| Based on field measurement, the CoupModel (Coupled heat and mass transfer model for soil-plant-atmosphere system) was applied to simulate water balance and heat transfer in three kinds of vegetation types, including broadleaved forest (Schima superba and Lithocarpus glaber), coniferous forest (Cunninghamia lanceolata and Pinus massoniana) and farmland(Zea mays) in Simian Mountain, which located in the terminal Three Gorges Reservoir Area of China. This study can provide foundation for the management afforestation and ecological reconstruction.(1) Soil particle composition, bulk density, porosity, water characteristic curve, saturated hydraulic conductivity, soil moisture and temperature among different vegetation types were studied. The results showed that the farmland had finer soil texture with lower porosity, while forest had a porous soil structure, which can increase water infiltration capacity. Forest played an important role on improving soil physical properties. Soil moisture and temperature had obvious seasonal variation characteristics, the average values of soil moisture were as follows:broad-leaved forest (10.47%)<coniferous forests (10.86%)<farmland (12.57%), soil moisture from the farmland was higher with less variability. Because of the better growth status of huge canopy, the soil surface always covered by forest, soil temperature from forestland was lower during the growing season, and its fluctuation was also much more moderate.(2) Simulated soil moisture, soil temperature and canopy capacity were fairly consistent with measured ones and the determination coefficient (R2) was0.69to0.99. It meant that the model had a good applicability in this region. And then, the OAT method (one factor at a time) was adopted to analyze sensitivities of the model parameters. The results of the sensitivity analysis indicated that, many parameters had great influence on the simulation of soil moisture, they were lambda, air entry, residual water, saturation, matrix conductivity and PsiRs-lp. While scaling coefficient, organic layer thick, plant albedo, light extinction coefficient, cond VPD and PsiRs-lp impacted on simulation of soil temperature.(3) Water balance simulation showed that water input/precipitation was2214mm for all the plots during the experimental period, but the water consumption (2224mm) was more than income in the broad-leaved forestland, this was the main reason causing soil water deficit. Evapotranspiration was main output of water balance with the percentage up to61%, and the figures were ranked as follows:broad-leaved forest (720mm/a)> coniferous forest (700mm/a)> farmland (601mm/a). In the growing season, leaf area index (LAI) determined the seasonal variation of ET, while weather condition determined its variation at a much smaller time scale such as one day. Annual simulated deep percolation decreased by60mm for broad-leaved forest and47mm for coniferous forest compared with that for farmland (452mm/a), and it was even greater in wet year. There was obvious difference between forestland and farmland for water conditions, the water balance of farmland was characterized by moisture surplus, while spring and autumn drought occurred in forestlands. This study indicated that a shift from cropland to forest would lead to an increase in evapotranspiration while a reduction in deep percolation or groundwater recharge, afforestation made the water balance process more complicated. Model results also indicated that vegetation species significantly influence the magnitude of water balance components, which call for further attention to the selection of tree-species when planning future afforestation projects.(4) Simulations of heat transfer showed that the plots received the amount of solar radiation was7096MJ-m-2during the experimental period of2008and2009. Compared with cropland, forestland received lardger amount of net radiation, this is mainly related with the surface reflectance. Latent heat was the main output in thermal dissipation, it ranged as follows:broadleaved forest (1702MJ·m-2)> coniferous forests (1642MJ-m’2)> farmland (1415MJ·m-2), respectively, and they accounted for83%,81%and73%of the net radiation. The sensible heat flux was higher in the the farmland than the forestland, for the bigger turbulent exchange and the higher vertical gradient of temperature in the farmland plot. During the experimental period, the soil heat flux was above zero for the farmland plot, it meant thatthe farmland soil needed to absorb heat from the environment. But the values of soil heat flux for the broad-leaved and coniferous forestland were only-35MJ·m-2and-28MJ·m-2. Simulation results also implied that it required2.33MJ energy for broadleaf-conifer forest and2.30MJ and energy for coniferous forest to evapotranspire1kg water, while for farmland, this value was2.28MJ. Forest consumed approximately60%of energy on transpiration, but for farmland, energy was consumed by plant transpiration and soil evaporation. Soil moisture conditions determined the distribution of heat flux, and in turn heat supply also impacted the amount of evapotranspiration, water and heat are interdependent in the soil-vegetation-atmosphere transfer system. After afforestation, water and heat balance was regulated by reflectivity or evapotranspiration of forest. Forest can improve the utilization efficiency of water and heat. |
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