Abstract: With the development of the society and economy，the contradiction between the growing demand of water and the shorter supply of water resources is increasingly intensified，which has resulted in the serious water shortage of many areas，and in turn，the serious water shortage can be the major restricting factor of the local social and economic development. The functions of ecosystem include soil conservation，water conservation，and climatic regulation，etc. Furthermore，in the arid and semi-arid regions，the water conservation areas play a very important role in alleviating the erosion of water and maintaining the amount and quality of water. This paper aims to explore how to calculate the amount of water conservation. The principle of water amount balance is the most used and effective way to calculate the amount of water conservation. In this paper，the amount of water yield（the amount of precipitation minus evapotranspiration）in the area of Ziwuling Ecological Area in Shaanxi Province，China was calculated with the InVEST model. Meanwhile，the amount of the surface runoff（other consumption）has been obtained by applying the SCS model. Based on the result of the two models，the amount of water conservation will be calculated. By analyzing the results of the calculation，the conclusions are as follows：（1）The average amount of water yield in this area is 139.4 mm per year，which has the trend of decreasing from the southeast to northwest. The cause of this general trend is that in the southeastern part of Ziwuling area，there concentrate the majority of the settlements and farmlands of the whole area. As is known，the precipitation is more likely to become the surface runoff than to infiltrate in the area of towns and settlements. On the other hand，the amount of water yield will be increased as a result of the decreasing amount of evaporation which is caused by the low content of water in the soil of farmlands. While in the northern area of Ziwuling，there are the densely distributed forests and shrubs，which has led to a significant evapotranspiration and less precipitation. All these factors are supposed to contribute to the low amount of water yield. （2）The average depth of surface runoff in the area is 71.1 mm per year，which has the trend of decreasing from the south to the north and it is roughly consistent with the specific distribution of water yield in the area. However，the distribution of the average depth of surface runoff displays a very complicated feature because the depth of surface runoff is also influenced by the following factors as the time distribution of precipitation，the land utilization types and the quality of the soil. （3）The average amount of water conservation in the area is approximately 300 million m3. And the average depth of water conservation is 69.79 mm，showing an outstanding feature of excellent water conservation function. Generally speaking，the depth of water conservation in the central part of the area of Ziwuling is greater than that of the north and the south part，which results from a concentration of forests and shrubs in the central part，and these two types of land possess a high capability of water conservation. （4）Based on the InVEST model and SCS model，the amount of water conservation and the discrepancy conservation capacities of different land uses can be acquired scientifically and quantificationally. （5）InVEST model is well-developed for calculating the water yeild，while SCS model requires further investigation because of its empirical relation among parameters．

Key words: Ziwuling Ecological Area, water conservation, water balance, InVEST model, SCS model