😅Hydrological Modeling Unit 5 – Evapotranspiration and Interception

Key Concepts and Definitions

• Evapotranspiration (ET) combines evaporation from land surfaces and transpiration from vegetation into a single term
• Potential evapotranspiration (PET) represents the maximum rate of ET that would occur given an unlimited water supply (reference crop evapotranspiration)
• Actual evapotranspiration (AET) is the real amount of ET occurring under specific conditions, limited by available water
• Interception is the process by which precipitation is caught and stored on vegetation surfaces before reaching the ground
  • Canopy interception refers to precipitation intercepted by leaves, branches, and stems of plants
  • Litter interception occurs when precipitation is caught by dead plant material on the ground (leaf litter, debris)
• Throughfall is the portion of precipitation that passes through the canopy and reaches the ground surface
• Stemflow describes the portion of intercepted water that flows down plant stems and trunks to the ground
• Canopy storage capacity represents the maximum amount of water that can be held on vegetation surfaces

Components of the Hydrologic Cycle

• Precipitation is the primary input of water to the land surface, including rain, snow, hail, and sleet
• Evapotranspiration returns water from the land surface to the atmosphere through evaporation and transpiration processes
  • Evaporation occurs from open water bodies, soil surfaces, and wet vegetation
  • Transpiration is the process by which water moves through plants and is released to the atmosphere through stomata
• Infiltration is the movement of water into the soil surface, influenced by soil properties and antecedent moisture conditions
• Surface runoff occurs when precipitation exceeds the infiltration capacity of the soil or when the soil becomes saturated
• Groundwater recharge is the process by which water moves downward through the soil profile to replenish aquifers
• Streamflow represents the integrated response of a watershed to precipitation inputs, including surface runoff and baseflow
• Soil moisture storage plays a crucial role in regulating ET rates and partitioning precipitation into runoff and infiltration

Evapotranspiration Processes

• Evaporation is the conversion of liquid water to water vapor, driven by the vapor pressure gradient between the surface and the atmosphere
• Transpiration involves the movement of water through plants, from roots to leaves, and its release through stomata
  • Stomatal conductance regulates the rate of transpiration based on environmental factors (light, temperature, humidity)
  • Plant water use efficiency describes the ratio of carbon assimilation to water loss through transpiration
• Aerodynamic resistance influences the turbulent transfer of water vapor from the surface to the atmosphere
• Surface resistance represents the combined resistance of soil, plant, and litter surfaces to evaporation
• Energy balance at the Earth's surface governs the partitioning of available energy into sensible heat flux, latent heat flux (ET), and ground heat flux
  • Net radiation is the primary energy source driving ET, determined by incoming and outgoing short- and longwave radiation
  • Latent heat flux is the energy consumed during the phase change of water from liquid to vapor during ET
• Advection can enhance ET rates by transporting dry, warm air from adjacent areas, increasing the vapor pressure deficit

Interception Mechanisms

• Canopy interception is influenced by vegetation characteristics such as leaf area index (LAI), canopy structure, and plant species
• Leaf surface properties, such as wettability and roughness, affect the ability of leaves to retain intercepted water
• Evaporation from wet canopy surfaces occurs at the potential rate, as intercepted water is readily available for evaporation
  • Canopy evaporation can be a significant component of total ET, especially in forested ecosystems
  • Intercepted water has a cooling effect on the canopy due to the latent heat of vaporization
• Throughfall is affected by canopy gap fraction, leaf angle distribution, and precipitation intensity
• Stemflow is influenced by bark characteristics (smoothness, water-holding capacity) and branch architecture
• Litter interception depends on the depth, density, and water-holding capacity of the litter layer
  • Litter evaporation can be a substantial component of total ET in ecosystems with thick litter layers (forests, grasslands)
• Canopy storage capacity varies with vegetation type, seasonality (leaf phenology), and precipitation event characteristics (duration, intensity)

