| Heihe River basin is the second longest inland river basin in China, and its water resource mainly originating from the upper reach, while the middle reach and lower reaches are water consumption area. The obvious shortage of the water supply from the upper reach has impact on economic water uses in the middle reach and ecologic water requirement in the lower reach. In1960s, the lower reach encountered serious ecological deterioration because of water decreases from the upper and middle reaches. The upper reach mainly include the Qilian Mountains areas, and the typical vegetation coverage in the mountains area is Qinghai spruce forest, subalpine scrub, alpine meadows and grasslands. In recent decades, vegetation coverage has changed dramatically in this area. For example, in the middle of the last century, the number of forest lands being converted into croplands; while after2000s, lots of grasslands had been converted to forest. The transformations will strongly change the hydrological processes in the upper reach. To discuss these changes, the primary task is to fully understand the hydrological processes under each of vegetation coverage. Therefore, we chose a small watershed (i.e., named Tianlaochi watershed) as the study area, and build several field survey plots to observe and analyze the mechanism of the hydrological processes under forest and grassland. In this research paper, the vegetation canopy interception, the water-holding capacity of the forest floor, the plant transpiration, the soil evaporation and the soil moisture characteristic have been analyzed and discussed, we found:1. The processes of canopy interception by the Qinghai spruce forest demonstrated that the relationships between the throughfall and gross precipitation were two-stepped. The first step was canopy unsaturated and the second step was canopy saturated,4.0mm is the threshold. Before the canopy saturated, the throughfall was free-throughfall-dominated. whereas when the canopy saturated, the throughfall was canopy-drop-dictated. The canopy interception rate before the canopy saturated was far higher than the rate after the canopy saturated. Our examination into the relationship between the interception and the10-min average intensity of precipitation showed the interception increased with increasing precipitation intensity, and the relationship was a divergent one. The divergent relationship was bracketed by an upper’dry line’indicating that100%of gross precipitation was intercepted before saturation, and by a lower’wet line’suggesting that the canopy reached the maximum canopy storage capacity and evaporation was the only component of the interception. Besides, the canopy interception of90throughfall collecting tanks under different canopy structures showed that the interception percentage increased with increasing canopy cover (or plant area index) under all of three rainfall-amount conditions, and the relationship also was a divergent one. The percentage of interception increased faster with increasing canopy cover (or plant area index) under intermediate rainfall conditions than that of under heavy rainfall conditions.The artificial precipitation experiment for the grassland canopy interception showed the processes of canopy interception by the grass were similarly to Qinghai spruce forest. The relationship between the throughfall and gross precipitation also had two steps (that is, the canopy unsaturated and saturated). There were different break points for the canopy saturated under the grazing prohibition (i.e.,1.8mm) and grazing conditions (i.e.,1.1mm). When the grass canopy unsaturated, the interception was linearly increased with the increasing gross precipitation, and then exponently increased after the canopy saturated. We examined the relationship between the percentage of interception and precipitation intensity, and found that the interception was uninfluenced by the precipitation intensity before the canopy saturated. After the canopy was saturated, interception increased with the increasing precipitation intensity when the intensity was<0.7-0.8mm/min; however, it appeared an opposite trend when the intensity was>1.0-1.2mm/min. Furthermore, our analyses suggested that the water storage capacity of the canopy was difference under different precipitation intensity. In the<0.3mm/min precipitation intensity degree, the water storage capacity for the grazing prohibited grass and grazed grass were1.95mm and1.29mm, respectively. These two values could be seemed as the actual maximum canopy water storage capacity.2. The different experimental methods yielded different water-holding capacity of the forest floor. The water balance method supported the average of the water-holding capacity of the moss was682.48%of its biomass, while this value was766.24%by using a direct soaking method. The water balance method was affected by several factors, such as the initial moisture content and density of the moss, which have positive effects to the water-holding capacity. Our results supported the rising trend between the biomass of moss and elevation. However, our results also illustrated a decreasing trend between the rate of the water-holding capacity and elevation; this could be attributed to decreased porosity of the moss along with the increased elevation. In our study area, with increasing elevation, the variation of the saturated water content of the moss were not obvious, and the average