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 (ΔS) 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 ΔS over the Loess Plateau from 1982 to 2009 from a water balance perspective, with a particular focus on the impact of vegetation on ΔS at different time scales. Results show that a 44.78% increase in vegetation coverage could lead to a 37.72% decrease in ΔS. Moreover, the better the vegetation growth, the lower the ratio of ΔS to precipitation (ΔS/P) in the Loess Plateau. ΔS/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 ΔS at a 10‐year scale has the least influence on water budget closure. These findings highlight the importance of considering time scales when ignoring ΔS in the water balance analysis, especially when the length of measurement is limited. Key Points We use simulated evapotranspiration (ET) from the PT‐JPL model, observed streamflow and precipitation to quantify ΔS from 1982 to 2009 on 16 catchments over the Loess Plateau, China We find that a 44.78% increase in vegetation coverage could reduce ΔS by 37.72% Neglecting ΔS on a time scale of about 10 years would have the least impact on the streamflow error simulated by the Budyko framework in the Loess Plateau