Technical NoteHow large are uncertainties in future projection of reference evapotranspiration through different approaches?

- Analyze the uncertainties in ET0 projection from estimation method and data quality.
- Provide reliable future projection of ET0 from GCMs.
- Discuss the implication of future ET0 change to regional water resources.

SummaryAs the indicator of atmospheric evaporative demand over a hypothetical reference surface, reference evapotranspiration (ET0) is an important input to hydrological models. Future projections of ET0 are of great importance in assessing the potential impacts of climate change on the hydrologic regime as well as water resources systems. Different estimating formulations and different input data reliabilities existing in practice determine there may be potential uncertainty in projection of future ET0 change. Here we investigated the difference of future ET0 response to climate change based on three approaches, i.e., more physically based Penman-Montieth equation with relatively uncertain downscaled data quality, more empirical temperature-based Hargreaves equation with more reliable downscaled input data, and statistical downscaling method with directly selecting ET0 as predictands. The Hanjiang River Basin, a headwater source of famous South to North Water Diversion Project (SNWDP) in China was chosen as example to illustrate this issue. Although similar increase processes of ET0 in the Hanjiang River Basin were suggested by three methods, the magnitude of ET0 increase differs substantially, indicating that uncertainty still exist despite of approximate performance of these methods in simulating general trends. Whilst increasing aridity index and decreasing water surplus over the period of 2011-2099 would inevitably cause negative impacts on the implementation of the SNWDP and effective adapting measures are thus expected.