Chapter 5: Evaporation, Transpiration, Interception, and Depression Storage

Evaporation

The process where water transitions from a liquid state to water vapor at a free surface.

Energy Transfer

Solar radiation warms the surface water.

Molecular Motion

Break free from the liquid's surface tension

Vapor Exchange

The molecules escape into the overlying atmosphere.

Energy Source

The remaining water cools down during the process.

Energy Transfer

Molecular Motion

Vapor Exchange

Energy Source

How Does Evaporation Occur? (4)

Temperature

Wind Speed

Humidity

Primary Factors Influencing Evaporation (3)

Atmospheric Pressure

Water Depth & Heat Storage

Dissolved Salts

Secondary Factors Influencing Evaporation (3)

Evaporation Pans

A shallow vessel filled with water and exposed to the weather

Water level is measured at regular intervals, typically daily, to determine the depth of water lost

Classic A Pan

A pan with 1210 mm diameter and 255 mm depth made of unpainted galvanized iron sheet, placed on a wooden platform 15 cm above the ground to allow air circulation

ISI Standard Pan

A modified form with a 1220 mm diameter, made of copper sheet tinned inside and painted white outside, placed on a square wooden platform 100 mm above ground level

Colorado Sunken Pan

A 920 mm square pan, 460 mm deep, buried into the ground to within 100 mm of the top

Water Balance Method

A straightforward method for estimating evaporation over monthly, seasonal, or annual periods, where all other water balance components are measured or estimated, and evaporation is calculated as the remaining value (residual)

Inputs include precipitation and inflow, while outputs include outflow and changes in storage

Energy Balance Method

Calculates evaporation based on the principle that energy inputs to a water body must equal energy outputs

Penman and Penman-Monteith Methods

Widely used to estimate evaporation and evapotranspiration from meteorological variables, combining energy balance and aerodynamic principles

Eddy Covariance (Mass Transfer) Method

Measures evaporation using sensors that detect changes in wind speed and water vapor above the water surface

Provides very accurate evaporation estimates, but expensive and requires skilled maintenance, so it is mostly used for research or large water bodies

Evaporation Pans

Water Balance Method

Energy Balance Method

Penman and Penman-Monteith Methods

Eddy Covariance (Mass Transfer) Method

Methods of Measuring Evaporation (5)

Floating Photovoltaic Systems

Install solar panels on reservoir surfaces to reduce evaporation while generating electricity.

Artificial Destratification

Uses compressed air bubbles to mix warm surface water with cooler deep water

Floating Covers and Modular Systems

Floating Photovoltaic Systems

Reservoir Design and Operational Management

Artificial Destratification

Strategies for Managing and Reducing Evaporation Losses

Transpiration

It is the loss of a plant’s water to its environment through evaporation

Stomatal Transpiration

Water vapor escapes through stomata (tiny pores in leaves).

This is the main type of transpiration, responsible for about 80–90% of water loss in plants

Cuticular Transpiration

Water is lost through the cuticle, the waxy outer covering of leaves.

This type occurs when the cuticle allows small amounts of water to pass through

Lenticular Transpiration

Water vapor escapes through lenticels, small openings in the woody stems of plants.

Stomatal Transpiration

Cuticular Transpiration

Lenticular Transpiration

Types of Transpiration (3)

Temperature

It strongly affects transpiration because warmer air can hold more water vapor

Humidity

It refers to the amount of moisture in the air

Wind

it increases transpiration by removing the moist air surrounding the leaf surface

Light Intensity

Stimulates the opening of stomata and provides the energy needed for evaporation

Plant Characteristics

Different plants lose water at different rates depending on their structure and physiology.

Soil Moisture

Affects how much water plants can absorb through their roots

Temperature

Humidity

Wind

Light Intensity

Plant characteristics

Soil Moisture

Factors that Influence Transpiration Rate

Transpiration

Major pathway by which water moves from land to the atmosphere, forming a critical link between land ecosystems and the atmosphere.

Evapotranspiration

Total water lost from soil and plants as vapor, and it is an important factor in planning irrigation schedules.

Drip Irrigation

Delivers water directly to the root zone, reducing surface evaporation

Mulching

Conserves soil moisture and reduces direct evaporation

Windbreaks

Reduce wind speed around crops, cutting transpiration losses in exposed fields

Drip Irrigation

Mulching

Windbreaks

Cover cropping and reduced tillage

Irrigation techniques and transpiration management (4)

Interception

Process where precipitation (rain, snow, or sleet) is caught and stored by vegetation such as leaves, branches, and stems before reaching the ground.

Canopy Interception

Rainwater captured by leaves, branches, and stems of vegetation.

Litter Interception

Rainwater is stored in dead leaves, twigs, and organic material on the forest floor.

Stemflow

Water that flows down plant stems or trunks and reaches the soil near the base of the plant.

Canopy Interception

Litter Interception

Stemflow

Types of Interception (3)

Vegetation Type

Vegetation Density

Rainfall Characteristics

Antecedent Moisture

Factors Affecting Interception Loss (4)

Depression storage

Amount of water that is retained during a storm event in the surface irregularities of the soil

Volume of water that temporarily accumulates in small natural or artificial depressions on the ground surface following rainfall, before any surface runoff begins.

Infiltration

Evaporation

Surface Sealing

Overland flow

Interaction with Other Hydrological Processes (4)