Ecohydrology (from Greek οἶκος, oikos, "house(hold)"; ὕδωρ, hydōr, "water"; and -λογία, -logia) is an interdisciplinary field studying the interactions between water and ecosystems. 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 on stream flow and function, and feedbacks between ecological processes and the hydrological cycle.
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.
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.
The principles of Ecohydrology are expressed in three sequential components:
- Hydrological: The quantification of the hydrological cycle of a basin, should be a template for functional integration of hydrological and biological processes.
- Ecological: 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.
- Ecological engineering: The regulation of hydrological and ecological processes, based on an integrative system approach, is thus a new tool for Integrated Water Basin Management.
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 water flux and carbon dioxide flux can have an impact on surrounding climate and weather.
Insufficient soil moisture produces stress in plants, and water availability is 1 of the 2 most important factors (temperature being the other) that determine species distribution. High winds, low atmospheric relative humidity, low carbon dioxide levels, 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 (1928; 1929a, b) 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 (Henckel 1964), while greater deficits disrupt normal plant processes.
An increase in moisture stress in the rooting medium as small as 5 atmospheres affected growth, transpiration, and internal water balance in seedlings, much more so in Norway spruce than in birch, aspen, and Scots pine (Jarvis and Jarvis 1963). The decrease in net assimilation rate was greater in the spruce than in the other species, and only the spruce showed no increase in water use efficiency as the soil became drier. The 2 conifers showed larger differences in water potential between leaf and substrate than did the hardwoods (Jarvis and Jarvis 1963). Transpiration rate decreased less in Norway spruce than in the other 3 species as soil water stress increased up to 5 atmospheres in controlled environments. In field conditions, Norway spruce needles lost 3 times as much water from the fully turgid state as did birch and aspen leaves, and twice as much as Scots pine before apparent closure of stomata (apparent because of the difficulty in determining that point) (Schneider and Childers 1941). Assimilation may therefore continue longer in spruce than in pine when plant water stresses are high, though spruce would 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, , or saturation, . These terms are related by porosity, , through the equation . The changes in soil moisture over time are known as soil moisture dynamics.
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:
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 () multiplied by its saturation () and the depth of the plant's roots (). The differential equation describes how the soil saturation changes over time. The terms on the right hand side describe the rates of rainfall (), interception (), runoff (), evapotranspiration (), and leakage (). 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".
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- Ecohydrology & Hydrobiology - International scientific journal on ecohydrology and aquatic ecology (ISSN 1642-3593). Editors: Maciej Zalewski, David M. Harper, Richard D. Robarts 
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