Which measurements can be taken to calculate the water balance in the nursery?

In the framework of the project work around the tree nursery in Schinznach, the water balance is to be measured and assessed. Therefore, certain values must be known, fortunately we already know some of the parameters of the water cycle. For example, the exchange with groundwater can be neglected, as only a certain quantity is taken and only something seeps away in the pond. Furthermore, there is no inflow in the form of a stream. The water for irrigation is taken from the groundwater and then preheated in the pond so that the plants do not get a shock. Therefore, it is known how much water comes into the system "tree nursery". What is not known is how much evaporates and how much remains in the canopy of leaves. According to that the following research is based on the question: Which remote sensing methods and indices are the easiest and most effective for measuring evapotranspiration?

NDVI (normalized difference vegetation index) is one of the most commonly used applications for UAS(Xue & Su, 2017). NDVI is easy to use as there are very light and high-resolution cameras UAS can be equipped with. It is defined as:

Because of the high reflectance of chlorophyll in the near infrared range this index is used to detect the plants greenness and vegetation cover in certain areas. One disadvantage of the NDVI is, that it doesn’t increase anymore above a certain biomass level, which is why more sensitive VI’s where developed like the EVI (enhanced vegetation index)(Xue & Su, 2017).

Another Index is the NDWI (normalized difference water index), it can measure the vegetation liquid water from space. It uses two narrow channels near 0.86 µm and 1.24 µm because these channels sense similar through vegetation canopy. NDWI is defined with the following formula:

 NDWI is a measure of liquid water molecules in the vegetation that interacted with solar radiation(Gao, 1996). Therefore it can be used to measure water stress and evaporation not only in the vegetation canopy but also in the soil in low depth(Nebo, Cesar & Richard, 2014). The disadvantage is that many UAS are not equipped with the necessary sensors and it can therefore not be used easily.

There are many more indices which build on the concept of NDVI and improve it in terms of error susceptibility and erroneous measurements, but they are all also a little more complicated and elaborate to process. Therefore, I recommend the NDVI as an index to measure the water balance in Schinznach. With the NDVI the vegetation cover can be measured relatively easily. If you now know which kind of species the vegetation consists of, you can find out how much this plant transpires and calculate the transpiration rate. Since NDVI is sensitive to the effects of soil brightness, soil colour, and leaf canopy shadow it needs to be calibrated with ground data(Glenn, Huete, Nagler & Nelson, 2008). To measure the evaporation, it would be useful to see if it is possible to get satellite data for the region using NDWI. Collecting our own data may turn out to be difficult because the necessary sensors may not be available.

Sighted Literature:

Gao, B. (1996). NDWI—A normalized difference water index for remote sensing of vegetation liquid water from space. Remote Sensing of Environment, 58 (3), 257–266. https://doi.org/10.1016/S0034-4257(96)00067-3

Glenn, E. P., Huete, A. R., Nagler, P. L. & Nelson, S. G. (2008). Relationship Between Remotely-sensed Vegetation Indices, Canopy Attributes and Plant Physiological Processes: What Vegetation Indices Can and Cannot Tell Us About the Landscape. Sensors (Basel, Switzerland), 8 (4), 2136–2160.

Nebo, J., Cesar, G. & Richard, B. (2014, April). Validation of remotely-sensed evapotranspiration and NDWI  using ground measurements at Riverlands, South Africa.

Xue, J. & Su, B. (2017). Significant Remote Sensing Vegetation Indices: A Review of Developments and Applications. Journal of Sensors. https://doi.org/10.1155/2017/1353691