| Summary: | Large ensembles of climate models are increasingly available either as ensembles of opportunity or perturbed physics ensembles, providing a wealth of additional data that is potentially useful for improving adaptation strategies to climate change. In this work, we propose a framework to evaluate the predictive capacity of 11 multi-model ensemble methods (MMEs), including random forest (RF), to estimate reference evapotranspiration (ET<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>0</mn></msub></semantics></math></inline-formula>) using 10 AR5 models for the scenarios RCP4.5 and RCP8.5. The study was carried out in the Segura Hydrographic Demarcation (SE of Spain), a typical Mediterranean semiarid area. ET<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>0</mn></msub></semantics></math></inline-formula> was estimated in the historical scenario (1970–2000) using a spatially calibrated Hargreaves model. MMEs obtained better results than any individual model for reproducing daily ET<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>0</mn></msub></semantics></math></inline-formula>. In validation, RF resulted more accurate than other MMEs (Kling–Gupta efficiency (KGE) <inline-formula><math display="inline"><semantics><mrow><mi>M</mi><mo>=</mo><mn>0.903</mn></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mi>S</mi><mi>D</mi><mo>=</mo><mn>0.034</mn></mrow></semantics></math></inline-formula> for KGE and <inline-formula><math display="inline"><semantics><mrow><mi>M</mi><mo>=</mo><mn>3.17</mn></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mi>S</mi><mi>D</mi><mo>=</mo><mn>2.97</mn></mrow></semantics></math></inline-formula> for absolute percent bias). A statistically significant positive trend was observed along the 21st century for RCP8.5, but this trend stabilizes in the middle of the century for RCP4.5. The observed spatial pattern shows a larger ET<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>0</mn></msub></semantics></math></inline-formula> increase in headwaters and a smaller increase in the coast.
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