Creating long term gridded fields of reference evapotranspiration in Alpine terrain based on a re-calibrated Hargreaves method
Hydrology and Earth System Sciences Discussions
A new approach for the construction of high resolution gridded fields of reference evapotranspiration for the Austrian domain on a daily time step is presented. Forcing fields of gridded data of minimum and maximum temperatures are used to estimate reference evapotranspiration based on the formulation of Hargreaves. The calibration constant in the Hargreaves equation is recalibrated to the Penman–Monteith equation, which is recommended by the FAO, in a monthly and station-wise assessment. This
... e assessment. This ensures on one hand eliminated biases of the Hargreaves approach compared to the formulation of Penman–Monteith and on the other hand also reduced root mean square errors and relative errors on a daily time scale. The resulting new calibration parameters are interpolated in time to a daily temporal resolution for a standard year of 365 days. The overall novelty of the approach is the conduction of surface elevation as a predictor to estimate the re-calibrated Hargreaves parameter in space. A third order spline is fitted to the re-calibrated parameters against elevation at every station and yields the statistical model for assessing these new parameters in space by using the underlying digital elevation model of the temperature fields. Having newly calibrated parameters for every day of year and every grid point, the Hargreaves method is applied to the temperature fields, yielding reference evapotranspiration for the entire grid and time period from 1961–2013. With this approach it is possible to generate high resolution reference evapotranspiration fields starting when only temperature observations are available but re-calibrated to meet the requirements of the recommendations defined by the FAO.