The hydrosphere is the combined mass of water found on, and above the surface of a planet, minor planet or natural satellite. It has been estimated there are 1386 million cubic kilometers of water on Earth. This includes water in liquid and frozen forms in groundwater, lakes, saltwater accounts for 97. 5% of this amount. Fresh water accounts for only 2. 5%, of this fresh water,68. 9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic, and mountain glaciers. 30. 8% is in the form of fresh groundwater, only 0. 3% of the fresh water on Earth is in easily accessible lakes and river systems. The total mass of the Earths hydrosphere is about 1.4 ×1018 tonnes, about 20 ×1012 tonnes of this is in Earths atmosphere. Approximately 75% of Earths surface, an area of some 361 million square kilometers, is covered by ocean, the average salinity of Earths oceans is about 35 grams of salt per kilogram of sea water. The hydrological cycle transfers water from one state or reservoir to another, reservoirs include atmospheric moisture, oceans, lakes, subterranean aquifers, polar icecaps and saturated soil.
Solar energy, in the form of heat and light, most evaporation comes from the oceans and is returned to the earth as snow or rain. Sublimation refers to evaporation from snow and ice, transpiration refers to the expiration of water through the minute pores or stomata of trees. Evapotranspiration is the used by hydrologists in reference to the three processes together, transpiration and evaporation. In his book Water, Marq de Villiers described the hydrosphere as a system in which water exists. The hydrosphere is intricate, interdependent, all-pervading and stable, de Villiers claimed that, On earth, the total amount of water has almost certainly not changed since geological times, what we had we still have. Water can be polluted and misused but it is neither created nor destroyed, there is no evidence that water vapor escapes into space. Every year the turnover of water on Earth involves 577,000 km3 of water and this is water that evaporates from the oceanic surface and from land. The same amount of falls as atmospheric precipitation,458,000 km3 on the ocean and 119,000 km3 on land.
The difference between precipitation and evaporation from the surface represents the total runoff of the Earths rivers. These are the sources of fresh water to support life necessities
Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not occur until the 18th century, the 19th century saw modest progress in the field after weather observation networks were formed across broad regions. Prior attempts at prediction of weather depended on historical data, Meteorological phenomena are observable weather events that are explained by the science of meteorology. Different spatial scales are used to describe and predict weather on local, Meteorology, atmospheric physics, and atmospheric chemistry are sub-disciplines of the atmospheric sciences. Meteorology and hydrology compose the interdisciplinary field of hydrometeorology, the interactions between Earths atmosphere and its oceans are part of a coupled ocean-atmosphere system. Meteorology has application in diverse fields such as the military, energy production, agriculture.
The word meteorology is from Greek μετέωρος metéōros lofty, high and -λογία -logia -logy, varāhamihiras classical work Brihatsamhita, written about 500 AD, provides clear evidence that a deep knowledge of atmospheric processes existed even in those times. In 350 BC, Aristotle wrote Meteorology, Aristotle is considered the founder of meteorology. One of the most impressive achievements described in the Meteorology is the description of what is now known as the hydrologic cycle and they are all called swooping bolts because they swoop down upon the Earth. Lightning is sometimes smoky, and is called smoldering lightning, sometimes it darts quickly along, at other times, it travels in crooked lines, and is called forked lightning. When it swoops down upon some object it is called swooping lightning, the Greek scientist Theophrastus compiled a book on weather forecasting, called the Book of Signs. The work of Theophrastus remained a dominant influence in the study of weather, in 25 AD, Pomponius Mela, a geographer for the Roman Empire, formalized the climatic zone system.
According to Toufic Fahd, around the 9th century, Al-Dinawari wrote the Kitab al-Nabat, ptolemy wrote on the atmospheric refraction of light in the context of astronomical observations. St. Roger Bacon was the first to calculate the size of the rainbow. He stated that a rainbow summit can not appear higher than 42 degrees above the horizon, in the late 13th century and early 14th century, Kamāl al-Dīn al-Fārisī and Theodoric of Freiberg were the first to give the correct explanations for the primary rainbow phenomenon. Theoderic went further and explained the secondary rainbow, in 1716, Edmund Halley suggested that aurorae are caused by magnetic effluvia moving along the Earths magnetic field lines. In 1441, King Sejongs son, Prince Munjong, invented the first standardized rain gauge and these were sent throughout the Joseon Dynasty of Korea as an official tool to assess land taxes based upon a farmers potential harvest. In 1450, Leone Battista Alberti developed a swinging-plate anemometer, and was known as the first anemometer, in 1607, Galileo Galilei constructed a thermoscope
Soil is a mixture of minerals, organic matter, gases and countless organisms that together support life on Earth. Soil is called the Skin of the Earth and interfaces with the lithosphere, the hydrosphere, the atmosphere, the term pedolith, used commonly to refer to the soil, literally translates ground stone. Soil consists of a phase of minerals and organic matter, as well as a porous phase that holds gases. Accordingly, soils are often treated as a system of solids, liquids. Soil is a product of the influence of climate, organisms, Soil continually undergoes development by way of numerous physical and biological processes, which include weathering with associated erosion. Given its complexity and strong internal connectedness soil has been considered as an ecosystem by soil ecologists. Most soils have a dry bulk density between 1.1 and 1.6 g/cm3, while the particle density is much higher. Little of the soil of planet Earth is older than the Pleistocene and none is older than the Cenozoic, Soil science has two basic branches of study and pedology.
