Ecohydrology and evapotranspiration

While a common simplification of the water cycle is to think about how raindrops ultimately find their way to the ocean, a far less popularized fate of precipitation is a return to the atmosphere within the same watershed where the it fell. Despite the fact that many watersheds export more water to the atmosphere than to runoff, evapotranspiration is seldom the focus of popularized perspectives on hydrological science. Furthermore, attempts to understand the role of evapotranspiration in the water budget of the landscape is one of the primary reasons that understanding of the water and energy budgets of the Earth's surface are so inextricably linked. In this module, we will cover the basics of the understanding the mechanical and thermal energy related to predicting evaporation, and expand on those theories to understand the approaches to predicting the role of plants in evapotranspiration exports from watersheds (3:00 min).

Evapotranspiration is not a flux that is easily directly measured over large areas. Because the other inputs, outputs, and changes in storage for a control volume are sometimes easier to measure or estimate, evapotranspiration is sometimes calculated by difference using characterization of the rest of the water budget (3:33 min).

Estimates of evapotranspiration in ungauged watersheds are often needed, where calculation by water budgets is either impossible or impractical. In the interest of estimating evapotranspiration independently of the water budget, let's build scaffolding for understanding the meteorological drivers of evapotranspiration based on principles of boundary theory and the mass transfer from open water due to evaporation (9:20 min).

A limitation of general application of evaporation mass transfer theory is the need to know the temperature of the water body that ultimately drives the temperature and saturation vapor pressure of the boundary layer. Let's explore the energy budget of a control volume of water to think about the drivers of temperature and the latent heat of vaporization necessary to evaporate water (9:53 min).

The Penman method (a.k.a. the combination method) for estimating evaporation is based on conceptually integrating the mass transfer and energy budget controls on evaporation, which eliminates the need to know the temperature of the water body being evaporated (8:00 min).

A sensitivity analysis with the Penman equation helps solidify connections between the algebra and the logical controls on evaporation it is intended to represent (6:17 min).

Plants play a critical role in enhancing the exposure of soil water to potential removal from the watershed by evaporation. Therefore, the role of plants in watersheds has become a critical topic in environmental hydrology (3:14 min).

Without even considering transpiration and soil water, plants initially play a critical role in the partitioning of gross precipitation into interception loss and net precipitation. The role of interception loss is typically nontrivial and a critical component of resolving water budgets for watersheds (4:56 min).

Except for soil water very near the surface, the water stored in soils would be relatively inaccessible to atmospheric evaporation without the physiological function of vascular plants (8:30 min).

Cohesion-tension theory in plant physiology is identical to the conceptualization of tension (or negative pressure head) in soils. Let's work through a thought exercise regarding the maximum height of trees to illustrate the similarities (3:35 min).

The influence of plants on evapotranspiration of water from soils can be summarized using metrics of atmospheric and canopy conductance (2:51 min).

Ultimately, canopy conductance can be integrated into the Penman model of evaporation to produce the Penman-Monteith model of evapotranspiration. The basic idea is to add a term to the Penman equation that characterizes the degree to which the canopy conductance of plants makes soil water available to the atmosphere for evaporation, where a large canopy conductance relative to the atmospheric conductance would effectively treat soil water like an open body of water (5:07 min).

The Penman-Monteith equation is essentially a predictor of the mechanical and thermal energy available for evapotranspiration, and does not take into account the actual availability of water to be evaporated. Budyko theory provides a simple approach to constraining actual evapotranspiration based on energy or water limitation on the effectiveness of potential evapotranspiration (12:00 min).

Let's consider the variation in the nature of vegetation that might drive differences in actual evapotranspiration across watersheds with the same potential evapotranspiration with respect to precipitation (4:07 min).

Finally, we can review how energy and water limitation of evapotranspiration are visible in global patterns of evapotranspiration (3:13 min).

Study guide

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Study guides are designed to summarize the vocabulary, concepts, and mathematics learned in this module.

Readings from Dingman (3rd ed)

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A list of associated readings from Physical Hydrology by S. Lawrence Dingman (3rd edition)

Slides used in videos

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A case study in the application of Budyko theory

Jones et al. 2012 in BioScience