1. Loge or not Loge? Some Questions with Answers about Soil Properties
Talk by Imtiaz Dharssi
September 2013
Based on work by
Dharssi, Verhoef, Vidale, Macpherson, Jones and Best
Met R&D Technical Report 528, 2009.

2. What Are Soil Properties?
Soil Properties Affect:
The ability of vegetation to extract soil water
The thermal properties of the soil and consequently the ground flux
The vertical movement of soil water and runoff
In UM jargon the soil properties are called the soil ancillaries
They are assumed to be determined by the soil texture
Time independent

3. Soil Properties Control Evaporation
Eveg ≈ Epotential x Soil_Moisture_Availability
Soil Moisture Availability
Soil Moisture
Zero
One
Wilting Point
Field Capacity

4. Once Upon A Time
1984: A paper is born! (Cited 838 times according to GS)

5. Cosby paper Uses Log Functions

6. A New Land Surface Model
Late 1990s: The UK Met Office develop a new land surface model
Met Office Surface Exchange Scheme (MOSES)
The Cosby equations are used to calculate the soil hydraulic properties
The new model assumes that the log functions used by the Cosby paper are natural log functions (i.e. Loge).

7. Why are UM soil properties so Different from ECMWF?
Much later: ~ 2006 UM
ECMWF
Field Capacity for Medium soil texture (m3/m3)
0.24
0.32

8. Do Cosby mean Loge or Log10?UM
Assume log≡Loge
Assume
log ≡ Log10
ECMWF
Field Capacity for Medium soil texture (m3/m3)
0.24
0.33
0.32

10. Cosby used Log10 not Loge From CH78: Ψ (cm) Loge Ψ Log10 Ψ From Cosby:
Silt Loam
78.6
4.4
1.9
Silty Clay
49.0
3.9
1.7
1.5

11. How we Tested the Bug Fix
Testing performed by scientists at:
Reading University, Climate Model
Met Office, Weather forecasting model
Hadley Centre, Climate Model
Centre for Ecology and Hydrology, Land Surface Model

12. Impact of Bug fix
All the tests showed a consistent and surprising result:
Model Soil moisture increased significantly
Model Evaporation decreased significantly
The bug fix significantly improved estimates of evaporation in the off-line land surface model
The bug fix made the NH summer warm bias worse

13. Soil Moisture Increases But Evaporation Decreases! How?
Soil Moisture Availability
Soil Moisture
Zero
One
Wilting Point
Field Capacity
Evaporation doesn’t depend on the absolute value of soil moisture!
Soil Moisture increased but the Soil Moisture Availability decreased!
The new Wilting point and Field capacity values are much higher
Crucially, after the bug fix the Field capacity minus Wilting point values increased significantly

14. Don’t Panic! The Bug Fix made the NH summer warm bias worse!
So we created a package of changes, including the bug fix, that significantly improved model performance as well as improving the science
most notably an improved parameterisation of soil thermal conductivity

15. Operational Verification of T2m T+72 RMS Error
Implementation

16. Conclusions: The small picture
We found a bug
We fixed it
After a lot of work, everyone was happy

17. Conclusions: The BIG picture
The soil properties are as important as soil moisture
Soil properties have a significant impact on the soil moisture dynamic range
Soil properties have a significant impact on evaporation
We should work with soil scientists at CSIRO (e.g. ASRIS) to improve the UM soil properties for Australia
The vertical variation of soil texture is ignored by most models but might be very important