On remote sensed NDVI and soil moisture correlations

Abstract

This report investigates the drivers of the observed interannual variability in atmospheric CO2. It is known that much of that variability is caused by the response of the terrestrial biosphere to climatic variations, but the dominant mechanisms are still poorly understood. In a paper by Poulter et al.[1] a significant carbon sink anomaly is described. This anomaly of an approximately 57% increase in carbon uptake, was predominately caused by semi-arid regions. These anomalies point to the important role of the climate sensitivity of semi-arid ecosystems, which had received little attention until then. In order to explain the variability caused by the semi-arid regions we look at the limiting factor for these regions. The availability of water. To see whether or not limited water availability causes a high variability we compare soil moisture deviations with NDVI deviations. NDVI is a measure of the photosynthetic capacity of vegetation. Using ESA CCI Soilmoisture and GIMMS AHVRR NDVI data sets we found a positive Pearson correlation for deviations from the 10 year average of 2002-2012. This correlation was found to have a higher coefficient when a time lag was introduced. The highest coefficients were found when soil moisture data was related with NDVI data measured 16 days later. The areas that showed the highest correlation coefficients were of the semi-arid nature. These areas are thus the most sensitive to soil moisture anomalies and cause a high variability in photosynthetic capacity.