Crop Water Use Efficiency Determination: Testing Parameters, Methods, and Applications

Crop water use efficiency (WUE) determination quantifies the relationship between crop productivity and the water consumed during the growing season, delivering critical data for irrigation optimization, drought-tolerance breeding, and sustainable agricultural water management. This article explains the core parameters, sample types, measurement methodologies, and applicable standards used by accredited laboratories and research institutions to evaluate water use efficiency at leaf, plant, field, and ecosystem levels.

Technical Overview

Crop water use efficiency (WUE) is a key agronomic and physiological indicator that expresses the amount of biomass, grain yield, or carbon fixed per unit of water consumed by a crop. At the field scale, WUE is commonly defined as the ratio of economic yield or total dry matter to evapotranspiration (ET), typically expressed in kg/m³ or kg per hectare per millimeter of water. At the leaf level, WUE is evaluated as the ratio of net photosynthetic rate to transpiration rate, expressed as µmol CO₂ per mol H₂O, or as the ratio of photosynthesis to stomatal conductance, which is known as intrinsic water use efficiency.

Reliable WUE determination is increasingly important because agriculture accounts for approximately 70% of global freshwater withdrawals. Accurate measurement supports cultivar selection, irrigation scheduling, water-saving engineering evaluation, and climate resilience research. The measurement framework integrates crop physiological measurements, soil water monitoring, and meteorological observations, with reference evapotranspiration typically estimated using the Penman–Monteith equation as recommended in the FAO Irrigation and Drainage Paper No. 56 guidelines. Determination is performed at four principal levels: single leaf, whole plant, field plot, and ecosystem scale, and accredited third-party laboratories as well as agricultural research institutions routinely conduct such analyses under recognized international and national standards.

Testing Samples

Leaf Samples

Fully expanded upper-canopy leaves, such as the flag leaf in cereals or the most recently matured leaf in broadleaf crops, are collected for in vivo gas exchange measurement. For isotope-based analysis, subsamples of the same leaves are oven-dried at 65–70 °C and finely ground for carbon isotope discrimination testing.

Whole-Plant and Biomass Samples

Aboveground biomass is sampled at defined growth stages and partitioned into stems, leaves, and reproductive organs. Belowground root biomass may be recovered from soil cores or monoliths and washed clean before drying and weighing to determine total dry matter accumulation.

Grain and Economic Yield Samples

Harvested grain, fruit, or forage samples are weighed fresh and after drying so that yield can be expressed on a standardized moisture basis. Grain moisture content is commonly determined according to ISO 712, which specifies the reference method for moisture determination in cereals and cereal products.

Soil Samples

Soil cores collected at fixed depth intervals are used to determine gravimetric water content according to ISO 11465 or ASTM D2216, and volumetric water content using coring sleeves in accordance with ISO 11461. These data establish soil water storage and its change over time, which are essential for field water balance calculations.

Stem, Xylem, and Water Samples

Stem samples, xylem sap, irrigation water, precipitation, and soil water extracts may be collected for hydrogen and oxygen isotope analysis when partitioning transpiration sources or validating water uptake depth.

Testing Items

Typical testing items for a comprehensive crop water use efficiency evaluation include the following:

• Instantaneous water use efficiency (net photosynthetic rate divided by transpiration rate)
• Intrinsic water use efficiency (net photosynthetic rate divided by stomatal conductance)
• Seasonal and integrated water use efficiency (dry matter or yield per unit of evapotranspiration)
• Carbon isotope discrimination (δ13C) of leaf, stem, and grain dry matter
• Gas exchange parameters: net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular CO₂ concentration
• Actual evapotranspiration and its soil evaporation and plant transpiration components
• Soil water storage, change in soil water storage, deep percolation, and surface runoff components of the field water balance
• Sap flow density and whole-plant transpiration
• Canopy temperature and crop water stress index (CWSI)
• Biomass moisture content, harvest index, and yield at standardized moisture basis

Testing Methods

Field Soil Water Balance Method

Evapotranspiration is computed as precipitation plus irrigation minus runoff, deep drainage, and the change in soil water storage measured between sampling dates. Soil water content is monitored with gravimetric oven-drying per ISO 11465 or ASTM D2216, or continuously with neutron probes, time-domain reflectometry (TDR), or frequency-domain reflectometry (FDR) sensors installed at multiple depths.

Weighing Lysimeter Method

Large precision weighing lysimeters containing undisturbed soil monoliths and growing crops provide direct, high-resolution measurements of actual evapotranspiration. This method is regarded as a reference standard for validating ET estimation models and sensor-based methods.

