NDWI – Agronomic performance

The NDWI (“Normalized Difference Water Index”) presents a strong association with the plant leaves water content. The NDWI responds faster than NDVI (Normalized Difference Vegetation Index) to changes in water availability. If the water content in the leaves decreases, the NDWI detects this variation faster than the NDVI, because plants spend a lot of energy creating chlorophyll and do not give it up quickly. Plants begin to divest in chlorophyll production only after maintaining the water deficit for a long period.
For perennial crops, NDVI variations depend on the number of plants per hectare, distance between lines and on the line, variety, etc. Normally, NDVI values are lower, because chlorophyll concentration in these crops is also lower, however the sprouting is also affected by differences in soil characteristics or by the pesticides placed on a fixed rate basis, etc.
NDWI can be very important to promote controlled water stress. The technical complexity of water management in vineyards is far superior to other crops because water stress is usually induced to obtain a higher productivity/quality ratio. Such decisions will depend on the company’s business model, that is, if they want more productivity, more quality or both. In this sense, NDWI and NDVI can be very interesting indicators about nutrition (NDVI in a perspective of more or less production), and about controlled water stress (NDWI from a higher or lower quality perspective).
Figure shows on the left side the average NDVI in August of several Tempranillo parcels of the same vineyard in two consecutive years (2017 and 2018) and on the right, the average NDWI for the same period and parcels. Mean values of NDVI and NDWI are different between the two consecutive years and in each year there are plots with different behaviors. If the company’s objective is to produce quantity, they may have to increase nutrition and water on the plots with lower NDVI and NDWI values to increase vegetative vigor and productivity. If the goal is to produce quality, they have to reduce nutrition and water in the plots with higher NDVI and NDWI to reach the desired quality. However, if they want both, they have to give more nutrition and water to the plots that naturally already have higher NDVI and NDWI values and reduce nutrition and water in those that already have lower NDVI and NDWI values.