# Ecohydrology

Ecohydrology (from Greek οἶκος, oikos, "house(hold)"; ὕδωρ, hydōr, "water"; and -λογία, -logia) is an interdisciplinary scientific field studying the interactions between water and ecological systems. It is considered a sub discipline of hydrology, with an ecological focus. These interactions may take place within water bodies, such as rivers and lakes, or on land, in forests, deserts, and other terrestrial ecosystems. Areas of research in ecohydrology include transpiration and plant water use, adaption of organisms to their water environment, influence of vegetation and benthic plants on stream flow and function, and feedbacks between ecological processes and the hydrological cycle.

## Key concepts

The hydrologic cycle describes the continuous movement of water on, above, and below the surface on the earth. This flow is altered by ecosystems at numerous points. Transpiration from plants provides the majority of flow of water to the atmosphere. Water is influenced by vegetative cover as it flows over the land surface, while river channels can be shaped by the vegetation within them. Ecohydrology was developed under the International Hydrological Program of UNESCO.

Ecohydrologists study both terrestrial and aquatic systems. In terrestrial ecosystems (such as forests, deserts, and savannas), the interactions among vegetation, the land surface, the vadose zone, and the groundwater are the main focus. In aquatic ecosystems (such as rivers, streams, lakes, and wetlands), emphasis is placed on how water chemistry, geomorphology, and hydrology affect their structure and function.

### Principles

The general assumptions of ecological hydrology is to decrease ecosystem degradation using concepts that integrate terrestrial and aquatic processes across scales. The principles of Ecohydrology are expressed in three sequential components:

1. Hydrological (Framework): The quantification of the hydrological cycle of a basin, should be a template for functional integration of hydrological and biological processes. This perspective includes issue of scale, water and temperature dynamics, and hierarchical interactions between biotic and abiotic factors.
2. Ecological (Target): The integrated processes at river basin scale can be steered in such a way as to enhance the basin’s carrying capacity and its ecosystem services. This component deals with aspects of ecosystem resilience and resistance.
3. Ecological Engineering (Method): The regulation of hydrological and ecological processes, based on an integrative system approach, is thus a new tool for Integrated Water Basin Management. This method integrates the hydrological framework and ecological targets to improve water quality and ecosystem services, using engineering methods such as levees, biomanipulation, reforestation, and other management strategies.

Their expression as testable hypotheses (Zalewski et al., 1997) may be seen as:

• H1: Hydrological processes generally regulate biota
• H2: Biota can be shaped as a tool to regulate hydrological processes
• H3: These two types of regulations (H1&H2) can be integrated with hydro-technical infrastructure to achieve sustainable water and ecosystem services

### Vegetation and water stress

A fundamental concept in ecohydrology is that plant physiology is directly linked to water availability. Where there is ample water, as in rainforests, plant growth is more dependent on nutrient availability. However, in semi-arid areas, like African savannas, vegetation type and distribution relate directly to the amount of water that plants can extract from the soil. When insufficient soil water is available, a water-stressed condition occurs. Plants under water stress decrease both their transpiration and photosynthesis through a number of responses, including closing their stomata. This decrease in the Canopy forest, canopy water flux and carbon dioxide flux can influence surrounding climate and weather.

Insufficient soil moisture produces stress in plants, and water availability is one of the two most important factors (temperature being the other) that determine species distribution. High winds, low atmospheric relative humidity, low carbon dioxide, high temperature, and high irradiance all exacerbate soil moisture insufficiency. Soil moisture availability is also reduced at low soil temperature. One of the earliest responses to insufficient moisture supply is a reduction in turgor pressure; cell expansion and growth are immediately inhibited, and unsuberized shoots soon wilt.

The concept of water deficit, as developed by Stocker in the 1920s,[1][2][3] is a useful index of the balance in the plant between uptake and loss of water. Slight water deficits are normal and do not impair the functioning of the plant,[4] while greater deficits disrupt normal plant processes.

