Lecture 13 Precipitation Interception (2)

1
Lecture 13 Precipitation Interception (2)
Interception Estimation

• General Comments
• General Models
• Hortons Model
• Merrians Model
• Jacksons Model
• Gashs Model

2
General Comments 

• Models are generally simpler than measurements
• Many models are developed with different
  assumptions and for different applications
•  

3
General Model
Interception water loss equals precipitation less
throughfall (TF) and stemflow (SF)
I P TF SF  
I Interception P Precipitation above
vegetation canopy TF Throughfall SF Stemflow
4
Empirical Models
Interception loss can also be modelled as a
linear function of precipitation
I
I aP b  
b
P
I Interception loss P Gross
rainfall a Slope (empirical coefficient) b Int
ercept (empirical coefficient)  
5
Relationship between rainfall and interception
6
Empirical Models (Jackson, 1975)
It is a semi-empirical logarithmic model
I
a
P
I Interception loss P Average rate of
rainfall during event T Duration of
event a,b,c Empirical coefficients  
I
a
lnP
7
Interception model of Horton (1919)
Interception loss equals the combined losses
from        Intercepted water during
precipitation event Intercepted water in canopy
storage (evaporated later)
I Interception loss t Duration of
rainfall S Interception storage
capacity E Rate of evaporation of intercepted
water  
8
Interception model of Horton (1919)(Modified)
Hortons model has been improved with the
following model
I Interception loss t Duration of
precipitation t Time until canopy
saturation S Interception storage
capacity E Rate of evaporation of intercepted
water
9
Merriam (1960)
Used an exponential equation that considered
diminished interception storage with increasing
precipitation
S
time
I Interception loss S Interception storage
capacity P Gross precipitation E Average
evaporation rate during event T Duration of
precipitation event
10
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)