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