An increase in moisture stress in the rooting medium as small as 5 atmospheres affects growth, transpiration, and internal water balance in seedlings, much more so in Norway spruce than in birch, aspen, or Scots pine.[5] The decrease in net assimilation rate is greater in the spruce than in the other species, and, of those species, only the spruce shows no increase in water use efficiency as the soil becomes drier. The two conifers show larger differences in water potential between leaf and substrate than do the hardwoods.[5] Transpiration rate decrease less in Norway spruce than in the other three species as soil water stress increases up to 5 atmospheres in controlled environments. In field conditions, Norway spruce needles lose three times as much water from the fully turgid state as do birch and aspen leaves, and twice as much as Scots pine, before apparent closure of stomata (although there is some difficulty in determining the exact point of closure).[6] Assimilation may therefore continue longer in spruce than in pine when plant water stresses are high, though spruce will probably be the first to “run out of water”.

### Soil moisture dynamics

Soil moisture is a general term describing the amount of water present in the vadose zone, or unsaturated portion of soil below ground. Since plants depend on this water to carry out critical biological processes, soil moisture is integral to the study of ecohydrology. Soil moisture is generally described as water content, ${\displaystyle \theta }$, or saturation, ${\displaystyle S}$. These terms are related by porosity, ${\displaystyle n}$, through the equation ${\displaystyle \theta =nS}$. The changes in soil moisture over time are known as soil moisture dynamics.

Recent global studies using water stable isotopes show that not all soil moisture is equally available for groundwater recharge or for plant transpiration.[7][8]

### Temporal and spatial considerations

Ecohydrological theory also places importance on considerations of temporal (time) and spatial (space) relationships. Hydrology, in particular the timing of precipitation events, can be a critical factor in the way an ecosystem evolves over time. For instance, Mediterranean landscapes experience dry summers and wet winters. If the vegetation has a summer growing season, it often experiences water stress, even though the total precipitation throughout the year may be moderate. Ecosystems in these regions have typically evolved to support high water demand grasses in the winter, when water availability is high, and drought-adapted trees in the summer, when it is low.

Ecohydrology also concerns itself with the hydrological factors behind the spatial distribution of plants. The optimal spacing and spatial organization of plants is at least partially determined by water availability. In ecosystems with low soil moisture, trees are typically located further apart than they would be in well-watered areas.

## Basic equations and models

### Water balance at a point

A fundamental equation in ecohydrology is the water balance at a point in the landscape. A water balance states that the amount water entering the soil must be equal to the amount of water leaving the soil plus the change in the amount of water stored in the soil. The water balance has four main components: infiltration of precipitation into the soil, evapotranspiration, leakage of water into deeper portions of the soil not accessible to the plant, and runoff from the ground surface. It is described by the following equation:

${\displaystyle nZ_{r}{\frac {ds(t)}{dt}}=R(t)-I(t)-Q[s(t),t]-E[s(t)]-L[s(t)]}$

The terms on the left hand side of the equation describe the total amount of water contained in the rooting zone. This water, accessible to vegetation, has a volume equal to the porosity of the soil (${\displaystyle n}$) multiplied by its saturation (${\displaystyle s}$) and the depth of the plant's roots (${\displaystyle Z_{r}}$). The differential equation ${\displaystyle ds(t)/dt}$ describes how the soil saturation changes over time. The terms on the right hand side describe the rates of rainfall (${\displaystyle R}$), interception (${\displaystyle I}$), runoff (${\displaystyle Q}$), evapotranspiration (${\displaystyle E}$), and leakage (${\displaystyle L}$). These are typically given in millimeters per day (mm/d). Runoff, evaporation, and leakage are all highly dependent on the soil saturation at a given time.

In order to solve the equation, the rate of evapotranspiration as a function of soil moisture must be known. The model generally used to describe it states that above a certain saturation, evaporation will only be dependent on climate factors such as available sunlight. Once below this point, soil moisture imposes controls on evapotranspiration, and it decreases until the soil reaches the point where the vegetation can no longer extract any more water. This soil level is generally referred to as the "permanent wilting point". This term is confusing because many plant species do not actually "wilt".

