Seasonal ensemble forecasts for soil moisture, evapotranspiration and runoff across Australia

Abstract

The seasonal predictability of hydrological variables at high resolution across Australia was assessed using a new national seasonal ensemble forecasting system for soil moisture, evapotranspiration and runoff. The system consists of a gridded water balance model (AWRA-L) forced with downscaled and calibrated seasonal climate forecasts from a climate modelling system, ACCESS-S1. Here, we evaluate the hydrological forecasts at one- to six-months lead time relative to a historical reference simulation forced with observed climate inputs. Deterministic and probabilistic skill, accuracy and reliability of the forecast ensemble were assessed, with a specific focus on forecasts of hydrological extremes. Additionally, we assess the performance of the hindcast for selected use cases, particularly focusing on agriculture and water management.Nationally, our assessment of the hydrological forecasts indicated good skill for all variables at one-month lead time, and up to three-months lead time for soil moisture and actual evapotranspiration in many key regions in Australia. Regarding hydrological extremes, we find particularly high skill at longer lead times for low soil moisture and actual evapotranspiration events, highlighting the potential value for applications such as hydrological drought prediction or bushfire risk assessments. Bias of the hydrological forecasts based on calibrated seasonal climate forecasts was shown to be close to zero for all variables, regions and lead times and insignificant, with a minor exception for potential evapotranspiration.More detailed evaluation across economically important regions and river basins, in particular, the Australian Wheatbelt and the Murray-Darling Basin (the largest river basin in Australia and an economically and environmentally critical region), highlights promising potential for the soil moisture and evapotranspiration forecast to be used in applications in agriculture, such as for optimising irrigation or scheduling sowing and harvesting dates. For hydrologically important headwater regions across the Great Dividing Range and the alpine regions of Victoria, the forecasts of runoff show good skill at longer lead times indicating the value of the forecasts for reservoir management.Overall, we conclude that the forecasting system shows sufficient skill for a wide range of applications and regions up to a lead time of three months. We outline limitations of the presented system and highlight potential future research directions.