Historical Water Storage Changes Over China's Loess Plateau

Since 1999, the Loess Plateau, China, has undergone one of the world's largest revegetation programs (Grain for Green Project, GfGP). Revegetation has profound impacts on hydrological cycle and water balance, especially in arid and semi-arid areas. As an essential component of water balance, the long-term change in water storage (Delta S) is generally treated as zero. However, it remains unclear how to define the time scale of "long-term," especially over regions undergoing dramatic vegetation restoration. In this study, we quantify Delta S over the Loess Plateau from 1982 to 2009 from a water balance perspective, with a particular focus on the impact of vegetation on Delta S at different time scales. Results show that a 44.78% increase in vegetation coverage could lead to a 37.72% decrease in Delta S. Moreover, the better the vegetation growth, the lower the ratio of Delta S to precipitation (Delta S/P) in the Loess Plateau. Delta S/P achieved a local minimum at the 4- to 5-year scale and reached a plateau after 10 years. We further use the Budyko framework to verify the above conclusions. We find that errors of simulated streamflow over 11/16 of catchments become stable after 4-5 years, and all 16 catchments become stable after 10 years. Ignoring Delta S at a 10-year scale has the least influence on water budget closure. These findings highlight the importance of considering time scales when ignoring Delta S in the water balance analysis, especially when the length of measurement is limited.