Environmental models simulate the functioning of environmental processes. The motivation behind developing an environmental model is often to explain complex behaviour in environmental systems, or improve understanding of a system. Environmental models may also be extrapolated through time in order to predict future environmental conditions, or to compare predicted behavior to observed processes or phenomena. However, a model should not be used for both prediction and explanation tasks simultaneously.

A model is a simplified representation of some aspect observed in the real world. A model is any consequence or interpretation taken from a set of observations or experience. Many problems in natural systems are so complex, nonlinear and multidimensional that they require a nonlinear approach. Traditionally, simplification is seldom dealt with in a nonlinear fashion. Linear correlation between different independent components has instead been used. This requires complex explanations and reduces our understanding of the fundamental dynamics behind complex problems. The understanding is then not the focus of the study but the constructed model itself.

Biogeochemical Models in Nature

We define biogeochemical models as models that describe the connection from biology through chemistry, to geology and back. Geology (the solid phase) is only visible to biology through its expression in chemistry. The only way that geology (solids) affects chemistry is through chemical weathering and ion exchange. Ion exchange is always in the form of reversible reactions, whereas chemical weathering reactions are almost always irreversible and slow. Decomposition of organic matter, however, we regard as part of the biotic cycle. For all biogeochemical models where the connection between biological and geological aspects is significant, the representation of the weathering process will be a vulnerable point for overall model performance.

Concerning global cycling of either S or C, etc. nor cycling within an individual water body, were modeling concepts addressed in more general terms. The term modeling is used in many ways. In general, it is a description of a real or hypothetical system. Models serve as a tool for solving specific problems. Therefore it is necessary to properly define the problem before choosing a model.

Ideally, a model should be capable of prediction. However, it can also serve the useful purpose of integrating information from different disciplines and in turn provide direction to the individual disciplines. Therefore, it is important that there are continuous interactions between the constructor of the model and the researchers providing the inputs.

Biogeochemical cycle

In ecology and Earth science a biogeochemical cycle or nutrient cycle is a pathway by which a chemical element or molecule moves through both biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. Water, for example, is always recycled through the water cycle, as shown in the diagram. The water undergoes evaporation, condensation, and precipitation, falling back to Earth clean and fresh. Elements, chemical compounds, and other forms of matter are passed from one organism to another and from one part of the biosphere to another through the biogeochemical cycle.

Carbon Cycle

• Carbon enters the atmosphere through respiration of living things, fires that burn organic compound and by diffusion from the ocean.

• Carbon is removed from the atmosphere through the photosynthesis of green plants algae, photosynthetic bacteria etc.

• When an organism dies, most of its organic materials decompose into inorganic compounds.

• Over years/decades /centuries, such storage occurs in wetlands including part of floodplain, lake basins etc.

• Some may be buried under the sediments and form part of sedimentary rock and becomes fossil fuels- coal, oil and natural gas.

Nitrogen cycle

Today nitrogen atom may be throbbing in the cells of meading grass, tomorrow it may be pulsating through the tissues of a living animal. He nitrogen atom afterwards may rise from decaying animal refuse and stream to the upper region of the atmosphere where it may be yoked with oxygen in a flash of lighting and return a plant food to the soil in a torrent of a rain or it may be directly absorbed from the atmosphere by the soil and there rendered available for plant food by the action of symbiotic bacteria. Hus each nitrogen atom has doubtless undergone a neverceasing cycle of changes through countless aeons of time.

Nitrogen and its compounds are essential for the maintenance of life processes in the biosphere. There is continuous exchange of nitrogen within ecosystem, operating the nitrogen cycle. Plants and animals continuously producing proteins which are organic compounds containing nitrogen. Plants absorb nitrates from the soil to produce plants. The death and decay of plants and animals as well, as excreta of animals comprise the major load of organic residues containing proteins to the soil. Various types of microorganism in the soil utilize these nitrogenous organic residues for their metabolism. The resulting reactions yield a chain of immediate products such as ammonia nitrites, nitrates. Plants absorb nitrates and re-enter the nitrogen cycle. Some soil microorganism break down soil nitrate into nitrogen by denitrification process while others transform nitrogen into soluble nitrogen compounds. In the total cycle about 4 to 7 tones of nitrogen per hectare is added to the soil each year. There is some loss through the leaching of nitrates into the fresh water courses and seas. However by all these processes the nitrogen cycle is balanced and thus the nitrogen concentration in the atmosphere is constant.(De, 2005)

