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40 2019 ND-CP 413905
| Biomass production
Plant growth depends on solar radiation, ambient temperature and soil moisture. The major driver for a biomass generation model is the global radiation. Five major steps are identified in such a model: 1. calculation of the global radiation, 2. calculation of the heat unit index and leaf area index, 3. calculation of Monteith biomass production, 4. determination of growth-limiting factors, and 5. validation of the biomass model through linkage with yield. Similar to reference evapotranspiration, global radiation is not widely measured on a regular basis. Values for global radiation can be calculated using any of the three most commonly used formulae (Ångström, Supit, Hargreaves). The heat unit index at a point in time is calculated from the summation of heat units (degree-days) from the start of the growing season to that point divided by the potential heat units for the particular plant or crop during the entire growing season. The heat unit index thus ranges from 0 at planting to 1 at harvest. The number of heat units per day is the average daily temperature minus the minimum temperature required for phenological growth. The potential heat units required for a crop are either read from the database or calculated from planting and harvest dates. The leaf area index (LAI) is the dimensionless ratio of leaf surface area (one-sided) of the vegetation to the ground area. It quantifies an important structural property of a plant canopy, and is related to a variety of canopy processes. LAI is used in terrestrial ecosystem models as an intermediate variable influencing water interception, photosynthesis, respiration and senescence. These plant processes are essential components in ecological and climate models that calculate terrestrial energy, carbon and water cycling processes and biogeochemistry of vegetation. The introduction of the LAI variable enables the biomass generation part of the plant growth model to run in forecasting mode. In a monitoring mode, biomass could be generated from the remotely sensed fraction of photosynthetically active radiation (FPAR). FPAR measures the proportion of available radiation that a canopy absorbs in the specific photosynthetically active wavelengths of the spectrum, i.e. 400–700 nm. FPAR is a radiation term and it relates directly to the remotely sensed simple ratio NDVI. The variable FPAR is then used to estimate photosynthetic activity and primary production. The relation between FPAR and LAI allows the LAI-based model (forecasting mode) to be compared to the FPAR-based model (monitoring mode). The potential increase in biomass is calculated from FPAR and the radiation use efficiency of the crop or plant using the Monteith equation. The potential increase in total biomass is adjusted according to both water- and temperature-related growth constraints. The water-related growth constraint models a reduction due to prolonged drought conditions, whereas growth reduction due to temperature is calculated from a temperature index based on base, optimum and maximum temperatures for phenological growth. The adjusted daily total biomass production is accumulated through the growing season. Stress caused by nutrient deficiencies, pests, diseases or weed competition is not considered. Land evaluation – towards a revised framework 94 The above-ground biomass is assumed to have a linear relationship with the adjusted total biomass. The fraction of the biomass partitioned to the root system is subtracted from the total biomass. The resulting above-ground biomass is used to adjust plant cover and leaf area index. At a certain point in the growing season (fraction of the heat unit index), senescence commences and leaf abscission is induced. As a result the canopy cover starts declining and the rate of biomass production is reduced. However, the production of plant residues increases and will have a positive impact on soil organic matter. For arable crops, yield is a commonly monitored variable. For purposes of model validation and post-harvest residue return, a link with yield is established through the harvest index. The harvested product is expressed in terms of above-ground biomass, or in some cases of total biomass. For cereals, a distinction is made between grain yield and straw yield. tải về 0.57 Mb. Chia sẻ với bạn bè của bạn:
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