Besides planting per se, most crop management can be simulated by modifying the levels of certain environmental factors, especially those associated with inputs. Planting is a special case because the selection of the cultivar, planting date, and spatial arrangement (e.g., density and row spacing) set the stage for simulating the crop in the environment. Cultivar selection is especially important and is discussed separately below.
The planting date provides a starting date for simulating plant processes, although simulations often start before planting in order to track effects of pre-planting irrigations or fertilizer applications or to estimate the soil conditions prior to planting. The population and planting arrangement are important because they influence early crop development. Furthermore, some models simulate the architecture of the canopy to estimate how different portions of the canopy intercept solar radiation and how much of the soil is covered by the crop (Boote and Pickering 1994). These calculations require information on the row spacing and orientation.
In many regions, the water available through precipitation either is insufficient to support crop growth or the temporal distribution of precipitation exposes crops to periods of water deficits. Thus, where feasible, producers provide supplemental water through irrigation. In models, irrigations have an effect that is similar to rainfall. An irrigation efficiency factor may be used to reduce the amount of water that enters the ground, and water temperature and nutrient concentration are sometimes considered. For climate change research, a major concern is not so much predicting crop responses to irrigation, but determining what are reasonable assumptions for the availability of water for irrigation (Chapter 10).
Fertilizer applications are characterized by the time of application, the amount of nutrient that is provided and the type of fertilizer, and the method and depth of incorporation. Detailed models consider the chemical form of the supplied nutrient in order to characterize how readily the nutrient becomes available to plants or is lost through leaching or volatilization.
Few models directly simulate the effects of tillage. Tillage effects are sometimes modeled by altering initial conditions such as soil bulk density and a portion of plant residue on the soil surface. Full simulation typically involves changing soil physical properties such as bulk density and mixing of different soil layers (e.g., Andales et al. 2000). Models also vary in their ability to simulate crop rotations or sequences of crops over time, which requires ensuring that the soil conditions at the end of a cropping season can be used for a subsequent fallow period or the next crop in the sequence or rotation.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.