Exploring the role of land restoration in the spatial patterns of deep soil water at watershed scales

Soil water is a key variable for re-vegetation and environmental restoration in water-limited terrestrial ecosystems such as the Chinese Loess Plateau. Large land restoration projects (e.g., the "Grain for Green" launched in 1999 and the "Gully Land Consolidation" launched in 2011) had substantial impacts on the storage, distribution, and spatial patterns of soil water, and these factors remain poorly understood across watershed scales. We measured the amount of water stored in soil layers from the surface down to 5 m depth and characterized the vertical distribution of gravimetric soil water content (SWC) among four land uses (cropland, shrubland, forestland, and orchard), two slope aspects (shady vs. sunny), and two landforms (slope vs. gully) in three watersheds on the Chinese Loess Plateau. All three of the watersheds were affected by Grain for Green project, two were affected by the Gully Land Consolidation project (named NG and GT-T watersheds) and one was unaffected by restoration efforts (named GT-U watershed). In the three watersheds, the slope and gully SWCs varied from 2.4 to 24.2% and from 4.8 to 46.6%, respectively, during the sampling period in October 2015 (end of the rainy season). The amount and vertical distribution of slope SWC differed significantly among the land uses and between shady and sunny slopes in the three watersheds (p < 0.05). The mean gully SWC (20.4%) was significantly higher than the mean slope SWC (8.7%) for each of the three watersheds (p < 0.01). Gullies filled by the Gully Land Consolidation project had a large capacity to store soil water by increasing the infiltration of precipitation, which accounted for 14.7% and 11.3% of the total annual rainfall in the NG and GT-T watersheds, respectively. Filled gullies can serve as large reservoirs of soil water to relieve the problem of water shortage and can also increase the amount of land available for cultivation to ease deficits in food production. A combination of the Grain for Green project on slopes for soil conservation and the Gully Land Consolidation project in gullies for storing more soil water and increasing farmland area is an effective land restoration strategy on the Chinese Loess Plateau and is helpful for managing water cycles in regions around the world with deep soils.