water during precipitation event

Intercepted water in canopy storage (evaporated later)

t

SEdtI0

Interception model of Horton (1919)(Modified)

I = Interception losst = Duration of precipitationt’ = Time until canopy saturationS = Interception storage capacityE = Rate of evaporation of intercepted water

Horton’s model has been improved with the following model:

'

'0

t t

tSEdtEdtI

Merriam (1960)

I = Interception lossS = Interception storage capacityP = Gross precipitationE = Average evaporation rate during eventT = Duration of precipitation event

Used an exponential equation that considered diminished interception storage with increasing precipitation

TES

PSI

exp1

time

S

Gash model (1979)

• A storm-by-storm accounting of interception loss• Most widely used model to date• Relies on several simplifying assumptions: (1) Rainfall represented by discrete storms and drying periods (2) Meteorological conditions constant during storms and canopy wetting (3) No drip from canopy during wetting (4) Canopy storage is perfectly saturated shortly after precipitation event

Should read Chapter 3.6 to understand the principles (no need to memorize the equations)