By Karin Kuipers (The Netherlands)
Drinking water companies lower the ground water level by extracting water. This reduces the soil moisture content in the root zone possibly resulting in drought stress for the vegetation. Drought stress results in a lower yield compared to crops which do not experience drought stress. Farmers are compensated by the water companies with drought damage settlements. The methods currently used to determine these settlements are not sufficient. Due to climate change the demand for alternative methods to estimate the drought damage settlements increases. This study investigated if the Normalized Difference Vegetation Index can contribute in determining the drought damage settlements.
The study areas chosen are the drinking water extractions Haarlo and Vessem. Haarlo is located in the East of the Netherlands and the main land use type is permanent grassland. Vessem is located in the south of the Netherlands with as main land use type temporary grassland. This latter study area also contains two fault lines. The main soil type in both study areas is Hn21 (veldpotzolgrond).
For both study areas quantile regressions, of the 0.9th quantile, have been applied on the monthly maximum NDVI values for 2013 against the distance to the drinking water extraction. Only the pixels with the main land use type on Hn21 have been selected. The trends found for the area inside and outside the influence of the drinking water extractions have been compared. For Haarlo the quantile regression was also performed for the NDVI against the drawdown. The same regressions were done with sub-study areas based on cardinal direction, soil type and, for Vessem, the fault lines. The temporal analyses consisted of comparing the NDVI though time for a pixel every half kilometre, determining if the number of grass cuts increased with distance from the extraction and if the recovery time after a grass cut decreased with distance.
A positive relation between the monthly maximum NDVI and the distance to the drinking water extraction was found for five out of seven months for Haarlo. In this study using sub-study areas was more suited for Vessem due to the presence of fault lines. This shows that, for the most suitable method depending on the distribution of the soil types and hydrological properties within a study area, NDVI data can be used to show relationships with the drinking water extraction.The temporal analyses were limited by the number of high quality images. Future development will enable the acquisition of a higher number of high quality images, making the method for determining the number of grass cuts and recovery time feasible.