1. Lecture 13 Precipitation Interception (2)
Interception Estimation
General Comments
General Models
Horton’s Model
Merrian’s Model
Jackson’s Model
Gash’s 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 = Intercept (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)
Horton’s 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) Most widely used model to date
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)