The dry summer 2018 in Aachen and Düren
During the summer of 2018, weather in Central Europe was characterized by intense sunshine and low precipitation values. Consequently, the activity of the vegetation was considerably reduced; meadows became brown and many trees lost a substantial number of leaves.
A method to roughly quantify the liveliness of plants is the so-called Normalized Difference Vegetation Index (NDVI). This vegetation index is based on the reflection of electromagnetic waves from plants. It utilizes the ratio between the red (0.6 - 0.7 µm) and the near infrared band (0.7 – 1.3 µm). Active plants are using the red and blue light for photosynthesis while the green light is mostly reflected. In consequence, the reflection in the red and blue spectrum is low. Radiation in the near infrared band is also not used by plants due to the long wavelength. In summary, living plants, which are actively conducting photosynthesis, have a low reflection in the red spectrum and a high reflection in the near infrared spectrum. Consequently, living plants are shown in positive NDVI values, while surface without vegetation have low values or even negative values.
The first time-series used is based on MODIS data and shows the development in Germany, the Netherlands, Belgium and Luxembourg from May to August 2018. Vegetation was developing above average in May 2018, as shown by the green pixels. The situation changed during the course of the summer. In the following, the vegetation status was below the average of the years 2002 to 2017. At the beginning of August, low NDVI values were recorded especially in NW Germany, Belgium, the Netherlands and parts of eastern Germany. However, the maps also indicate a shift in the affected areas throughout the summer.
For a detailed spatial analysis of the area around Aachen, we utilized data from Landsat satellites with a higher spatial resolution of 30 m. Similar to the results from MODIS data, large parts of the region are affected by far lower than average NDVI values at the beginning of August 2018. However, there are some remarkable spatial differences. Especially low NDVI values are reached in the area around the village of Breinig and Walheim SE of Aachen. Here, meadows at elevated plains above the river level were especially affected. Low values were also obtained for the agricultural areas in the north (Jülicher Börde) and east (Zülpicher Börde) of the study area. In these areas, there are also patches with positive deviation from the long-term mean. The low values in red are related to weak development of the plants due to the dryness, while the positive green values are caused by either an earlier than average sowing of successive crops or long term changes in the usage of the land.
There are some special areas in the vicinity of Aachen and Düren which are related to the open pit mining in the area. East of Aachen positive values show the re-cultivation around the Blaustein Lake. Further to the east, a green band of positive NDVI values marks the new river bed of the river Inde. This river was artificially shifted to the west due to its interference with the open pit mining. The last image is also highlighting re-cultivation at the northern edge of the open pit mining area of Hambach.
The last image shows a comparison of the NDVI in the summer 2018 (left) and 2013 (right). Summer 2013 was an average summer with a vegetation status close to the long-term average, while the summer of 2018 is characterized by strong negative values.
Also for the summer of 2019, the MODIS data for some areas in Germany show a significantly worse vegetation development than the average of the years 2002 to 2017. However, the negative development is by far not as pronounced as summer 2018. What is particularly striking in 2019 is the drought phase in July, which affected Northern Germany in particular.