saturated water contents of the moss was7.40kg/m23. Our observation data revealed that the variation of grass evapotranspiration rate shows a single-peak changing tendency in a sunny day. The evapotranspiration rate from the grassland was higher than the bare land; the difference of the two could be considered as the grass transpiration. The highest transpiration rate appeared at15:00, while the rate was low during the morning and evening. The total transpiration of the grass in a sunny day was0.727mm. The correlation coefficient and path coefficients between the transpiration rate and environmental factors indicated that the air temperature had the most positive direct effect on transpiration rate, and the subsequent factors were soil temperature and soil moisture. In addition, the air pressure, relative humidity and wind speed had an indirect effect on transpiration rate because the air temperature had relatively close relationship to these factors.We used an SF type sap flow systems to measure sap flow of individual Qinghai spruce tree. The data showed that the daily sap flow density had significant singlet variation tendency. In the sunny days, the maximal sap flow density ranged from4.20g·cm-2h-1to4.66g·cm-2h-1. The daily highest sap flow density was positively correlated with the photosynthetically active radiation (PAR); the following factors were air temperature above the canopy and soil temperature. However, the soil moisture was not relevant with the daily sap flow. Furthermore, in all the seven factors, the air temperature above the canopy had the maximal positive direct effect on the sap flow density, while the soil temperature had a negative effect. Our results showed the daily transpiration of the Qinghai spruce stand was between0.023kg/d and12.10kg/d, with the average being6.0kg/d. There was a huge difference between a sunny day and a rainy day. During the growing season, the daily transpiration started to increase at the beginning of May. reached its maximum at the end of the July or the beginning of August, and returned to its minimal value at the end of the growing season. The total transpiration of an individual stand during the growing season was 803.75kg. On the basis of the analyzing result, we compared the daily transpiration and the soil moisture and found that during the growing season, these two factors had similar varying tendency, and the reason may be the melting of frozen soil. The correlation coefficient showed the daily transpiration was most sensitive to the soil moisture, and then the PAR and the air temperature above the canopy.4. In this study, the Lysimeter was used to observe the soil evaporation. The results indicated that the daily soil evaporation from the number7and8Lysimeter was1.17mm/d and1.70mm/d under the canopy, as the canopy characters strongly affected the soil evaporation. After removal the impact of forest canopy, the soil evaporation was2.2mm/d and2.44mm/d from the two Lysimeter. Under the same meteorological conditions, the soil texture could introduce10.9%difference of the soil evaporation, but this effect was less than the canopy characters. Besides, the soil evaporation from the grassland Lysimeter showed that the meteorological factors could cause29.6%of the difference, while the soil evaporation was3.76mm/d in2011and2.90mm/d in2012, respectively. During the observation period, the soil evaporation from grassland was higher than the Qinghai spruce forest soil. However, they were all relative with the potential evaporation from the20cm evaporation pan. For example, the average soil evaporation of the Qinghai spruce forest soil under the canopy and open area were1.44mm/d and2.32mm/d, respectively, which accounted for38.1%and83.7%of the average potential evaporation during the same time. The soil evaporation from the open area was2.2times higher than that of under the canopy. Also the grassland soil evaporation suggested that it was1.2times higher than that from the open area. The vegetation could make a greater impact to the soil evaporation.5. Our experimental data showed the soil porosity of the Qinghai spruce forest was about53%higher than that of the grassland soil. As a result, the total soil saturated water was249.4mm in the0-30cm of Qinghai spruce forest soil, and was only161.8mm in the0-30cm of grassland soil. There were significant differences in Qinghai spruce forest and grassland soil. In general, under the same soil water potential the Qinghai spruce forest soil had more water than the grassland soil. Our examination showed that the soil holding water of Qinghai spruce forest soil and grassland soil were0.648cm3/cm3and0.403cm3/cm3. respectively. The Qinghai spruce forest soil had higher value than the grassland.During the growing season, the average soil water content in the Qinghai spruce forest soil and grassland soil were0.35cm3/cm3and0.23cm3/cm3in2011, while these values were0.41cm3/cm3and0.26cm3/cm3in2012. Compared with the grassland, the soil water content of Qinghai spruce forest was about50%higher. Although the transpiration from the Qinghai spruce stand was higher than the grass, but the soil evaporation from the Qinghai spruce soil was much lower than the grassland soil. In additional, the forest floor could improve the water retention capacity of soil, as a result, the soil water content in the Qinghai spruce forest was higher than that of grassland. |
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