Edaphology is concerned with the influence of soils on living things, pedology is focused on the formation and classification of soils in their natural environment. In engineering terms, soil is referred to as regolith, or loose material that lies above the solid geology. Soil is commonly referred to as earth or dirt, technically, as soil resources serve as a basis for food security, the international community advocates its sustainable and responsible use through different types of soil governance. Soil is a component of the Earths ecosystem. The worlds ecosystems are impacted in far-reaching ways by the carried out in the soil, from ozone depletion and global warming, to rainforest destruction. Following the atmosphere, the soil is the next largest carbon reservoir on Earth, as the planet warms, soils will add carbon dioxide to the atmosphere due to its increased biological activity at higher temperatures. Thus, soil carbon losses likely have a positive feedback response to global warming.
Since soil has a range of available niches and habitats. A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial, Soil has a mean prokaryotic density of roughly 108 organisms per gram, whereas the ocean has no more than 107 procaryotic organisms per milliliter of seawater. Since plant roots need oxygen, ventilation is an important characteristic of soil and this ventilation can be accomplished via networks of interconnected soil pores, which absorb and hold rainwater making it readily available for plant uptake
A mineral is a naturally occurring chemical compound, usually of crystalline form and abiogenic in origin. A mineral has one specific chemical composition, whereas a rock can be an aggregate of different minerals or mineraloids, the study of minerals is called mineralogy. There are over 5,300 known mineral species, over 5,070 of these have been approved by the International Mineralogical Association, the silicate minerals compose over 90% of the Earths crust. The diversity and abundance of species is controlled by the Earths chemistry. Silicon and oxygen constitute approximately 75% of the Earths crust, which translates directly into the predominance of silicate minerals, minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various species, changes in the temperature, pressure, or bulk composition of a rock mass cause changes in its minerals. Minerals can be described by their various properties, which are related to their chemical structure.
Common distinguishing characteristics include crystal structure and habit, lustre, colour, tenacity, fracture, more specific tests for describing minerals include magnetism, taste or smell and reaction to acid. Minerals are classified by key chemical constituents, the two dominant systems are the Dana classification and the Strunz classification, the silicate class of minerals is subdivided into six subclasses by the degree of polymerization in the chemical structure. All silicate minerals have a unit of a 4− silica tetrahedron—that is, a silicon cation coordinated by four oxygen anions. These tetrahedra can be polymerized to give the subclasses, disilicates, inosilicates, other important mineral groups include the native elements, oxides, carbonates and phosphates. The first criterion means that a mineral has to form by a natural process, stability at room temperature, in the simplest sense, is synonymous to the mineral being solid. More specifically, a compound has to be stable or metastable at 25 °C, modern advances have included extensive study of liquid crystals, which extensively involve mineralogy.
Minerals are chemical compounds, and as such they can be described by fixed or a variable formula, many mineral groups and species are composed of a solid solution, pure substances are not usually found because of contamination or chemical substitution. Finally, the requirement of an ordered atomic arrangement is usually synonymous with crystallinity, crystals are periodic, an ordered atomic arrangement gives rise to a variety of macroscopic physical properties, such as crystal form and cleavage. There have been recent proposals to amend the definition to consider biogenic or amorphous substances as minerals. The formal definition of an approved by the IMA in 1995, A mineral is an element or chemical compound that is normally crystalline. However, if geological processes were involved in the genesis of the compound, Mineral classification schemes and their definitions are evolving to match recent advances in mineral science
Plants are mainly multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. The term is generally limited to the green plants, which form an unranked clade Viridiplantae. This includes the plants and other gymnosperms, clubmosses, liverworts and the green algae. Green plants have cell walls containing cellulose and obtain most of their energy from sunlight via photosynthesis by primary chloroplasts and their chloroplasts contain chlorophylls a and b, which gives them their green color. Some plants are parasitic and have lost the ability to produce amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although reproduction is common. There are about 300–315 thousand species of plants, of which the great majority, green plants provide most of the worlds molecular oxygen and are the basis of most of Earths ecologies, especially on land. Plants that produce grains and vegetables form humankinds basic foodstuffs, Plants play many roles in culture.
