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. Endorheic and arid regions around the world are suffering from seriousdrought problems. In this study, a drought forecasting system based on eightstate-of-the-art climate models from the North American Multi-Model Ensemble(NMME) and a Distributed Time-Variant Gain Hydrological Model (DTVGM) wasestablished and assessed over the upstream and midstream of Heihe River basin(UHRB and MHRB), a typical arid endorheic basin. The 3-month StandardizedPrecipitation Index (SPI3) and 1-month Standardized Streamflow Index (SSI1)were used to capture meteorological and hydrological drought, and valuesbelow −1 indicate drought events. The skill of the forecasting systems wasevaluated in terms of anomaly correlation (AC) and Brier score (BS) or Brierskill score (BSS). The predictability for meteorological drought wasquantified using AC and BS with a “perfect model” assumption, referring tothe upper limit of forecast skill. The hydrological predictability was todistinguish the role of initial hydrological conditions (ICs) andmeteorological forcings, which was quantified by root-mean-square error(RMSE) within the ESP (Ensemble Streamflow Prediction) and reverse ESPframework. The UHRB and MHRB showed season-dependent meteorological droughtpredictability and forecast skill, with higher values during winter andautumn than that during spring. For hydrological forecasts, the forecastskill in the UHRB was higher than that in MHRB. Predicting meteorologicaldroughts more than 2 months in advance became difficult because of complexclimate mechanisms. However, the hydrological drought forecasts could showsome skills up to 3–6 lead months due to memory of ICs during cold and dryseasons. During wet seasons, there are no skillful hydrological predictionsfrom lead month 2 onwards because of the dominant role of meteorologicalforcings. During spring, the improvement of hydrological drought predictionswas the most significant as more streamflow was generated by seasonalsnowmelt. Besides meteorological forcings and ICs, human activities havereduced the hydrological variability and increased hydrological droughtpredictability during the wet seasons in the MHRB.

Seasonal drought predictability and forecast skill in the semi-arid endorheic Heihe River basin in northwestern China