The development and validation of a national-scale process-based hydrological model for Finland

As climate change and increasing human activity continue to reshape the hydrological cycle worldwide, the need for accurate and adaptable forecasting and simulation models has never been more critical. In response to this global challenge, we present the WSFS-P (Watershed Simulation and Forecasting System – version P) hydrological model, designed to simulate and forecast discharge, water levels and other key hydrological variables. The WSFS-P model was especially developed to handle the unique challenges that sub-Arctic and boreal climates provide, especially in relation to seasonal snow dynamics. Covering all the watersheds of Finland and also including transboundary watersheds, the WSFS-P supports operational forecasting, research and planning on both national and international scales. Representing a significant shift from the original conceptual WSFS model (WSFS-O), the WSFS-P integrates advanced sub-models, including an energy-balance snow model, an evapotranspiration model based on the Penman-Monteith equation, a two-layer soil moisture model and a groundwater model, all utilising spatially distributed soil and land-use data. The model has been calibrated using a national discharge and water-level observation network, and in this paper, the calibration is validated at 57 discharge observation stations across Finland. On average, the model achieves a Nash-Sutcliffe coefficient (NSE) of 0.83 in the calibration period 1990–2015 (range: 0.66–0.93) and 0.78 in the validation periods 1981–1990 and 2015–2021 (range: 0.53–0.93). These results confirm the model’s robustness and accuracy, making it a valuable tool for operational hydrological forecasting and research. Overall, the WSFS-P provides reliable simulations of water levels and discharges, contributing to improved flood and drought predictions, which are critical for informed water resource management and climate adaptation planning.