| The Qilian Mountains is located at the intersection of the Tibetan Plateau, the Loess Plateau and the Mongolia-Xinjiang Desert, northwestern China. Its ecological environment is not only restricted by regional conditions but also sensitive to global change. In order to address the subsistence needs, local residents have exerted all kinds of serious interferences to regional ecosystem of the Qilian Mountains for a long time, which lead to grassland degradation, soil erosion intensification, and severe forest destruction. There are close interactions and coupling relationships between forest ecosystem and the ecological and hydrological processes in the whole Qilian Mountains. On the one hand, forest ecosystem defends the safety of permanent snow and glaciers by controlling the energy balance and the water cycle; on the other hand, it plays the role of water conservation, purifying water, protecting soil and water resources, and maintaining biological diversity by regulating hydrological paths (e.g., intercepting and regulating the mountainous precipitation and melt-water). However, the mechanism of the ecological and hydrological processes in the forest ecosystem has not been well understood due to the complex influences on the interaction between forest and water resouces. Therefore, this study aims to investigate the ecological and hydrological processes of Qinghai spruce (Picea crassifolia) forests. Based on the analysis of ecological processes and hydrological processes and their interactions, the mechanism of hydrological cycle in the Qinghai spruce forests and its regulation on the whole mountainous hydrological processes could be quantitatively revealed. Furthermore, the occurrence rules and internal relationships of various hydrological phenomena in the Qilian Mountains can be thoroughly understood, which can provide scientific basis for establishing a rational management pattern on water resources. The main results are as follows:(1) The daily maximum and average air temperature in the study area have obvious seasonal variations. In the Qinghai spruce forest, the relative humidity is basically around 60%, wind speed is less than 0.5 m-s-1, and the highest understory solar radiation intensity is only 86.6 W-m-2. Precipitation in the study area also has a significant seasonal distribution. Rainfall during the growing season (from May to September) of 2008 accounts for 78.7% of annual precipitation, and the rainfall events mainly occur as low rainfall amount and intensity. The vertical distribution of soil temperature and soil moisture in different months show different variation trends, while soil temperature and volume water content of different soil depths during the whole year increase firstly and then decrease.(2) During the observation period (from June 12 to October 8 in 2008), the total throughfall, rainfall interception and stemflow of the Qinghai spruce forest were 212.6 mm,64.5 mm and 3.4 mm, and accounted for 75.8%,23.0% and 1.2% of the total precipitation, respectively. Throughfall increased with the increase of rainfall and had a great spatial variability. Under the conditions of no rainfall before a rain event, stemflow in the Qinghai spruce forest began to yield when rainfall amount reached 5.6 mm. The stemflow amount increased with the increase of rainfall and stemflow rate of 34 rain events averaged 0.58%. Canopy interception rate gradually decreased and finally stabilized with the increase of rainfall, and the averaged proportion of canopy interception was 33.9%.(3) This study simulated canopy transpiration of the Qinghai spruce forest during the growing season of 2008 using an improved Penman-Monteith equation and the univariate sensitivity analysis was performed to test the sensitivity of variables affecting the canopy transpiration. The total transpiration of the Qinghai spruce forest in the whole growing season of 2008 is 160.8 mm and the daily canopy transpiration rate averages 1.05 mm. The modeled daily canopy transpiration starts to increase at the beginning of May, reaches its maximum in the middle of July, and returns to its minimal value towards the end of the growing season. Our sensitivity test shows that among the factors affecting the canopy transpiration, the sequential order of the importance is as follows:net radiation received by canopy> leaf area index> daily air temperature> wind speed>relative humidity. We consider that less than±10% change in the estimated daily canopy transpiration resulted from about±10% change in any of the contributing factors is acceptable and thus the model-based estimation of the daily canopy transpiration is acceptable.(4) The total soil evaporation (estimated by using an improved Penman-Monteith equation) of the Qinghai spruce forest in the whole growing season of 2008 is 51.9 mm and the daily soil evaporation rate averages 0.34 mm. Meanwhile, the daily soil evaporation rates of non-rainy days, observed by small man-made lysimeters, average 1.25 mm and the daily maximum of soil evaporation is 2.82 mm. During the observation period, the measured soil evaporation by lysimeters on 38 non-rainy days was higher than that estimated by the improved Penman-Monteith, but the variation trends were consistent with each other.(5) Based on the improved Penman-Monteith equation, the estimated total evapotranspiration of the Qinghai spruce forest throughout the entire 2008 growing season was 313.6 mm. Canopy interception evaporation, canopy transpiration, and soil evaporation were 100.9 mm,160.8 mm and 51.9 mm, and accounted for 32.2%, 51.3% and 16.5% of the total evapotranspiration, respectively. The estimated evapotranspiration of the Qinghai spruce forest during the whole growing season was not obviously higher or lower than that observed by the eddy correlation system in the study area, and the differences between them were much smaller on sunny days. Specially, the modeled monthly evapotranspiration starts to increase at May, reaches its maximum in July, and decrease towards September.(6) In the study area, most of the rainfall reaches the Qinghai spruce forest floor via forest canopy and is stored in the soil. Meanwhile, seasonal frozen soil melts gradually with the increase of air temperature, which also results in sharply increase in soil moisture. Therefore, the storage of soil water during the whole growing season of 2008 increased 191.5 mm. Furthermore, the evapotranspiration deduced from water balance equation was 187.5 mm and 306.7 mm without and with the consideration of the seasonal frozen soil, respectively. They were lower than that estimated based on the improved Penman-Monteith equation as much as 126.1 mm and 6.9 mm, respectively. |
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