Infrared Gas Exchange Analysis

Portable photosynthesis systems equipped with infrared gas analyzers measure leaf-level net CO₂ assimilation, transpiration, and stomatal conductance under controlled light, humidity, temperature, and CO₂ concentration. Measurements are typically performed during mid-morning hours under representative weather conditions, with sufficient biological replication across plants and canopy positions.

Stable Carbon Isotope Discrimination (δ13C)

Dried and ground leaf or grain samples are analyzed by elemental analysis coupled with isotope ratio mass spectrometry (EA-IRMS). In C₃ crops, carbon isotope discrimination integrates stomatal behavior over the entire growth period and is widely used as a time-integrated proxy for intrinsic water use efficiency in breeding programs. Isotope results are reported against internationally recognized reference scales.

Sap Flow Measurement

Thermal dissipation probes, heat pulse, or heat ratio sensors installed on plant stems quantify sap flow density, which is scaled to whole-plant and stand-level transpiration. This method separates plant transpiration from total evapotranspiration and is well suited to orchard, vineyard, and row crop studies.

Eddy Covariance and Bowen Ratio Methods

At the ecosystem scale, eddy covariance systems measure turbulent fluxes of water vapor and CO₂ to determine actual evapotranspiration and canopy-level water use efficiency. The Bowen ratio energy balance method offers a complementary micrometeorological approach for field-scale energy partitioning.

Canopy Temperature and Remote Sensing Methods

Infrared thermometry and thermal imaging quantify canopy temperature and crop water stress index as rapid phenotyping proxies for water use efficiency. Unmanned aerial vehicles and satellite platforms equipped with multispectral and thermal sensors support spatial evapotranspiration mapping using surface energy balance models for regional WUE assessment.

Container Gravimetric Method

In greenhouse and growth chamber studies, potted plants are weighed daily to determine water consumption, and water use efficiency is calculated from dry matter accumulation per unit of water transpired, following experimental protocols consistent with national irrigation testing specifications.

Applications

Determination of crop water use efficiency serves a broad range of scientific, commercial, and regulatory purposes:

• Screening germplasm and selecting drought-tolerant, water-efficient cultivars in breeding programs
• Optimizing irrigation scheduling and evaluating regulated deficit irrigation strategies
• Comparing water-saving irrigation technologies such as drip, sprinkler, and alternate wetting and drying systems
• Assessing agronomic practices including mulching, tillage regimes, planting density, and fertilizer management
• Supporting genotype-by-environment trials and regional adaptation studies under climate change scenarios
• Quantifying carbon–water coupling in ecosystem research and evapotranspiration partitioning studies
• Providing technical data for agricultural water footprint accounting, water rights allocation, and sustainability reporting
• Evaluating compliance with water-saving irrigation engineering requirements under relevant GB/T technical codes and international guidance

FAQ

What is the difference between instantaneous, intrinsic, and integrated water use efficiency? Instantaneous WUE is the ratio of photosynthesis to transpiration at a single moment; intrinsic WUE relates photosynthesis to stomatal conductance and removes the direct effect of humidity; integrated WUE expresses final yield or biomass per unit of water consumed across the whole growing season.

What are typical WUE values for major field crops? Under favorable management, wheat commonly reaches approximately 1.0–1.5 kg/m³, maize approximately 1.5–2.5 kg/m³, and rice generally below 1.0 kg/m³ when flooded. Values vary substantially with climate, soil, cultivar, and irrigation regime.

Why is carbon isotope discrimination used to estimate WUE? In C₃ species, discrimination against the heavier carbon-13 isotope during photosynthesis decreases when stomata partially close under water stress. Because leaf or grain δ13C integrates this signal over weeks to months, it provides a robust, time-averaged surrogate for repeated instantaneous gas exchange measurements.

Which standards are applicable to WUE testing? Relevant references include ISO 712 for cereal moisture determination, ISO 11465 and ISO 11461 for soil water content, ASTM D2216 for laboratory determination of soil moisture by mass, FAO Paper No. 56 for evapotranspiration computation, and applicable GB/T technical codes for water-saving irrigation engineering and irrigation experimentation.

How much sample is required for δ13C analysis? Laboratories typically require 20–50 mg of oven-dried, finely ground (<0.15 mm) leaf or grain material per determination, with 2–5 mg actually consumed during elemental analyzer–isotope ratio mass spectrometry measurement. Replicate field sampling is recommended to capture spatial variability.

What quality control measures are applied? Calibrated sensors and instruments, certified reference materials for isotope analysis, blanks and duplicates, standardized measurement windows for gas exchange, and moisture-corrected yield reporting are standard quality assurance practices used by accredited third-party laboratories to ensure data comparability and traceability.