## References

1. ^ Stocker, O. 1928. Des Wasserhaushalt ägyptischer Wüsten- und Salzpflanzen. Bot. Abhandlungen (Jena) 13:200.
2. ^ Stocker, O (1929a). "Das Wasserdefizit von Gefässpflanzen in verschiedenen Klimazonen". Planta. 7 (2–3): 382–387. doi:10.1007/bf01916035.
3. ^ Stocker, O. 1929b. Vizsgálatok Különbözö termöhelyn nött Novények víshiányának nagyságáról. Über die Hóhe des Wasserdefizites bei Pflanzen verschiedener Standorte. Erdészeti Kisérletek (Sopron) 31:63-–76; 104-114.
4. ^ Henckel, P.A. (1964). "Physiology of plants under drought". Annu. Rev. Plant Physiol. 15: 363–386. doi:10.1146/annurev.pp.15.060164.002051.
5. ^ a b Jarvis, P.G.; Jarvis, M.S. 1963. The water relations of tree seedlings. I. Growth and water use in relation to soil potential. II. Transpiration in relation to soil water potential. Physiol. Plantarum 16:215–235; 236–253.
6. ^ Schneider, G.W.; Childers, N.F. (1941). "Influence of soil moisture on photosynthesis, repiration and transpiration of apple leaves". Plant Physiol. 16 (3): 565–583. doi:10.1104/pp.16.3.565. PMC 437931. PMID 16653720.
7. ^ Good, Stephen P.; Noone, David; Bowen, Gabriel (2015-07-10). "Hydrologic connectivity constrains partitioning of global terrestrial water fluxes". Science. 349 (6244): 175–177. doi:10.1126/science.aaa5931. ISSN 0036-8075. PMID 26160944.
8. ^ Evaristo, Jaivime; Jasechko, Scott; McDonnell, Jeffrey J. (2015). "Global separation of plant transpiration from groundwater and streamflow". Nature. 525 (7567): 91–94. doi:10.1038/nature14983. PMID 26333467.
• García-Santos, G.; Bruijnzeel, L.A.; Dolman, A.J. (2009). "Modelling canopy conductance under wet and dry conditions in a subtropical cloud forest". Journal Agricultural and Forest Meteorology. 149 (10): 1565–1572. doi:10.1016/j.agrformet.2009.03.008.
• Ecohydrology in a montane cloud forest in the National Park of Garajonay, La Gomera (Canary Islands, Spain). García-Santos, G. (2007), PhD Dissertation, Amsterdam: VU University. http://dare.ubvu.vu.nl/handle/1871/12697
• "Guidelines for the Integrated Management of the Watershed – Phytotechnology & Ecohydrology", by Zalewski, M. (2002) (Ed). United Nations Environment Programme Freshwater Management Series No. 5. 188pp, ISBN 92-807-2059-7.
• "Ecohydrology. A new paradigm for the sustainable use of aquatic resources", by Zalewski, M., Janauer, G.A. & Jolankai, G. 1997. UNESCO IHP Technical Document in Hydrology No. 7.; IHP - V Projects 2.3/2.4, UNESCO Paris, 60 pp.
• Ecohydrology: Darwinian Expression of Vegetation Form and Function, by Peter S. Eagleson, 2002. [1]
• Ecohydrology - why hydrologists should care, Randall J Hunt and Douglas A Wilcox, 2003, Ground Water, Vol. 41, No. 3, pg. 289.
• Ecohydrology: A hydrologic perspective of climate-soil-vegetation dynamics, Ignacio Rodríguez-Iturbe, 2000, Water Resources Research, Vol. 36, No. 1, pgs. 3-9.
• Ecohydrology of Water-controlled Ecosystems : Soil Moisture and Plant Dynamics, Ignacio Rodríguez-Iturbe, Amilcare Porporato, 2005. ISBN 0-521-81943-1
• Dryland Ecohydrology, Paolo D'Odorico, Amilcare Porporato, 2006. ISBN 1-4020-4261-2 [2]
• Ecohydrology of terrestrial ecosystems, Paolo D'Odorico, Francesco Laio, Amilcare Porporato, Luca Ridolfi, Andrea Rinaldo, and Ignacio Rodriguez-Iturbe, Bioscience, 60(11): 898–907, 2010 [3].
• Eco-hydrology defined, William Nuttle, 2004. [4]
• "An ecologist's perspective of ecohydrology", David D. Breshears, 2005, Bulletin of the Ecological Society of America 86: 296-300. [5]
• Ecohydrology - An International Journal publishing scientific papers. Editor-in-Chief: Keith Smettem, Associate Editors: David D Breshears, Han Dolman & James Michael Waddington [6]
• Ecohydrology & Hydrobiology - International scientific journal on ecohydrology and aquatic ecology (ISSN 1642-3593). Editors: Maciej Zalewski, David M. Harper, Richard D. Robarts [7]
• García-Santos, G.; Marzol, M. V.; Aschan, G. (2004). "Water dynamics in a laurel montane cloud forest in the Garajonay National Park (Canary Islands, Spain)". Hydrol. Earth Syst. Sci. 8 (6): 1065–1075. doi:10.5194/hess-8-1065-2004.
Behavioral modeling in hydrology

