The Loess Plateau is located in a semi-arid and semi-humid region. The main land use types are farmland and grassland [1
]. At the same time, this area is the largest loess-covered area in the world. With the impact of climate change (extreme precipitation and drought) and human activities (overgrazing and deforestation) [2
], a series of ecological and environmental problems, such as soil erosion, drought, and water scarcity, and siltation of rivers and lakes on the Loess Plateau have been aggravated [3
]. Therefore, the Chinese government implemented the “Grain for Green Project” (GFGP) in the 1990s in the Loess Plateau. The implementation of the GFGP has led to dramatic changes in land use/land cover, with the vegetation cover increasing by 25% from 1999 to 2013 [4
], and the change rate of normalized difference vegetation index (NDVI) in the new century is 10 times higher than that before 1999 [1
]. With the evapotranspiration increase and soil drying induced by vegetation recovery, observed stream flow has also declined by approximately 50% from the 1950s to 2015 [5
]. The improved vegetation cover, in combination with engineering measures (e.g., check dam, terrace, level furrow, and fish-scale pits), has improved soil properties, reduced soil erosion, and saved incoming water to prevent sediment load from entering the river [6
]. As a result, sediment discharge into the Yellow River has declined by approximately 90% over the past 60 years [5
]. In general, the vegetation restoration effectively reduced the surface runoff and river sediment load [7
], but it also brought out some problems of water resources.
Direct local-scale observations suggest a considerable reduction in surface water yield over the region after revegetation, including soil desiccation, stunted or “small old” trees, and even plant mortalities. Extra planted vegetation consumes water, and when evapotranspiration exceeds precipitation, a soil water imbalance occurs, which plays a crucial role in the formation of dried soil layers. The dried soil layer is, in turn, deemed a serious obstacle to sustainable vegetation restoration, suggesting a negative feedback of revegetation on local water yield and subsequently on vegetation growth. For a long time, scholars believed that the Loess Plateau tends to be warm and dry, but in recent years, especially since the 1990s, there has been a significant warming and wetting trend in this region [2
]. The debate on the ecological sustainability of the uneven distribution of water resources on the Loess Plateau continues, but the sustainability of afforestation in the Loess Plateau region needs to be judged according to the local hydrology and meteorology. The knowledge of the relationship between local precipitation and afforestation characteristics would indeed be highly valuable for a more comprehensive evaluation of the impacts of revegetation on surface water yield, thus guiding the ecological restoration implementation.
Precipitation plays a critical role in the human life and hydrological cycle [12
]. The change in precipitation leads to an aggravation of natural hazards such as floods, drought, and soil erosion [14
]. At present, there are many studies on the influence of atmospheric circulation [15
], meteorological factors [16
], and aerosol factors [17
] on precipitation. However, there are few studies paying attention to the impact of vegetation restoration and the local microclimate on precipitation. Gao et al. [2
] analyzed the changes in actual evapotranspiration (AET), normalized difference vegetation index (NDVI), and precipitation in the Loess Plateau during 1990–2014, and they found that the precipitation increase may cause an increase in AET, and in turn, the AET increase may also influence the precipitation. Li et al. [9
] analyzed the impacts of the Chinese Grain for Green program and climate change on vegetation in the Loess Plateau during 1982–2015, and found that precipitation has mainly contributed to the increase in NDVI in the southeastern region; meanwhile, they found that with the increase in vegetation coverage, the change in underlying surface also had a feedback effect on regional precipitation. Therefore, to further analyze the precipitation change and to identify its potential driving factors in the Loess Plateau, it is urgent to comprehensively consider both climate factors and vegetation restoration factors, and quantitatively analyze the main contribution factors of precipitation under the condition of global climate change and fast land use/cover change. At present, the analysis methods of precipitation driving factors mainly include regression analysis [19
], machine learning [15
], and principal component analysis [20
]. Regression analysis cannot solve the problem of multicollinearity among predictors. Principal component analysis is mainly used to extract important information to explain data structure [21
], while principal component regression analysis can be used for driver factor analysis and is also widely used in predictive analysis. Chen et al. [21
] analyzed the spatial variation in driving factors of irrigation water consumption based on principal component regression, and pointed out that the water consumption structure, irrigation technique, and planting structure were major influential factors in most provinces of China. Tang et al. [22
] integrated principal component analysis with statistically based models for analysis of causal factors and landslide susceptibility mapping; the results showed that rainfall and land use were essential in predicting both loess landslide and rockfall occurrences. Zeng [23
] analyzed the influence of NDVI, EVI, precipitation, and temperature on surface water fluctuation based on regression analysis, and pointed out that vegetation cover is an important factor in controlling permanent water changes. Therefore, we can use principal component regression analysis to eliminate multicollinearity in independent variables, and the weights coefficient can be used to measure the influence of the factors to precipitation.