They are used as ornaments and, until recently and in variety, they have served as the source of most medicines. The scientific study of plants is known as botany, a branch of biology, Plants are one of the two groups into which all living things were traditionally divided, the other is animals. The division goes back at least as far as Aristotle, who distinguished between plants, which generally do not move, and animals, which often are mobile to catch their food. Much later, when Linnaeus created the basis of the system of scientific classification. Since then, it has become clear that the plant kingdom as originally defined included several unrelated groups, these organisms are still often considered plants, particularly in popular contexts. When the name Plantae or plant is applied to a group of organisms or taxon. The evolutionary history of plants is not yet settled. Those which have been called plants are in bold, the way in which the groups of green algae are combined and named varies considerably between authors.
Algae comprise several different groups of organisms which produce energy through photosynthesis, most conspicuous among the algae are the seaweeds, multicellular algae that may roughly resemble land plants, but are classified among the brown and green algae. Each of these groups includes various microscopic and single-celled organisms
Hydrology is the scientific study of the movement and quality of water on Earth and other planets, including the water cycle, water resources and environmental watershed sustainability. A practitioner of hydrology is a hydrologist, working within the fields of earth or environmental science, physical geography, geology or civil, Hydrology subdivides into surface water hydrology, groundwater hydrology, and marine hydrology. Domains of hydrology include hydrometeorology, surface hydrology, drainage-basin management and water quality and meteorology are not included because water is only one of many important aspects within those fields. Hydrological research can inform environmental engineering and planning, the term hydrology comes from Greek, ὕδωρ, hýdōr, and λόγος, lógos, study. Chemical hydrology is the study of the characteristics of water. Ecohydrology is the study of interactions between organisms and the hydrologic cycle, hydrogeology is the study of the presence and movement of groundwater.
Hydroinformatics is the adaptation of technology to hydrology and water resources applications. Hydrometeorology is the study of the transfer of water and energy between land and water surfaces and the lower atmosphere. Isotope hydrology is the study of the signatures of water. Surface hydrology is the study of processes that operate at or near Earths surface. Drainage basin management covers water-storage, in the form of reservoirs, water quality includes the chemistry of water in rivers and lakes, both of pollutants and natural solutes. Determining the water balance of a region and predicting flood and drought risk. Real-time flood forecasting and flood warning, designing irrigation schemes and managing agricultural productivity. Part of the module in catastrophe modeling. Designing dams for water supply or hydroelectric power generation, designing sewers and urban drainage system. Analyzing the impacts of antecedent moisture on sanitary sewer systems, predicting geomorphologic changes, such as erosion or sedimentation.
Assessing the impacts of natural and anthropogenic environmental change on water resources, assessing contaminant transport risk and establishing environmental policy guidelines. Hydrology has been a subject of investigation and engineering for millennia, for example, about 4000 BC the Nile was dammed to improve agricultural productivity of previously barren lands
Clay is a fine-grained natural rock or soil material that combines one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to water content and become hard, brittle. Depending on the content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red. Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, some overlap in size and other physical properties. The distinction between silt and clay varies by discipline and soil scientists usually consider the separation to occur at a particle size of 2 µm, sedimentologists often use 4–5 μm, and colloid chemists use 1 μm.
Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils Atterberg limits, ISO14688 grades clay particles as being smaller than 2 μm and silt particles as being larger. These solvents, usually acidic, migrate through the rock after leaching through upper weathered layers. In addition to the process, some clay minerals are formed through hydrothermal activity. There are two types of deposits and secondary. Primary clays form as residual deposits in soil and remain at the site of formation, secondary clays are clays that have been transported from their original location by water erosion and deposited in a new sedimentary deposit. Clay deposits are associated with very low energy depositional environments such as large lakes. Depending on the source, there are three or four main groups of clays, montmorillonite-smectite and chlorite. Chlorites are not always considered to be a clay, sometimes being classified as a group within the phyllosilicates.