Sulphur cycle

Plants and animals depends on continuous supply of sulphur and its compounds for synthesis of some amino acids and proteins. Some sulphur bacteria serve as the media for exchange of sulphur within ecosystem. The sulphur cycle illustrates the circulation of sulphur and its compound in the environment. Sulphates are converted into plant and animal material by bacterial action . In polluted water under anaerobic condition hydrogen sulphide is produced by bacteria giving deposits of iron sulphide.in unpolluted waters under aerobic conditions the sulphur bacteria transform sulphides into sulphates fo further production of protein.(De, 2005)

Oxygen cycle

Oxygen the byproduct of photosynthesis, is involved in the oxidation of carbohydrates with release of energy, carbon dioxide and water. Its primary role in biological oxidation is that of a hydrogen acceptors. The breakdown and decomposition of organic molecules proceeds primarily by dehydrogenation. Hydrogen is removed by enzymatic activity from organic molecules in a series of reaction and is finally accepted by the oxygen, forming water. Though oxygen is necessary for life, but being very active chemically, molecular oxygen may be toxic to living body cells. Therefore for the protection from toxic effects of molecular oxygen, cells posses the cellular organelles called peroximes which mediate oxidative reaction resulting in the production of hydrogen peroxide which in turn used through the mediation of other enzymes an acceptor in oxidizing other compounds.

The major supply of free oxygen which supports life occurs in the atmosphere. There are two significant sources of atmospheric oxygen. One is the photo dissociation of water vapour in which most of the hydrogen is released scopes into outer space .The other source is photosynthesis active only since life began on earth. Because photosynthesis and respiration are cyclic involving both the release and utilization of oxygen, one would seem to balance the other, and no significant quantity of oxygen would accumulate in the atmosphere.

The cycling of oxygen is very complex. as a constituent of carbon dioxide , it circulates freely through the biosphere. Some carbon dioxides combined with calcium to form carbonates. Oxygen combined with nitrogen to form nitrates, with iron to ferric oxides and with many other minerals to form to form other oxides. In these states oxygen is temporarily withdrawn from circulation. In photosynthesis the oxygen freed is split from the water molecule. This oxygen is then reconstituted into water during plant and animal respiration. Some part of the atmospheric oxygen that reaches the higher levels of the troposphere is reduced to ozone by high energy ultraviolet radiation.

Water cycle

The hydrological cycle is acontinous natural process which helps in exchange of water between the atmosphere, the land, the sea, living plants and animals. About one third of the solar flux is absorbed by the earth is used ti drive the hydrological cycle - massive evaporation of water from the ocean, cloud formation and precipitation which provides us with our supply and reserves of fresh water.

Water as rain, hail or snow is precipitated on all land and water surfaces. Water on land surfaces seeps into the soil as ground water. Below the ground there is a natural water level or water table. The soil below the water table is sustained by the underlying clay and rock strata. Ground water does not remain stationary but moves in various directions; water moves up above the water table by capillary action and there maintains a continuous supply of water to the surface layer of soil, where it is absorbed by plant roots in absence of rain .another important ground water resource is the aquifers. The latter exist above impermeable rock strata- water percolates through the porous rocks and forms underground lakes or reservoir. From these aquifers water can be extracted by sinking wells, tube wells and pumping it to the surface.

In the event of good rainfall all the rain water precipitated on land does not percolated into the soil. Surface water or run off flows into streams, rivers, lakes and catchment areas or reservoirs. The land surfaces and all water surfaces on the earth lose water by evaporation by solar energy. Normal evaporation from the ocean exceeds precipitation by rain into the seas by about 10%. This access 10% ultimately moves as water vapour over land surface and thereby balance the hydrological cycle and meets our requirements for additional water. Plant absorbs capillary ground water but give off excess water through leaves by the process of transpiration. This is an important process which ensures conduction of water and dissolved mineral salts throughout the plant.

Concluding Remarks

Environmental models seek to recreate occurs during some events in nature. Environmental modeling is very important to deter mine chemical exposure concentration to aquatic organisms and or human in the past present or future.

The chosen model defines the data needed to create the learning process required to produce the answer. No modeling starts by assembling 'all' data, and just adding more does not give more clarity, but less. If asked for a value judgment, no biogeochemical model is 'best'. The best model is the model that answers the question asked with the necessary amount of accuracy with the smallest cost or effort.

The Writer is an Environmental Analyst and Associate Member, Bangladesh Economic Association.