In the past decades, jointly influenced by climate change and ecological restoration, the spatial and temporal pattern of precipitation in the Loess Plateau was undergoing a significant change. However, the potential influencing factors and their influence on local precipitation are still unclear and not well revealed. Therefore, this study aims to analyze the main influencing factors of precipitation in the condition of vegetation recovery and climate change. Based on previous studies and the local conditions of the Loess Plateau, we selected five potential influencing factors, including potential evapotranspiration (PET), normalized difference vegetation index (NDVI), precipitable water (PW), surface temperature (ST), and water vapor transport (WVT). The paper is structured as follows: (1) To collect the precipitation data and analyze the spatial and temporal characteristics of precipitation before and after the GFGP in the Loess Plateau; (2) to analyze the factors and reveal the spatial and temporal characteristics of each factor; (3) to use the statistical methods to quantitatively analyze the main influencing factors of precipitation before and after the GFGP, and discuss the impact of vegetation restoration on precipitation. The above findings in this research may provide the basic reference for the precipitation prediction in the future. Meanwhile, it may be a great significance for the ecological restoration and water resource management of the Loess Plateau.
Since the implementation of the Grain for Green Project (GFGP) in 1999, precipitation has increased significantly and the region features a warmer and wetter climate. In order to quantitatively analyze the impact of vegetation restoration on precipitation, this study mainly analyzed the temporal and spatial variation of precipitation and the potential influencing factors before and after the GFGP in the Loess Plateau. This study compared the influence degree of NDVI to precipitation before and after the GFGP, found that vegetation restoration had a great influence on precipitation after the GFGP, and speculated that vegetation recovery may have a positive feedback effect on regional precipitation. The main findings are concluded as follows:
(1) Precipitation in the Loess Plateau increased significantly after the GFGP. Before the GFGP, precipitation at 72.6% of all stations was in a decreasing trend, while 84.5% of stations showed an increasing trend after the GFGP and in the central and north stations at a 90% confidence level.
(2) With the implementation of the GFGP and climate change, the annual precipitation (PREA) increased at a rate of 4.96 mm/a, and the summer precipitation (PRES) increased at a rate of 5.63 mm/a. The PREA and PRES both showed increasing trends with a 95% confidence level. The spatial distribution of precipitation decreased from the southeast to northwest. After the GFGP, the PREA and PRES in the southeast increased significantly by around 20–50 mm, while the change in the winter precipitation (PREW) was not significant.
(3) The influence degree of NDVI on regional precipitation increased after the GFGP. Specifically, the annual precipitation (PREA) and the summer precipitation (PRES) were mainly influenced by NDVI, with relative influence degrees of 30.00% and 30.62%, respectively, and the PW was the second influencing factor. The winter precipitation (PREW) was more influenced by PW and PET, with relative influence degrees of 33.03% and 27.80%, respectively. According to the above results, we speculate that the trend of warming and wetting of the Loess Plateau is not only closely related to global climate change on the one hand, but also significantly affected by the changes in the local climate brought by vegetation restoration on the other.
(4) In this study, the implementation of the Grain for Green Project (GFGP) was used as the time node to analyze the contribution rates of five main factors to precipitation changes. However, statistical methods were used to analyze the feedback of vegetation restoration to precipitation, which makes the interaction mechanism between them difficult to explain. Therefore, it should be necessary to further study the ecological and hydrological processes in this region.