In hydrology, behavioral modeling is a modeling approach that focuses on the modeling of the behavior of hydrological systems.

The behavioral modeling approach makes the main assumption that every system, given its environment, has a most probable behavior. This most probable behavior can be either determined directly based on the observable system characteristics and expert knowledge or, the most frequent case, has to be inferred from the available information and a likelihood function that encodes the probability of some assumed behaviors.

This modeling approach has been proposed recently by Sivapalan et al. (2006) in watershed hydrology.

Benthos

Benthos is the community of organisms that live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.

Many organisms adapted to deep-water pressure cannot survive in the upperparts of the water column. The pressure difference can be very significant (approximately one atmosphere for each 10 metres of water depth).Because light is absorbed before it can reach deep ocean-water, the energy source for deep benthic ecosystems is often organic matter from higher up in the water column that drifts down to the depths. This dead and decaying matter sustains the benthic food chain; most organisms in the benthic zone are scavengers or detritivores.

The term benthos, coined by Haeckel in 1891, comes from the Greek noun βένθος "depth of the sea". Benthos is also used in freshwater biology to refer to organisms at the bottom of freshwater bodies of water, such as lakes, rivers, and streams. There is also a redundant synonym, benthon.

Drylands

Drylands are defined by a scarcity of water. Drylands are zones where precipitation is balanced by evaporation from surfaces and transpiration by plants (evapotranspiration). The United Nations Environment Program defines drylands as tropical and temperate areas with an aridity index of less than 0.65. The drylands can be classified into four sub-types: dry sub-humid lands, semi-arid lands, arid lands, and hyper-arid lands. Some authorities consider Hyper-arid lands as deserts (United Nations Convention to Combat Desertification) although a number of the world's deserts include both hyper arid and arid climate zones. The UNCCD excludes hyper-arid zones from its definition of drylands.

Drylands cover 41.3% of the earth’s land surface, including 15% of Latin America, 66% of Africa, 40% of Asia and 24% of Europe. There is a significantly greater proportion of drylands in developing countries (72%), and the proportion increases with aridity: almost 100% of all Hyper Arid lands are in the developing world. Nevertheless, the United States, Australia and several countries in Southern Europe also contain significant dryland areas.Drylands are complex, evolving structures whose characteristics and dynamic properties depend on many interrelated links between climate, soil, and vegetation.

Geobiology

Geobiology is a field of scientific research that explores the interactions between the physical Earth and the biosphere. It is a relatively young field, and its borders are fluid. There is considerable overlap with the fields of ecology, evolutionary biology, microbiology, paleontology, and particularly biogeochemistry. Geobiology applies the principles and methods of biology and geology to the study of the ancient history of the co-evolution of life and Earth as well as the role of life in the modern world. Geobiologic studies tend to be focused on microorganisms, and on the role that life plays in altering the chemical and physical environment of the lithosphere, atmosphere, hydrosphere and/or cryosphere. It differs from biogeochemistry in that the focus is on processes and organisms over space and time rather than on global chemical cycles.

Geobiological research synthesizes the geologic record with modern biologic studies. It deals with process - how organisms affect the Earth and vice versa - as well as history - how the Earth and life have changed together. Much research is grounded in the search for fundamental understanding, but geobiology can also be applied, as in the case of microbes that clean up oil spills.Geobiology employs molecular biology, environmental microbiology, chemical analyses, and the geologic record to investigate the evolutionary interconnectedness of life and Earth. It attempts to understand how the Earth has changed since the origin of life and what it might have been like along the way. Some definitions of geobiology even push the boundaries of this time frame - to understanding the origin of life and to the role that man has played and will continue to play in shaping the Earth in the Anthropocene.

