Evaluation of reference evapotranspiration techniques for seasonal forecasting applications in Croatia

Seasonal climate predictions are becoming important to increase preparedness and adapt agricultural decision making before and within the growing season. Evapotranspiration represents a major component of the water cycle and agricultural water balance, and is thus of key relevance also for assessment of crop growth and irrigation water needs. This study investigates the comparison of different empirical methods for reference evapotranspiration calculation with the reference Penman Monteith method, implemented in the AquaCrop model, over different climate zones of Croatia using national high resolution high quality meteorological dataset in comparison with EOBS and AgERA5 datasets. This research aims to address the appropriateness of using the method with limited data to reduce uncertainty in crop yield modelling with seasonal climate forecasts.

The motivation for the research stems from the need to use a reduced number of parameters for ETo calculation, which as such enters the crop yield model. Estimation of seasonal predictions of ETo relies on the skill of prediction of multiple meteorological variables; in the case of Penman Monteith, the estimates would rely on the skill of temperature, vapour pressure, winds speed and global solar radiation prediction. Reducing the number of input variables for estimating the reference evapotranspiration might reduce the overall reliance on the limited skill of seasonal predictions of above-mentioned meteorological variables. 

In testing the usefulness of the methods, the interpolated field of the national data network, EOBS dataset, and AgERA5 for the period from 1981 to 2020 are used. Preliminary results show that for the continental area of Croatia the Trajkovic method is satisfactory for ETo calculation, and the EOBS dataset provides better results than AgERA5 when compared to high resolution national dataset. In addition, we analyse the ETo seasonal prediction skill by comparing both methods, original Penman Monteith and Trajkovic method with reduced number of input variables. This will give us an important insight on the relevance of the meteorological dataset choice as well as the ET0 method selection for seasonal prediction purposes. Since there is no comprehensive research on the reference crop evapotranspiration in Croatia, this research fills the gap of knowledge at the local level, but also contributes to the global picture