Measurement Techniques and Instruments

• Lysimeters directly measure ET by monitoring changes in soil water content over time
  • Weighing lysimeters use a balance to measure changes in mass, representing water loss through ET
  • Drainage lysimeters collect percolated water and measure the difference between inputs (precipitation) and outputs (percolation, ET)
• Eddy covariance systems measure vertical fluxes of water vapor and heat using high-frequency measurements of wind speed, temperature, and humidity
  • Evapotranspiration is calculated as the covariance between vertical wind speed and water vapor concentration
  • Energy balance closure is used to assess the quality of eddy covariance measurements
• Bowen ratio energy balance method estimates ET by measuring gradients of temperature and humidity above the surface
• Sap flow sensors measure the velocity of water movement in plant stems, providing estimates of transpiration rates
  • Heat pulse velocity and thermal dissipation methods are commonly used sap flow techniques
• Remote sensing techniques, such as satellite-based vegetation indices (NDVI) and land surface temperature, can provide spatially distributed estimates of ET
• Pan evaporation measures the evaporation rate from a standardized open water surface, serving as a proxy for potential evapotranspiration
• Throughfall and stemflow collectors, such as tipping buckets and spiral gauges, measure the quantity and spatial variability of water reaching the ground

Modeling Approaches and Equations

• Penman-Monteith equation combines energy balance and aerodynamic principles to estimate evapotranspiration
  • Requires inputs of net radiation, air temperature, humidity, wind speed, and surface and aerodynamic resistances
  • FAO-56 reference evapotranspiration equation is a standardized form of the Penman-Monteith equation for a reference grass surface
• Priestley-Taylor equation simplifies the Penman-Monteith equation by assuming a constant relationship between sensible and latent heat fluxes
• Hargreaves equation estimates potential evapotranspiration based on air temperature and extraterrestrial radiation
• Gash analytical model simulates rainfall interception by considering canopy storage capacity, evaporation rate, and throughfall
  • Sparse and revised versions of the Gash model have been developed for different canopy structures and precipitation regimes
• Rutter numerical model simulates the time-varying processes of interception, evaporation, and drainage from the canopy
• Soil-Vegetation-Atmosphere Transfer (SVAT) models couple land surface processes, including ET, with atmospheric boundary layer dynamics
  • Examples include NOAH, CLM, and JULES land surface models
• Crop coefficient (Kc) approach relates actual crop evapotranspiration to reference evapotranspiration using empirical coefficients based on crop type and growth stage

Environmental Factors and Influences

• Solar radiation is the primary energy source for evapotranspiration, with higher radiation levels leading to increased ET rates
• Air temperature affects the vapor pressure deficit and the rate of evaporation, with higher temperatures generally increasing ET
  • Temperature also influences the duration of the growing season and the timing of plant phenological stages
• Humidity, or vapor pressure deficit, determines the gradient for water vapor transfer from the surface to the atmosphere
  • Lower humidity levels (higher vapor pressure deficit) enhance ET rates by increasing the evaporative demand
• Wind speed affects the turbulent mixing of air and the transfer of water vapor away from the surface
  • Higher wind speeds generally increase ET rates by reducing the boundary layer resistance
• Soil moisture availability controls the actual ET rate, as water stress can limit plant transpiration and soil evaporation
  • Soil texture, structure, and organic matter content influence soil water retention and availability for ET
• Vegetation characteristics, such as plant species, rooting depth, and stomatal behavior, regulate transpiration rates and intercept precipitation
  • Land cover changes, such as deforestation or urbanization, can significantly alter ET patterns and water balance components
• Topography, including elevation, slope, and aspect, affects the spatial distribution of ET by influencing solar radiation, temperature, and wind exposure
• Seasonality and climate variability, such as wet and dry seasons or El Niño/La Niña events, can lead to significant variations in ET rates and water availability

Applications in Hydrological Modeling

• Quantifying the water balance of watersheds, including the partitioning of precipitation into ET, runoff, and groundwater recharge
• Estimating irrigation water requirements for agricultural water management and scheduling
• Assessing the impacts of land use and land cover changes on hydrological processes and water resources
  • Evaluating the hydrological consequences of deforestation, afforestation, or urbanization
  • Quantifying the role of green infrastructure (urban parks, green roofs) in mitigating urban heat island effects and reducing runoff
• Investigating the feedback between ET and atmospheric processes, such as boundary layer development and precipitation recycling
• Studying the effects of climate change on ET patterns and water availability
  • Assessing the potential impacts of rising temperatures, altered precipitation patterns, and increased climate variability on ET and water resources
  • Evaluating the role of ET in the water-energy-carbon nexus and its implications for ecosystem functioning and services
• Improving the representation of ET processes in land surface models, regional climate models, and Earth system models
  • Developing and testing parameterizations for canopy interception, soil evaporation, and plant transpiration
  • Assimilating ET observations (in-situ measurements, remote sensing data) to constrain and validate model simulations
• Informing water resources planning and management decisions, such as reservoir operations, water allocation, and drought mitigation strategies