Experimental design for measuring and calculating evapotranspiration in the tree nursery

Due to the small timeframe for sampling in the tree nursery in Schinznach a well structured sampling design and time management plan is of great importance. On site plant specific Transpiration can be measured by a weighing experiment and the output of this measurement will be used to calculate crop specific Evapotranspiration by the help of a temperature based calculation model which will be explained below.

Weighing Experiment

·      The largest crops in the tree nursery will be class divided by their size on one hand and whether they are coniferous or broadleaved woody plants on the other hand.

-       The crops will be divided by size in two classes (>0.5 meter, <0,5 meter).

-       Thuja occidentalis and Taxus baccata could be well fitting representatives for the coniferous plants, Prunus laurocerasus for the broadleaved woody plants.

·      Five plants of every class are going to be labelled and weighed individually in the morning and in the evening. In the meantime, they`re going to be put back in the cultivating area to make sure they have the same conditions like the whole cultivation area.

·      Exclusion criterias are covered cultivation areas, Plants weighing more than 20kg including the pot (due to the limit of the scale) and Plants growing on non drained areas

material:

-       Scales, small and big (up to 20kg)

-       Adhesive tape and waterproof marker for the labelling (different colours)

-       Hand truck for transporting big plants

-       Cable reel

Evapotranspiration calculation:

·      The measured values for crop-specific Evapotranspiration are going to be used to adjust the reference crop coefficients. If there are for any reasons crops who can’t be measured, values from literature have to be used.

·      Due to the insufficient or missing radiation data of the tree nursery, the evapotranspiration will be calculated by using a temperature based model such as the “Blaney – Criddle” model will be used. The required data for the calculation will be provided by the measuring station “Campus Grüental”. Only the temperature data (T_mean ) will be taken from the measuring station of the tree nursery.

·      The evaporation of the pond will be calculated by using literature-values for water bodies, as there are not big differences in different water bodies.

Blaney – Criddle equation

ET_o = p ·(0.457·T_mean + 8.128)

 Where:

-       ET_o is the reference evapotranspiration [mm day⁻¹] (monthly)

-       T_mean is the mean daily temperature [°C] given as T_mean = (T_max + T_min )/ 2

-       p is the mean daily percentage of annual daytime hours.