Ecohydrology is a sub-discipline of hydrology that focuses on ecological processes involved in the hydrological cycle. These processes generally occur within the soil and plant foliage, and so emphasis is put on transpiration and thermodynamic energy balance at the land surface. Water covers 70% of the Earths surface. ... (Ecology is sometimes used incorrectly as a synonym for the natural environment or environmentalism. ... The water cycle is known scientifically as the hydrologic cycle. ... SOiL is a five-piece nu metal band from Chicago, formed in 1997. ... This article is about the leaf, a plant organ. ... Transpiration is a continuous process caused by the evaporation of water from leaves of plants and its corresponding uptake from roots in the soil. ... Energy balance has the following meanings in several fields: In physics, energy balance is a systematic presentation of energy flows and transformations in a system. ...

Key Concepts

The hydrologic cycle describes the flow and movement of water through the plant-atmosphere-soil continuum. The interactions among vegetation, the land surface, and the vadose zone are the primary subject of ecohydrology. The water cycle—technically known as the hydrologic cycle—is the circulation of water within the earths hydrosphere, involving changes in the physical state of water between liquid, solid, and gas phases. ... The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the water table, and is thus not considered groundwater (vadose is Latin for shallow). It comprises the unsaturated portion of the soil, regolith or bedrock, as well as the saturated capillary fringe...

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. A rainforest is a forested biome with high annual rainfall. ... Semi-arid generally describes regions that receive low annual rainfall (25 to 50 cm /10 to 20 in) and generally have scrub or grass vegetation. ... Savanna is a grassland dotted with trees, and occurs in several types of biomes. ... Transpiration is a continuous process caused by the evaporation of water from leaves of plants and its corresponding uptake from roots in the soil. ... Leaf. ... This is not about surgically created bowel openings; see stoma (medicine) In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. ... A dense growth of softwoods (a forest) in the Sierra Nevada Range of Northern California A forest is an area with a high density of trees (or, historically, a wooded area set aside for hunting). ... In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks. ... Find more information on Weather by searching one of Wikipedias sister projects: Dictionary definitions from Wiktionary Textbooks from Wikibooks Quotations from Wikiquote Source texts from Wikisource Images and media from Commons News stories from Wikinews Weather is an all-encompassing term used to describe all of the many and...

Soil Moisture Dynamics

Soil moisture is a general term describing the amount of water present in the vadose zone, or unsaturated portion of soil belowground. 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, θ, or saturation, S. These terms are related by porosity, n, through the equation θ = nS. The changes in soil moisture over time are known as soil moisture dynamics. This concept emphasizes that soil moisture is not constant but can fluctuate. These fluctuations have been the subject of recent research[1]. Soil moisture is a ratio which quantifies the amount of water contained within a soil column. ... The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the water table, and is thus not considered groundwater (vadose is Latin for shallow). It comprises the unsaturated portion of the soil, regolith or bedrock, as well as the saturated capillary fringe... Water content is a ratio used in hydrogeology and soil mechanics to indicate the amount of water a porous medium contains. ... In chemistry, saturation has three different meanings: In physical chemistry, saturation is the point at which a solution of a substance can dissolve no more of that substance. ... Used in geology, building science and hydrogeology, the porosity of a porous medium (such as rock or sediment) is the proportion of the non-solid volume to the total volume of material, and is defined by the ratio: where Vp is the non-solid volume (pores and liquid) and Vm...

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 availablity is high, and drought-adapted trees in the summer, when it is low. A Mediterranean climate is one that resembles those of the lands bordering the Mediterranean Sea. ... A drought or an extreme dry periodic climate is an extended period where water availability falls below the statistical requirements for a region. ...

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: See: espionage, urban exploration, entryism, heating, ventilation, and air conditioning. ... Evapotranspiration (ET) is the sum of evaporation and plant transpiration. ... Run-off or runoff may refer to one of the following. ...

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 (n) multiplied by its saturation (s) and the depth of the plant's roots (Z_r). The differential equation ds(t) / dt describes how the soil saturation changes over time. The terms on the right hand side describe the rates of rainfall (R), infiltration (I), runoff (Q), evapotranspiration (E), and leakage (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. Water content is a ratio used in hydrogeology and soil mechanics to indicate the amount of water a porous medium contains. ... In mathematics, a differential equation is an equation in which the derivatives of a function appear as variables. ...

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 dependant on climate factors such as available sunlight. Once below this point, soil moisture imposes controls on evaporation, and it decreases linearly until the soil reaches the point where the vegetation can no longer extract any more water. This soil saturation level is referred to as the wilting point. The following image illustrates the water losses at a point. When bacteria or fungi clog a plants water-conducting or vascular system, they can cause permanent wilting and death. ...

References

• Ecohydrology: Darwinian Expression of Vegetation Form and Function, by Peter S. Eagleson, 2002. [2]
• 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. [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]