1. flux tower representativeness >croplands< news from the workbench Martin Wattenbach Astley Hastings Pete Smith

2. 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)

3. What is our population Corine 2000 Croplands: Area = 126.5 Mha in EU25 plus Norway and Switzerland

4. 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….

5. 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

6. 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)http://www- eosdis.ornl.gov/NPP/html_docs/npp_est.html NPP=f ( soil, crop, climate, management, altitude, slope…)

7. What is our sample

8. What is our population ?

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

10. 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

11. The 2003 anomaly

12. Statistical parameters Sample: mean annual NPP (2000-2004 ex. 2003) LSD (difference 2003 to mean NPP) Mean=0.3816 kgC/m2 year-1Mean=-0.0081 kgC/m2 year-1 Standard deviation=0.0957 kgC/m2 year-1 Standard deviation=0.0392 kgC/m2 year-1

13. Sample size n - sample size t - t value for 95% confidence and two samples s - standard deviation of the sample LSD - least significant difference

14. 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

15. HOWEVER….. This is only the mean !

16. What about spatial variance of the 2003 anomaly ?

17. Spatial variance for the current network

18. Possible Improvement ?

19. Spatial variance of the perfect grid

20. 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

21. Thank you for your attention Wheat Fields by Jacob Isaackszon van RUISDAEL

22. methods Nearest neighbour Semivariogram Covariance Predictive error If we like to predict spatial pattern how many points do we need What would we like to predict ? What is the optimum distribution T-test and f-test for different networks