Background The networking and large-scale development of highway have caused the serious soil erosion, naturally the investment for the control of soil and water loss in highway construction is huge. Therefore, scientific evaluation of ecological benefits from soil and water conservation in highway construction projects would provide the basis for the assessments of soil and water conservation in production and construction projects.

Methods Evaluation indexes were firstly determined and scaled using the dispersion range model, then principal component analysis was used to obtain the comprehensive score of each evaluation index, further the analytic hierarchy process was further applied to calculate the weight of the evaluation indexes, and finally the ecological benefits were calculated by multiplying scaled evaluation indexes and their weights. The ecological benefits of different functional zones during pre-construction period, vegetation restoration period, and use period after highway built-up were all evaluated for national highway project G40 from Xi'an to Shangluo.

Results 1) The runoff modulus index had the highest comprehensive score in the pre-construction period, with the soil erosion modulus index in the vegetation restoration period, and the displacement index and sediment concentration of discharged drainage index in the use period after highway built-up. 2) The ecological benefits in northern Qinling Mountains areas and southern Qinling Mountains areas were 0.455 and 0.487 respectively in the pre-construction period, indicating that the ecological system approached stable during this period.3) The ecological benefits in the northern Qinling Mountains areas were all smaller than those in southern Qinling Mountains areas in the vegetation restoration period, indicating that the ecological system in the northern Qinling Mountains areas was more vulnerable. 4) The ecological benefits of different functional zones during the use period after highway built-up were generally higher than those in the pre-construction period, such as the ecological benefit of mainline project area during the pre-construction period was 0.455 and it increased to 0.578 during the use period after highway built-up, and the ecological benefits of mainline project areas were always higher than other functional zones.

Conclusions The ecological benefits of different functional zones in the national highway from Xi'an to Shangluo were calculated, and the results showed the soil and water conservation measures taken by the project, especially the mainline project area, have resulted in obvious ecological benefits.