Ecohydrology

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.

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.

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.

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 $θ = n S$ . 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].

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.

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 ( $n$ ) multiplied by its saturation ( $s$ ) and the depth of the plant's roots ( $Z r$ ). The differential equation $d s (t) / d t$ 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.

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 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.