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, the soil carbon sponge 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. 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.

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:

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

The ecological hydrology in a specific system can be assessed by answering a few basic questions Where does the water come from and where does it go? This is defined as the flowpath taken by the water entering the watershed being assessed. How long does the water stay in a specific flux or pool of water? This is defined as residence time, in which the rate the water enters, exits, or is stored can be observed. What reactions and changes does the water undergo through those processes? This is defined as biogeochemical reactions, which have the potential to change the solutes, nutrients, or compounds in the water. Many methods are used to observe and test watersheds for the answers to these questions. Namely, hydrographs, environmental and injected tracers, or equations such as Darcy's law. These three factors are interactive and interdependent. The connectivity of a watershed often defines how these traits will interact. As seasonal or event-scale flows occur, changes in connectivity of a watershed affect flowpath, residence time, and biogeochemical reactions. Places of high reaction activity in a specific place or time are called hot spots or hot moments (Pedroli, 1990)(Wand et al., 2015)(Krause et al., 2017)(Fisher et al., 2004)(Trauth et al., 2014)(Covino, 2016).

A fundamental concept in ecohydrology is that the development of the soil carbon sponge and 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.^{[citation needed]}