Regionalization of the apple tree water balance based on modified agrometeorological model

The agrometeorological model named AVISO is developed and operated at the Czech Hydrometeorological Institute as a tool for calculation the evapotranspiration and soil profile water content. The model uses modified evapotranspiration equation developed by Penman and Monteith. The basic calculated surfaces are water level, bare soil, grass and a few field crops. As part of the research project, some AVISO model procedures and canopy parameters were modified for usage in fruit orchard. Data and observation from apple, cherry and apricot orchards from 2020 to 2022 were used to set up the model and to validate the model results. By determining the hydrolimits of the soil and the characteristics of the root zone of the trees, the water capacity of the model soil profile was estimated. The AVISO model results were compared with measured soil moisture, which was converted to the average soil profile saturation determined in percentage of available water capacity. When tuning the model, emphasis was placed on the decreasing part of the soil water content curve interpreting the intensity of evapotranspiration. In many iterations of the calculation, the results of the model were compared with the measurements using the correlation coefficient, the average bias and the mean absolute error. The average model-measurement correlation values for months in the growing season (April–September) in 2020 and 2021 were between 0.30 and 0.99. The bias error values were between -13.14 and +38.31 mm. The second comparison option was to use the precipitation sums for time periods determined by the almost identical moisture at the beginning and the end of the period. The amount of precipitation in the given section thus theoretically coincided with the sum of evapotranspiration. Correlations of model evapotranspiration and evapotranspiration calculated on the basis of this simple precipitation balance reached values of up to 0.9 for some variants of orchards. Despite some weaknesses, the model results are satisfactory in some experimental stands, especially in apple orchards. The modified model was used to calculate the long term average water balance of apple trees within the territory of the Czech Republic for the growing season for the period 1991-2020. Composition of seven maps was produced - six maps for monthly data and one for entire growing season. The values of water balance for summer months are in range from -77 to +125 mm, sum for growing season is in range from -323 to +563 mm. The lowest values indicating the threat of drought are in the South Moravia region. Among other things, improved model and maps can help to optimize water management and irrigation in orchards.