Comparison of interpolation methods for the prediction of reference evapotranspiration - An application in greece

The objective of this paper is to evaluate four interpolation methods, concerning their suitability for spatial prediction of long-term mean daily reference evapotranspiration (calculated by Penman-Monteith equation) for each month in Greece. The methods studied were ordinary kriging (OK) and inverse distance squared (IDS) and their modifications in which elevation data was incorporated into the interpolation process. The modified methods were named residual kriging (RK), and gradient-plus-inverse distance squared (GIDS). Apart from interpolation methods, two different approaches were studied in order to define what the proper sequence of steps is in the case of the interpolation of reference evapotranspiration. More particularly the 'calculate first, interpolate later' (CI) procedure was compared to the reverse procedure, namely 'interpolate first, calculate later' (IC). The assessment criteria of the methods accuracy were: mean error (ME), mean absolute error (MAE) and root mean squared error (RMSE). The results revealed that the incorporation of elevation significantly improved the performance of interpolation methods. On the contrary, procedures CI and IC were very similar since they had no effect on the performance of the four interpolation methods studied.