Objective To study the change of stand structure with age and its hydrological influence, and provide a theoretical basis for forest vegetation construction and management in the semi-arid area, Liupan Mountains.

Method The main plantation type -- Larix principis-rupprechtii (Larch) plantation was selected in Diediediegou small watershed in the semi-arid area, Liupan Mountains. Based on daily meteorological data from 1993—2002, both the coupled stand structure model and BROOK90, an ecohydrological model were applied to simulate the variation of stand structures (average tree height, diameter at breast height (DBH), canopy density and leaf area index (LAI)) with plantation age, and then its effects on each water balance component (interception, transpiration, soil evaporation and runoff) were obtained.

Result The Larch plantation could be divided into rapid growth period (5～15 years old), slow growth period (15～40 years old) and stable period (over 40 years old) with the increase of age. During the rapid growth period, the average tree height, average DBH, canopy density and canopy LAI increased rapidly with the increase of age, with an average annual growth rate of 0.48 m·a⁻¹, 0.54 cm·a⁻¹, 0.03 and 0.19, respectively. During the slow growth period, the average stand tree height, DBH and canopy density increased slowly, while the LAI increased first and then decreased. But during the stable period, the stand structures did not change significantly. In the fast growth period, the average annual interception and transpiration increased rapidly with the increase of age, and the rate was 1.91 and 24.13 mm·a⁻¹, respectively. Meanwhile, the average annual soil evaporation and water yield decreased rapidly with the increase of age, and the decrease rate was 10.58 and 14.88 mm·a⁻¹, respectively. In the slow growth period, the change of average annual transpiration, soil evaporation and runoff with age slowed down, and tended to be stable when the age was 30 years. The average annual change rates were 0.62, −0.75 and −0.76 mm·a⁻¹, respectively.

Conclusion In the semi-arid area of Liupan Mountains, the stand structure of Larch plantations changes continuously when the stand age is less than 30 years, especially less than 15 years, which significantly affectes the eco-hydrological function. And when the stand age is over 30 years old, the stand structure is stable and the water balance components tend to be stable too, which indicates that it is no longer necessary to consider the influence of stand age in the integrated forest-water management.