Groundwater flow

In hydrogeology, groundwater flow is defined as the "...part of streamflow that has infiltrated the ground, has entered the phreatic zone, and has been discharged into a stream channel, or springs and seepage water." It is governed by the groundwater flow equation.

Groundwater is water that is found underground in cracks and spaces in the soil, sand and rocks. An area where water fills these spaces is called a phreatic zone or saturated zone. Groundwater is stored in and moves slowly through the layers of soil, sand and rocks called aquifers. The rate of groundwater flow depends on the permeability (the size of the spaces in the soil or rocks and how well the spaces are connected) and the hydraulic head (water pressure).

Hind Al-Abadleh is a multi-award-winning professor of chemistry at Wilfrid Laurier University in Waterloo, Ontario, Canada. She studies the physical chemistry of environmental interfaces, aerosols and climate change.

Hydropedology

Hydropedology is an emerging field formed from the intertwining branches of soil science and hydrology. Similar to hydrogeology, hydroclimatology, and ecohydrology, the emphasis is connections between hydrology and other of the earth's spheres. In this case, hydropedology focuses on the interface between the hydrosphere and the pedosphere.

Limnological Review

Limnological Review (ISSN 1642-5952) is an official journal of Polish Limnological Society and publishes original papers that deal with

theoretical and applied freshwater research, including such topics as limnology, ecohydrology, chemistry, physics, biology, sedymentology, hydrogeology and environmental engineering.

Malala-Ambilikala Lagoons

Malala-Ambilikala lagoons (Sinhala: මලල-ඇඹිලිකල කලපු) are two interconnected coastal water-bodies located inside the Bundala National Park, Hambantota District in the Southern Province, Sri Lanka. It is 260 km (160 mi) from Colombo to the arid south. The Malala-Ambilikala lagoons are two of the three key lagoons located within the Bundala Ramsar wetlands.

Martin Doyle (ecologist)

Martin W. Doyle is an American professor of ecohydrology, and geomorphology at the Independent Institute and the Nicholas Institute for Environmental Policy Solutions of Duke University.

Particle (ecology)

In marine and freshwater ecology, a particle is a small object. Particles can remain in suspension in the ocean or freshwater. However, they eventually settle (rate determined by Stokes' law) and accumulate as sediment. Some can enter the atmosphere through wave action where they can act as cloud condensation nuclei (CCN). Many organisms filter particles out of the water with unique filtration mechanisms (filter feeders). Particles are often associated with high loads of toxins which attach to the surface. As these toxins are passed up the food chain they accumulate in fatty tissue and become increasingly concentrated in predators (see bioaccumulation). Very little is known about the dynamics of particles, especially when they are re-suspended by dredging. They can remain floating in the water and drift over long distances. The decomposition of some particles by bacteria consumes a lot of oxygen and can cause the water to become hypoxic.

Permanent wilting point

Permanent wilting point (PWP) or wilting point (WP) is defined as the minimal amount of water in the soil that the plant requires not to wilt. If the soil water content decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours. The physical definition of the wilting point, symbolically expressed as θpwp or θwp, is defined by convention as the water content at −1,500 kPa (−15 bar) of suction pressure, or negative hydraulic head.

Peter S. Eagleson

Peter S. Eagleson (born February 27, 1928) is an American hydrologist, author of Dynamic Hydrology and Ecohydrology: Darwinian Expression of Vegetation Form and Function. He has taught at the Massachusetts Institute of Technology since 1952 and is currently a Professor Emeritus. He has held professional positions including member of the National Academy of Engineering (since 1982) and President of the American Geophysical Union from 1986-1988. He has won many awards including the Stockholm International Water Institute's World Water Prize in 1997.

Eagleson's research interests include dynamic hydrology, hydroclimatology, and forest ecology. His early research was on sediment transport and wave theory. He published multiple articles and book chapters about these subjects. It was not until 1964 that he significantly narrowed his focus to hydrology. In 1967 Eagleson along with some of his students, published six papers in Water Resources Research. These papers immediately impacted the field of hydrology.Eagleson has taught at MIT since 1952. He has held a chair as Professor of Civil and Environmental Engineering since 1965.

