Assessing the impacts of climate and land use land cover changes on hydrological droughts in the Yellow River Basin using SWAT model with time-varying parameters

Climate and land use land cover (LULC) are two important factors for changes in hydrological droughts in the Yellow River Basin (YRB). Quantifying their impacts on hydrological droughts is of great importance for drought prevention and mitigation. To better characterize hydrological droughts, we proposed a new drought index named Standard Surface water and Groundwater Index (SSGI) that considers both surface water and groundwater conditions as well as their connections using copulas. The SWAT model is used and a time-varying parameters approach is adopted to improve model simulation accuracy during different periods with significant changes in land surface characteristics due to human activities. The impacts of climate and LULC changes on hydrological droughts were then quantified. Results indicate the following: 1) time-varying parameter is not only effective but also necessary to ensure the validity of the model when dealing with significant changes in physical environment, while model parameters obtained from the baseline period calibration can degrade model performance during other periods, especially in the middle and downstream of the YRB due to LULC changes; 2) hydrological droughts became increasingly serious during the last few decades, especially in part of the basin located in the Loess Plateau areas where the ecosystem is extremely fragile; 3) climate and LULC changes have both contributed to the changes in hydrological droughts but their contributions can vary significantly spatially.