flux tower representativeness

>croplands<news from the workbench

Martin Wattenbach

Astley Hastings

Pete Smith

What does is mean: representativeness ?

• If we study a sample of individuals to represent a population we need to decide how many measurements or observations we need to make to get a result that we can be confident is representative of the population we are trying to study (Townend 2002)

What is our population

Corine 2000

Croplands:Area = 126.5 Mha in EU25 plus Norway and Switzerland

What are the criteria to characterise European croplands

• Soil type– SOC– Clay– …..

• Climatic region – Temperature– Climatic water balance…..

• Management– Crops – Fertilization– Tillage– Irrigation….

• Relief• Altitude• Groundwater level….

What we are really looking for:

• CarboEurope-IP aims to understand and quantify the present terrestrial carbon balance of Europe and the associated uncertainty at local, regional and continental scale.

• We need an integrator

Integrator NPP

• Net primary productivity (NPP) is defined as the net flux of carbon from the atmosphere into green plants per unit time. NPP refers to a rate process, i.e., the amount of vegetable matter produced (net primary production) per day, week, or year (http://www-eosdis.ornl.gov/NPP/html_docs/npp_est.html)

• NPP=f ( soil, crop, climate, management, altitude, slope…)

What is our sample

What is our population ?

What is our population (mean NPP 2000-2004 ex. 2003)

Are we taking enough samples ?

• The question can only be answered if we know what we need to detect !

• We need to define something like the least significant difference (LSD)

• One possible LSD could be the inter annual variability

• A special case of that variability is the detection of extreme events – e.g. the 2003 anomaly

The 2003 anomaly

Statistical parameters

Sample: mean annual NPP

(2000-2004 ex. 2003)

LSD (difference 2003 to mean NPP)

Mean=0.3816 kgC/m2 year-1 Mean=-0.0081 kgC/m2 year-1

Standard deviation=0.0957

kgC/m2 year-1

Standard deviation=0.0392 kgC/m2 year-1

Sample size

205.0,22 )(2LSD

stn n

n - sample sizet - t value for 95% confidence and two sampless - standard deviation of the sampleLSD - least significant difference

Do we have enough flux towers (23) ?

Number of samples with 50% power

n = 1004

Lower limits (based on standard deviation) = 31 – 74

Number of samples with 80% power

n = 2053

Lower limits (based on standard deviation) = 64 – 151

HOWEVER…..

This is only the mean !

What about spatial variance of the 2003 anomaly ?

Spatial variance for the current network

Possible Improvement ?

Spatial variance of the perfect grid

conclusions

• Our network might be not big enough to get a result that we can be confident is representative of the population (NPP change in Europe)

• The network might also not have the appropriate density to detect the spatial variability

• Upscaling simply based on the current network design and geostatistics might therefore not be possible

• Simulation models can help to bridge the gap because the “perfect grid” could be also partly virtual

• If we like to improve the network geoprocessing may help to find gaps in the network

• However the results are quite preliminary and final conclusions are not possible

Thank you for your attention

Wheat Fields by Jacob Isaackszon van RUISDAEL