Photic zone

The photic zone, euphotic zone (Greek for "well lit": εὖ "well" + φῶς "light"), or sunlight (or sunlit) zone is the uppermost layer of water in a lake or ocean that is exposed to intense sunlight. It corresponds roughly to the layer above the compensation point, i.e. depth where the rate of carbon dioxide uptake, or equivalently, the rate of photosynthetic oxygen production, is equal to the rate of carbon dioxide production, equivalent to the rate of respiratory oxygen consumption, i.e. the depth where net carbon dioxide assimilation is zero.

It extends from the surface down to a depth where light intensity falls to one percent of that at the surface, called the euphotic depth. Accordingly, its thickness depends on the extent of light attenuation in the water column. Typical euphotic depths vary from only a few centimetres in highly turbid eutrophic lakes, to around 200 meters in the open ocean. It also varies with seasonal changes in turbidity.

Since the photic zone is where almost all of the photosynthesis occurs, the depth of the photic zone is generally proportional to the level of primary production that occurs in that area of the ocean. About 90% of all marine life lives in the photic zone. A small amount of primary production is generated deep in the abyssal zone around the hydrothermal vents which exist along some mid-oceanic ridges.

The zone which extends from the base of the euphotic zone to about 200 metres is sometimes called the disphotic zone. While there is some light, it is insufficient for photosynthesis, or at least insufficient for photosynthesis at a rate greater than respiration. The euphotic zone together with the disphotic zone coincides with the epipelagic zone. The bottommost zone, below the euphotic zone, is called the aphotic zone. Most deep ocean waters belong to this zone.

The transparency of the water, which determines the depth of the photic zone, is measured simply with a Secchi disk. It may also be measured with a photometer lowered into the water.

Ramsar site

A Ramsar site is a wetland site designated to be of international importance under the Ramsar Convention.The Convention on Wetlands, known as the Ramsar Convention, is an intergovernmental environmental treaty established in 1971 by UNESCO, which came into force in 1975. It provides for national action and international cooperation regarding the conservation of wetlands, and wise sustainable use of their resources.Ramsar identifies wetlands of international importance, especially those providing waterfowl habitat.

As of 2016, there were 2,231 Ramsar sites, protecting 214,936,005 hectares (531,118,440 acres), and 169 national governments are currently participating.

Rekawa Lagoon

Rekawa Lagoon (Sinhala: රැකව කලපුව) is a coastal waterbody located in Hambantota Districtt in the Southern Province, Sri Lanka and it is located 200 km (120 mi) south of Colombo. The lagoon possesses a rich biodiversity with a variety of flora (ex; mangroves) and fauna (ex; fish, crustaceans, birds).

Shahbaz Khan (hydrologist)

Shahbaz Khan (born in Jhelum, Pakistan) is an Australian climatologist and hydrologist, currently Director of the UNESCO Cluster Office in Jakarta and Regional Bureau for Science in Asia and the Pacific, serving as UNESCO Representative to Indonesia, Brunei Darussalam, Malaysia, the Philippines and Timor Leste. In his previous role at UNESCO he was Chief of Section on Sustainable Water Resources Development and Management at UNESCO, Paris. His work at UNESCO includes the Water Education for Sustainable Development, Hydrology for Environment, Life and Policy (HELP), Ecohydrology, Water and Ethics, Energy and Food Nexus within the International Hydrological Programme (IHP). He advises UN member states on environmental policies, review of curricula, and securing multilateral support for research and education projects especially in the Asia-Pacific region.

Sioma

Sioma is a town on the west bank of the Zambezi River in the Western Province of Zambia. Since 2012 it has been the capital of the Sioma District.

Subsurface flow

Subsurface flow, in hydrology, is the flow of water beneath earth's surface as part of the water cycle.

In the water cycle, when precipitation falls on the earth's land, some of the water flows on the surface forming streams and rivers. The remaining water, through infiltration, penetrates the soil traveling underground, hydrating the vadose zone soil, recharging aquifers, with the excess flowing in subsurface runoff. In hydrogeology it is measured by the Groundwater flow equation.

Aquatic ecosystems

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