Abstract. The general relationships between vegetation and water yield under different climatic regimes are well established at a small watershed scale in the past century. However, applications of these basic theories to evaluate the regional effects of land cover change on water resources remain challenging due to the complex interactions of vegetation and climatic variability and hydrologic processes at the large scale. The objective of this study was to explore ways to examine the spatial and temporal effects of a large ecological restoration project on water yield across the Loess Plateau region in northern China. We estimated annual water yield as the difference between precipitation input and modelled actual evapotranspiration (ET) output. We constructed a monthly ET model using published ET data derived from eddy flux measurements and watershed streamflow data. We validated the ET models at a watershed and regional levels. The model was then applied to examine regional water yield under land cover change and climatic variability during the implementation of the Grain-for-Green (GFG) project during 1999-2007. We found that water yield in 38 % of the Loess Plateau area might have decreased (1-48 mm per year) as a result of land cover change alone. However, combined with climatic variability, 37 % of the study area might have seen a decrease in water yield with a range of 1-54 mm per year, and 35 % of the study area might have seen an increase with a range of 1-10 mm per year. Across the study region, climate variability masked or strengthened the water yield response to vegetation restoration. The absolute annual water yield change due to vegetation restoration varied with precipitation regimes with the highest in wet years, but the relative water yield changes were most pronounced in dry years. We concluded that the effects of land cover change associated with ecological restoration varied greatly over time and space and were strongly influenced by climatic variability in the arid region. The current regional vegetation restoration projects have variable effects on local water resources across the region. Land management planning must consider the influences of spatial climate variability and long-term climate change on water yield to be more effective for achieving environmental sustainability.

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The general relationships between vegetation and water yield under different climatic regimes are well established at a small watershed scale in the past century. However, applying the basic theories to evaluate the regional effects of land cover change on water resources has been rarely done due to the complex interactions of vegetation and climatic variability and hydrologic processes at the large scale. The objective of this study was to explore ways to examine the spatial and temporal effects of a large ecological restoration project on water yield across the Loess Plateau region in Northern China. We estimated annual water yield as the difference between precipitation input and modeled actual evapotranspiration (ET) output. We constructed a monthly ET model using published eddy flux ET measurements, ET estimates derived from local watershed streamflow data. We validated the ET models at a watershed and regional levels. The model was then applied to examine regional water yield under land cover change and climatic variability during the implementation of the Grain-for-Green (GFG) project during 1999–2007. We found that water yield in 38% of the Loess Plateau area as a whole might have decreased (1–48 mm yr^−1) as a result of land cover change alone. However, combined with climatic variability, 37% of the study area might have seen a decrease in water yield with a range of 1–54 mm yr^−1, and 35% of the study area might have seen an increase with a range of 1–10 mm yr^−1. Across the study region, climate variability masked or strengthened the water yield response to vegetation restoration. The absolute annual water yield change due to vegetation restoration varied with precipitation regimes with the highest in wet years, but the relative water yield changes were most pronounced in dry years. When compared to findings at the plot or catchment-scale, this study suggested that regional hydrologic effects of vegetation restoration practices had a rather complex pattern due to both spatial differences in climatic regimes and vegetation response. We concluded that the effects of land cover change associated with ecological restoration varied greatly over time and space and were strongly influenced by climatic variability in the arid region. The current regional vegetation restoration projects have variable effects on local water resources across the region. Land management planning must consider the influences of spatial climate variability and long-term climate change on water yield to be more effective and achieve environmental sustainability