Objective  Water retention is one of the most important ecosystem services closely related to human beings, and it is also an important index to evaluate the regional ecological environment benefits. This study took the amount of water retention as an index to explore the ecological benefits of the implementation of the Beijing-Tianjin Sandstorm Source Control Project Area (BTSSCPA).

  Method  We evaluated the effects of land use/cover change on water retention services using regional land use/cover and meteorological data from 2000 to 2018.

  Result  (1) The land use/cover in the BTSSCPA indicated a spatial heterogeneity with large area of grassland, cropland and woodland, and the amount of water retention increased from northwest to southeast. (2) From 2000 to 2018, forest land increased by 14.22%, and the amount of regional water retention also showed an increasing trend. The increase was relatively large in mountainous and hilly area in northern Shanxi Province of northern China, water conservation zone of Yanshan Mountain region, Horqin Sandy Land, Ordos Plateau of northern China and southern Greater Hinggan Mountains of northeastern China. (3) The water retention service of regional ecosystem varied with different periods, sub-regional distribution and land type transformation, and the water retention service of woodland was the highest, and it was dominant in all sub-regions. (4) From 2000 to 2010, the transfer of grassland to woodland made the greatest contribution to the increase of water retention, reaching 49.32 × 10⁸ m³. The conversion of a large area of grassland to woodland increased the water retention capacity most significantly in the southern Greater Hinggan Mountains, reaching 16.11 × 10⁸ m³. (5) From 2010 to 2018, the transfer of grassland to woodland made the greatest contribution to the increase of water retention, reaching 3.87 × 10⁸ m³. The contribution of reclamation, afforestation and grass planting of wasteland to water retention concentrated on the southern Ordos Plateau, increasing by 1.31 × 10⁸ m³. Returning farmland to forest land and grassland made the greatest contribution to water conservation zone of Yanshan Mountain region, up to 1.84 × 10⁸ m³.

  Conclusion  Since the implementation of the BTSSCPA, the measures such as returning farmland to forest land, wasteland afforestation and grassland management have effectively improved the water retention service in the study area. The main types of land use/cover transfer that leading to the increase of water retention are the transferring of unused land to woodland and grassland and the transferring of grassland and cropland to woodland. The areas with the greatest contribution to the increase of water retention by land use/cover transfer are mainly in the southern Greater Hinggan Mountains, water conservation zone of Yanshan Mountain region, northern Shanxi Mountains and other southeastern regions. The regional land use/cover pattern needs to be further optimized. The implementation of the next phase of the project program should be based on the ecological characteristics of each district, to implement the project of returning farmland to forest and grassland in areas with weak water retention service capacity, to improve cultivation measures for soil and water conservation on cropland that needs to be preserved, to scientifically implement sand sealing, forestry and grass cultivation or artificial afforestation in areas with large unused land area according to the carrying capacity of water resources, and to strengthen the management and protection, so that the high ecological benefit areas in the study area will continue to develop steadily, while the low ecological benefit areas will focus on treatment and efficient development.