3.4 Evapotranspiration in water balance calculations

Evapotranspiration in Water Balance Calculations

Evapotranspiration (ET) is the combined water loss from evaporation off surfaces and transpiration through plants. In water balance calculations, ET is often the largest and hardest-to-measure outflow term, which makes it a major source of uncertainty. Getting ET right is essential for everything from sizing reservoirs to scheduling irrigation to predicting how a watershed responds to climate shifts.

Importance of Evapotranspiration

ET plays a central role in the hydrologic cycle because it returns a huge fraction of precipitation back to the atmosphere. In arid and semi-arid regions, ET can consume 90% or more of annual precipitation, leaving very little for runoff or groundwater recharge.

Because ET draws from the same pool of water that feeds streams and aquifers, high ET rates directly reduce water availability for other components of the water balance. This matters for:

• Water resource management — knowing how much water leaves as ET tells you how much remains for human use and ecological flows
• Agriculture — crop water demand is driven by ET, so accurate estimates guide irrigation planning
• Ecosystem health — shifts in ET affect soil moisture, wetland persistence, and vegetation patterns

Evapotranspiration in the Water Balance Equation

The fundamental water balance equation is:

$P = Q + ET + \Delta S$

where $P$ is precipitation, $Q$ is runoff, $ET$ is evapotranspiration, and $\Delta S$ is the change in water storage (soil moisture, groundwater, snowpack, etc.).

If you know any three of these terms, you can solve for the fourth. In practice, $P$ and $Q$ are measured directly with rain gauges and stream gauges, while $\Delta S$ is estimated or assumed negligible over long periods. That leaves ET as the residual, which is why independent ET estimates are so valuable for checking whether your water balance closes.

Methods for estimating ET fall into three broad categories:

• Direct measurements — lysimeters (weigh a block of soil to track water loss) and eddy covariance towers (measure turbulent water vapor fluxes above a surface)
• Indirect/physically-based methods — energy balance, mass transfer, and combination approaches that use meteorological data
• Empirical equations — Penman-Monteith (the FAO standard, uses radiation, temperature, humidity, and wind speed), Priestley-Taylor (simplified energy-based approach), and Hargreaves (temperature-only, useful when data are limited)

Spatial and temporal scales matter when choosing a method:

• Point scale (a single plant or soil column) — lysimeters, sap flow sensors
• Field scale (an agricultural plot or small catchment) — eddy covariance, soil water balance
• Watershed or regional scale (river basins) — remote sensing, distributed hydrologic models

Temporal resolution ranges from hourly or daily (for irrigation scheduling) to monthly, seasonal, or annual (for water resources planning and climate change assessments). Over longer time periods, $\Delta S$ tends toward zero, simplifying the balance to $ET \approx P - Q$.

Sensitivity to Evapotranspiration Changes

Small shifts in ET can ripple through the entire water balance. Increased ET reduces runoff, lowers groundwater recharge, and depletes soil moisture storage. Decreased ET does the opposite: more water reaches streams, aquifers fill faster, and water tables can rise.

Factors that drive ET changes:

• Climate variables — temperature, humidity, wind speed, and solar radiation all affect the atmospheric demand for water (often expressed as potential ET)
• Vegetation characteristics — plant type, growth stage, and leaf area index control how much water plants pull from the soil
• Soil properties — texture, moisture content, and salinity influence how readily water moves to roots and to the surface
• Land use and management — irrigation adds water, mulching reduces soil evaporation, deforestation removes transpiring surfaces

Sensitivity analysis helps you quantify these effects. Two common approaches:

1. Scenario-based modeling — run a hydrologic model under different ET assumptions (e.g., +10%, -10%) and compare the resulting runoff and storage
2. Elasticity coefficients — express the percent change in runoff per percent change in ET. For example, an elasticity of -2.0 means a 10% increase in ET produces roughly a 20% decrease in runoff

These tools are especially useful for evaluating how climate change or land use conversion might alter a basin's water balance.

Applications of Evapotranspiration Concepts

Water resources management. ET estimates feed directly into assessments of water availability and demand. Water managers incorporate ET into allocation decisions, drought contingency plans, and projections of how land use change or warming temperatures will shift the supply-demand balance.

Irrigation scheduling. ET-based scheduling is the standard approach for optimizing water use in agriculture. The process works like this:

1. Calculate reference ET ($ET_0$) from weather data using an equation like Penman-Monteith
2. Multiply $ET_0$ by a crop coefficient ($K_c$) that reflects the specific crop and its growth stage to get crop ET
3. Compare crop ET to available soil moisture and decide when and how much to irrigate

This approach prevents both over-watering (which wastes water and leaches nutrients) and under-watering (which reduces yield).

Ecosystem studies. ET is a key term in both the water balance and the energy balance of ecosystems. Tracking ET patterns across landscapes helps researchers assess ecosystem services like water yield and carbon sequestration, and understand how vegetation, soil, and water interact in natural and managed systems.