Why Water Productivity Matters

Water is one of the most valuable inputs in desert agriculture. Here in Yuma, Arizona, most of our lettuce is irrigated using water from the Colorado River, and every drop counts. One way to measure this efficiency is through Crop Water Productivity (CWP), often called “more crop per drop.”

What is Crop Water Productivity?
CWP tells us how much crop yield we get for each unit of water used.

CWP = Crop Yield/Crop Water Use  

• Crop Yield: The total harvested product (tons or kilograms per acre).
• Crop Water Use: The total water consumed by the crop through evapotranspiration (ETa), which includes both soil evaporation and plant transpiration.

A higher CWP means more yield per unit of water an important goal in our desert environment.

Why It’s Important in Yuma

Yuma’s hot, dry, and often windy conditions cause high water loss from soil and plants. Improving CWP helps ensure that every acre-foot of water supports strong, profitable production. Measuring CWP helps identify where we can save water through irrigation timing, soil management, or better agronomic practices.

Physical vs. Economic CWP
CWP can be viewed in two ways:
Physical CWP: How much lettuce (by weight) is produced per unit of water.

Economic CWP: The market value produced per unit of water. What Affects Crop Water Productivity Several factors influence how efficiently crops use water:

• Climate: High temperatures and low humidity increase ET.
• Soil: Soils rich in organic matter hold water longer.
• Crop Variety: Some varieties are more tolerant to water stress.
• Growth Stage: Lettuce needs the most water during the heading stage.
• Irrigation Management: Proper scheduling avoids both over- and under-watering.
• Field Practices: Mulching, reduced tillage (does not apply to lettuce), and good weed control help conserve water.

Why Compare Organic and Conventional Systems?
Interest in organic lettuce production is growing in the Yuma Valley, but questions remain about its efficiency under desert conditions. Organic systems build soil health with compost and cover crops, which can improve water-holding capacity. However, conventional systems often achieve higher yields due to faster growth and more readily available nutrients. Comparing both systems helps us understand which approach makes better use of limited water resources in Yuma’s unique climate.

Field Study in Yuma Ag Center, 2024–2025
During the 2024–2025 season, a trial was conducted at the Yuma Agricultural Center to measure and compare CWP between organic and conventional iceberg lettuce systems. Both systems were grown under similar conditions using subsurface drip irrigation. This information was used to calculate and compare CWP for each system.

Crop Water Productivity Results
The results from the trial are shown in Figure 1 below. CWP values are expressed as kilograms of lettuce produced per cubic meter of water used (kg/m³). Overall, conventional lettuce produced about 40–50% more yield per unit of water compared to organic systems. This difference was mainly due to stronger plant growth and faster canopy development under conventional fertilization and management. However, organic systems still provide long-term benefits. Healthier soils in organic plots can store more moisture, reduce runoff, and support sustainable water use over time. 

Figure 1. Crop Water Productivity (CWP) for organic and conventional iceberg
lettuce systems at the Yuma Ag Center, 2024–2025 season.

Take Home Messages  

• Conventional lettuce achieved higher CWP due to higher yield and faster growth.
• Organic lettuce used less total water but produced a lower yield.
• Healthy, high-organic-matter soils can gradually improve water efficiency in organic systems.
• Careful irrigation timing and soil moisture monitoring are key to improving water productivity in all systems.