There are approximately 30 different types of clays in these categories. Varve is clay with visible annual layers, which are formed by deposition of those layers and are marked by differences in erosion. This type of deposit is common in glacial lakes
In earth science, erosion is the action of surface processes that remove soil, rock, or dissolved material from one location on the Earths crust, transport it away to another location. Eroded sediment or solutes may be transported just a few millimetres, the rates at which such processes act control how fast a surface is eroded. Feedbacks are possible between rates of erosion and the amount of eroded material that is carried by, for example. Processes of erosion that produce sediment or solutes from a place contrast with those of deposition, while erosion is a natural process, human activities have increased by 10-40 times the rate at which erosion is occurring globally. At well-known agriculture sites such as the Appalachian Mountains, intensive farming practices have caused erosion up to 100x the speed of the rate of erosion in the region. Excessive erosion causes both on-site and off-site problems, on-site impacts include decreases in agricultural productivity and ecological collapse, both because of loss of the nutrient-rich upper soil layers.
In some cases, the end result is desertification. Off-site effects include sedimentation of waterways and eutrophication of bodies, as well as sediment-related damage to roads. Intensive agriculture, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils. Rainfall, and the surface runoff which may result from rainfall, produces four types of soil erosion, splash erosion, sheet erosion, rill erosion. Splash erosion is generally seen as the first and least severe stage in the erosion process. In splash erosion, the impact of a falling raindrop creates a crater in the soil. The distance these soil particles travel can be as much as 0.6 m vertically and 1.5 m horizontally on level ground. If the soil is saturated, or if the rate is greater than the rate at which water can infiltrate into the soil.
If the runoff has sufficient flow energy, it will transport loosened soil particles down the slope, sheet erosion is the transport of loosened soil particles by overland flow. Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically of the order of a few centimetres or less and this means that rills exhibit hydraulic physics very different from water flowing through the deeper, wider channels of streams and rivers. Gully erosion occurs when water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow
Soil retrogression and degradation
Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a stable soil. Retrogression is primarily due to erosion and corresponds to a phenomenon where succession reverts the land to its natural physical state. Degradation is an evolution, different from natural evolution, related to the local climate and it is due to the replacement of primary plant communities by the secondary communities. This replacement modifies the humus composition and amount, and affects the formation of the soil and it is directly related to human activity. Soil degradation may be viewed as any change or ecological disturbance to the soil perceived to be deleterious or undesirable, at the beginning of soil formation, the bare rock out crops is gradually colonized by pioneer species. They are succeeded by herbaceous vegetation and finally forest, in parallel, the first humus-bearing horizon is formed, followed by some mineral horizons. Each successive stage is characterized by an association of soil/vegetation and environment.
After a certain time of parallel evolution between the ground and the vegetation, a state of balance is reached. This stage of development is called climax by some ecologists and natural potential by others, succession is the evolution towards climax. Regardless of its name, the stage of primary succession is the highest natural form of development that the environmental factors are capable of producing. The cycles of evolution of soils have very variable durations, between tens, hundreds, or thousands of years for quickly evolving soils to more than a million years for slowly developing soils. The same soil may achieve several successive steady state conditions during its existence, as exhibited by the Pygmy forest sequence in Mendocino County, soils naturally reach a state of high productivity, from which they naturally degrade as mineral nutrients are removed from the soil system. Thus older soils are vulnerable to the effects of induced retrogression and degradation. There are two types of factors influencing the evolution of a soil.
These two factors are significant to explain the evolution of soils of short development. A first type of factor is the climate of an area. A second type of factor is more local, and is related to the original rock and this type of factor explains appearance of specialized associations. The destruction of the vegetation implies the destruction of evoluted soils, cycles of succession-regression of soils follow one another within short intervals of time or long intervals of time
Chalk is a soft, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is a salt called calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the accumulation of minute calcite shells shed from micro-organisms called coccolithophores. Flint is very common as bands parallel to the bedding or as embedded in chalk. It is probably derived from sponge spicules or other organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified, Chalk as seen in Cretaceous deposits of Western Europe is unusual among sedimentary limestones in the thickness of the beds. Most cliffs of chalk have very few obvious bedding planes unlike most thick sequences of such as the Carboniferous Limestone or the Jurassic oolitic limestones. This presumably indicates very stable conditions over tens of millions of years, Chalk has greater resistance to weathering and slumping than the clays with which it is usually associated, thus forming tall steep cliffs where chalk ridges meet the sea.
Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, because chalk is well jointed it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons. Chalk is mined from chalk deposits both above ground and underground, Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks. Abandoned chalk mines remain a popular tourist attraction due to their massive expanse, the Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, the Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage. Some of the highest chalk cliffs in the occur at Jasmund National Park in Germany. Ninety million years ago what is now the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea.
Chalk was one of the earliest rocks made up of particles to be studied under the electron microscope. Their shells were made of calcite extracted from the rich sea-water, as they died, a substantial layer gradually built up over millions of years and, through the weight of overlying sediments, eventually became consolidated into rock. Later earth movements related to the formation of the Alps raised these former sea-floor deposits above sea level, the chemical composition of chalk is calcium carbonate, with minor amounts of silt and clay. It is formed in the sea by plankton, which fall to the sea floor and are consolidated and compressed during diagenesis into chalk rock
A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separate from the other eukaryotic life kingdoms of plants, a characteristic that places fungi in a different kingdom from plants and some protists, is chitin in their cell walls. Similar to animals, fungi are heterotrophs, they acquire their food by absorbing dissolved molecules, growth is their means of mobility, except for spores, which may travel through the air or water. Fungi are the principal decomposers in ecological systems and this fungal group is distinct from the structurally similar myxomycetes and oomycetes. The discipline of biology devoted to the study of fungi is known as mycology, in the past, mycology was regarded as a branch of botany, although it is now known fungi are genetically more closely related to animals than to plants. Abundant worldwide, most fungi are inconspicuous because of the size of their structures.
Fungi include symbionts of plants, animals, or other fungi and they may become noticeable when fruiting, either as mushrooms or as molds. Fungi perform a role in the decomposition of organic matter and have fundamental roles in nutrient cycling. Since the 1940s, fungi have been used for the production of antibiotics, Fungi are used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, the fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans, losses of crops due to fungal diseases or food spoilage can have a large impact on human food supplies and local economies. The fungus kingdom encompasses a diversity of taxa with varied ecologies, life cycle strategies. However, little is known of the biodiversity of Kingdom Fungi.
Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, phylogenetic studies published in the last decade have helped reshape the classification within Kingdom Fungi, which is divided into one subkingdom, seven phyla, and ten subphyla. The English word fungus is directly adopted from the Latin fungus, used in the writings of Horace, a group of all the fungi present in a particular area or geographic region is known as mycobiota, e. g. the mycobiota of Ireland. Like plants, fungi grow in soil and, in the case of mushrooms, form conspicuous fruit bodies. The fungi are now considered a kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago. Fungi have membrane-bound cytoplasmic organelles such as mitochondria, sterol-containing membranes and they have a characteristic range of soluble carbohydrates and storage compounds, including sugar alcohols and polysaccharides
Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use. Soil classification is a subject, from the structure of the system itself, to the definitions of classes. Soil classification can be approached from the perspective of soil as a material, typically geotechnical engineers, classify soils according to their engineering properties as they relate to use for foundation support or building material. Modern engineering classification systems are designed to allow a transition from field observations to basic predictions of soil engineering properties. The most common engineering classification system for soils in North America is the Unified Soil Classification System, the USCS has three major classification groups, coarse-grained soils, fine-grained soils, and highly organic soils. The USCS further subdivides the three major soil classes for clarification and it distinguishes sands from gravels by grain size, and further classifying some as well-graded and the rest as poorly-graded.
Silts and clays are distinguished by the soils Atterberg limits, moderately organic soils are considered subdivisions of silts and clays, and are distinguished from inorganic soils by changes in their plasticity properties on drying. The European soil classification system is similar, differing primarily in coding and in adding an intermediate-plasticity classification for silts and clays. The USCS and additional engineering description is standardized in ASTM D2487, differing concepts of pedogenesis, and differences in the significance of morphological features to various land uses can affect the classification approach. Despite these differences, in a system, classification criteria group similar concepts so that interpretations do not vary widely. This is in contrast to a system approach to soil classification. Natural system approaches to classification, such as the French Soil Reference System are based on presumed soil genesis. Another approach is numerical classification, called ordination, where individuals are grouped by multivariate statistical methods such as cluster analysis.
This produces natural groupings without requiring any inference about soil genesis, in soil survey, as practiced in the United States, soil classification usually means criteria based on soil morphology in addition to characteristics developed during soil formation. Criteria are designed to guide choices in use and soil management. As indicated, this is a system that is a hybrid of both natural and objective criteria. USDA soil taxonomy provides the criteria for differentiating soil map units. This is a revision of the 1938 USDA soil taxonomy which was a strictly natural system