Abstract: Under the support of GIS, the water conservation quantity of the Loess Plateau ecosystem in northern Shaanxi Province was estimated by the methods of water balance and volume conversion, and the water conservation value of the ecosystem was computed by cost accounting and marketing price methods. The results showed that in 1978-2000, the water conservation quantity and value of the ecosystem tended to increase first and decrease then. In 1978-1990, the water conservation quantity increased from 15.98 ×10⁹ t·a^-1 to 19.19×10⁹ t·a^-1, and water conservation value increased from 10.71×10⁹ yuan·a^-1 to 12.86 ×10⁹ yuan·a^-1, with an average annual increment of 1.39%; while in 1990-2000, the water conservation quantity decreased from 19.19 ×10⁹ t·a^-1 to 17.96 ×10⁹ t·a^-1, and water conservation value decreased from 12.86 ×10⁹ yuan·a^-1 to 12.03 ×10⁹ yuan·a^-1, with an average annual decrement of 0.68%. The water conservation quantity and value of the Loess Plateau ecosystem in northern Shaanxi Province had different spatial distribution patterns, showing a trend of high in north and south parts and low in central part.

Key words: Heat technique, Stem sap flow, Transpiration