The concept and theory of green and blue waters are important for water resources assessment and management, especially in semi-arid and sub-humid zones. Most previous studies have been at farm-field scales; to better manage green water at drainage-basin scales, it is important to study how the transformation of precipitation to green water is influenced by climate and human activity. An index of the green water coefficient (C_gw) at the drainage-basin scale is adopted in the present study, defined as the ratio of the annual amount of green water to the annual precipitation within a drainage basin. Based on hydrological and meteorological data from the Yellow River basin and using statistical methods, we analyzed the temporal variation of C_gw in relation to climate change and human activity. During 1950-2011, the C_gw over the Hekouzhen-Longmen drainage basin showed an increasing trend. Apart from changes in air temperature and precipitation, the implementation of large-scale soil and water conservation measures is an important factor. Soil and water conservation measures reduce the transformation rate from precipitation to runoff (blue water) and increase the transformation rate from precipitation to green water. The increase in the green water coefficient implies a weakening of runoff on hill slopes and flows in the river, thus reducing soil erosion and sediment yield. On the other hand, the increased green water also indicates higher transpiration by the improved vegetation, and the latter may increase the vegetation's protection of land surfaces against erosion, also reducing sediment yield. The sediment yield (Q_s) over the Hekouzhen-Longmen drainage area has negatively correlated with C_gw (R²=0.53), indicating that 53% of the decrease in Q_s can be explained by the increase in C_gw. After the implementation of soil and water conservation measures, the increased terracing land and the land created by check-dams increased the productive green water, increasing grain yield. We found that grain yield was positively correlated with C_gw and with the areas of terrace land and the land created by check-dams. The increased proportion of productive green water was an important factor for the increased grain yields. The green water coefficient may be used as an indicator of the environmental quality of a drainage basin. At the same precipitation, an increased C_gw means that the environmental quality has improved, and vice versa. According to the variation in C_gw, the changes in the environmental quality of the Hekouzhen-Longmen drainage area has been be divided into three stages. In Stage 1 (1950-1969), C_gw showed a decreasing trend, meaning that the environmental quality was lowered due to serious soil and water losses. In Stage 2, C_gw showed an increasing trend, meaning that the environmental quality improved due to the large-scale practice of soil and water conservation measures. In Stage 3, C_gw increased more rapidly, indicating that the environmental quality was further improved due to the large-scale ecological restoration focused on "returning farmland to forests (and grasses)". Therefore, the application of the green water coefficient is useful for the assessment of generalized water resources at the drainage-basin scale and for a better understanding of the water-resource effect induced by soil and water conservation measures.