Hydrological Response of the Duero River Basin under Present and Future Climate

Climate change has lead to a decrease of precipitation and an increase of temperature in the Mediterranean Basin during the last fifty years. These changes are expected to be more intense over the course of the 21^th century according to global climate projections. As a consequence, water resources are expected to decrease, particularly in the Iberian Peninsula. The Duero River Basin, as part of this region, constitutes a basin of great interest given that is the largest basin of this area, and therefore the identification of the streamflow changes is of paramount importance in order to develop adequate water management policies and strategies that minimize the already evident effects of climate change.

This study is focused on the hydrological response of the Duero River Basin under present and future climate. For this end, firstly, the implementation of the Variable Infiltration Capacity (VIC) model in the Basin has been carried out. The VIC model has been calibrated with a dataset of daily observations of precipitation, temperature and streamflow selecting 2000-2009 as the calibration period and the Nash-Sutcliffe Efficiency (NSE) as the objective function. The Shuffled-Complex-Evolution Algorithm was used for optimizing the NSE by comparing and adjusting the streamflow simulations to the observed streamflow. Precipitation and temperature data were extracted from SPREAD/STEAD, a dataset that covers the Peninsular Spain at 5 km x 5 km of spatial resolution. Streamflow data were gathered for the subwatersheds corresponding to the headwater reservoirs that regulate the streamflow generated in the mountainous areas of the Basin. These data were provided by the Spanish Center for Public Work Experimentation and Study (CEDEX, Centro de Estudios y Experimentación de Obras Públicas). Subsequently, the VIC model has been validated for the period 2009-2011 in order to verify that the model outputs fit well with the observational data out of the calibration period. The goodness-of-fit for both the calibration and validation periods was described through the objective function itself and the coefficient of determination of the linear adjustment of the simulated streamflow to the observed time series.

After the calibration and validation of the VIC model for present climate, secondly, the effect of climate change on the Duero River Basin has been analyzed by developing several simulations of the streamflow for future climate. Precipitation and temperature data were obtained in this case from future projections taken from high resolution (0.088º) simulations carried out with the Weather Research and Forecasting (WRF) model for the Iberian Peninsula driven by the bias-corrected outputs from CESM1 (version 1 of NCARs Community Earth System Model) and MPI-ESM-LR (Max Planck Institute - Earth System Model - Low Resolution). These last simulations were driven under two different Representative Concentration Pathway (RCP) scenarios, RCP4.5 and RCP8.5, and for the periods 2021-2050 and 2071-2100. The hydrological response of the Duero River Basin to the climate change was evaluated through two different approaches: 1) an analysis of the changes of the water balance indices of the law of conservation of mass applied to the hydrological system defined by each subwatershed and normalized by the precipitation; and 2) the determination of the delta changes of the water balance components in the future with respect to the present. The Mann-Whitney-Wilcoxon test was applied to estimate the statistical significance of the delta changes.

The first results of this work show that the VIC model outputs are in good agreement with the observed streamflow for both the calibration and validation periods. Thus, the calibrated parameters can be used for simulating the streamflow of the studied subwatersheds in time periods other than the calibration one. In the climate change context a generalized decrease in the streamflow is expected in the Duero River Basin. This behavior is due to a mark reduction of the precipitation in the future in some cases, while in other cases the streamflow decline is explained by a combined effect of an increase of the actual evapotranspiration and a decrease of the precipitation. All these results could be of interest for water policy makers and practitioners in the next decades.

Keywords: Duero River Basin, VIC model, Calibration and validation, Climate change.

ACKNOWLEDGEMENTS: All the simulations were conducted in the ALHAMBRA cluster (http://alhambra.ugr.es/) of the University of Granada. This work was partially funded by the Spanish Ministry of Economy and Competitiveness projects CGL2013-48539-R and CGL2017-89836-390 R, with additional support from the European Community Funds (FEDER). The first author was supported by the Ministry of Education, Culture and Sport of Spain (FPU grant FPU17/02098).