1. 5.1 Consumptive use of crops

 

Definition of Consumptive Use of Crops

Consumptive use of crops refers to the total amount of water consumed by plants during their growth period through processes such as evaporation from soil surfaces and transpiration from plant leaves. It is a critical concept in irrigation and agriculture, defining the water demand of crops and guiding irrigation management practices to ensure optimal growth and productivity.

Concept of Consumptive Use of Crops

In agriculture and irrigation engineering, the term "consumptive use" refers to the amount of water that a crop utilizes or consumes during its growth period through the processes of evaporation from the soil and transpiration from the plant's leaves. Understanding consumptive use is crucial for efficient water management and irrigation planning. Here’s a detailed explanation of this concept:

Components of Consumptive Use

1. Evaporation (E): This is the loss of water from the soil surface due to direct evaporation into the atmosphere. Evaporation depends on factors such as temperature, humidity, wind speed, and soil characteristics (like moisture content and texture).

2. Transpiration (T): Transpiration is the process by which plants absorb water from the soil through their roots and release it into the atmosphere through their leaves. It is influenced by factors including plant type, leaf surface area, environmental conditions (temperature, humidity, wind), and plant physiological activity.

Total Consumptive Use (ET)

The total consumptive use, often denoted as ET (Evapotranspiration), is the sum of evaporation and transpiration:

$ET = E + T$

Importance and Applications

• Water Management: Understanding the consumptive use of crops helps in estimating the total water demand for irrigation purposes. This knowledge guides farmers and irrigation planners in determining how much water to apply to fields to meet crop requirements and maintain optimal growth.

• Irrigation Scheduling: By estimating ET rates for different crops and growth stages, farmers can schedule irrigation events to coincide with periods of peak water demand, ensuring that plants receive adequate moisture for healthy growth and yield.

• Crop Selection: Knowledge of consumptive use assists in selecting suitable crops for specific climates and soil types, considering water availability and potential irrigation requirements.

• Water Conservation: Efficient water management practices, based on consumptive use estimates, promote water conservation by minimizing losses due to over-irrigation or inefficient water application.

Factors Affecting Consumptive Use

Several factors influence the consumptive use of crops:

• Climate: Temperature, humidity, wind speed, and solar radiation affect evaporation and transpiration rates.

• Crop Type: Different crops have varying water requirements and transpiration rates based on their growth habits, leaf structure, and physiological characteristics.

• Soil Characteristics: Soil texture, depth, and moisture-holding capacity impact evaporation rates and water availability to plants.

• Management Practices: Irrigation methods, scheduling, and soil moisture monitoring influence how effectively water is used by crops.

Estimation of Consumptive Use

Consumptive use can be estimated using empirical methods based on climate data, crop coefficients (which reflect the relative water requirements of different crops), and soil characteristics. Advanced techniques may involve satellite data and modeling to predict ET rates more accurately over large areas.

Example Scenario

For example, if a field of maize has an estimated consumptive use (ET) of 6 mm per day during the peak growing season, irrigation scheduling would aim to replenish this amount of water regularly to maintain optimal soil moisture levels and support crop growth.

In conclusion, consumptive use is a fundamental concept in irrigation and agriculture, determining the water requirements of crops and guiding irrigation management practices to optimize water use efficiency, crop productivity, and sustainable agricultural practices.

Let's consider this numerical example to illustrate the concept of consumptive use for a specific crop, such as wheat, in a hypothetical scenario.

Example: Consumptive Use Calculation for Wheat

Scenario:

Crop: Wheat (Triticum aestivum)

Location: Central Valley, California, USA

Growth Conditions:

• Planting Date: November 1st
• Harvest Date: June 1st
• Climate: Mediterranean climate with hot, dry summers and mild, wet winters.

Steps to Calculate Consumptive Use:

1. Determine Evapotranspiration (ET):

   • Evaporation (E): Estimated based on local climate data and soil conditions. Let's assume average daily evaporation is 4 mm/day during the growing season.

   • Transpiration (T): Calculated using crop coefficients (Kc) and reference evapotranspiration (ETo) values. For wheat, assume Kc values range from 0.8 to 1.2 throughout its growth stages.

2. Total Consumptive Use (ET):

   • Calculate daily consumptive use $ET = E + T$.

3. Example Calculation:

   Let's assume:

   • Average daily evaporation (E) = 4 mm/day
   • Average transpiration (T) = 6 mm/day (considering Kc values and local ETo)

   Therefore, $ET = E + T = 4 \text{ mm/day} + 6 \text{ mm/day} = 10 \text{ mm/day}$.

4. Total Consumptive Use over the Growing Season:

   • Duration: From November 1st to June 1st (7 months)

   • Total Consumptive Use $= ET \times \text{Number of Days}$

   • Assuming an average ET of 10 mm/day over the 7-month growing season:

     $\text{Total Consumptive Use} = 10 \text{ mm/day} \times 7 \times 30 \text{ days/month} = 2100 \text{ mm}$.

   • Convert to meters (since 1 mm = 0.001 m):

     $\text{Total Consumptive Use} = 2100 \text{ mm} \times 0.001 = 2.1 \text{ meters}$.

Practical Application:

• Irrigation Management: Based on the calculated consumptive use, farmers can schedule irrigation to ensure wheat plants receive sufficient water throughout their growth stages, adjusting irrigation frequency and duration as needed based on weather conditions and soil moisture levels.

• Water Conservation: Efficiently managing water resources based on consumptive use estimates helps minimize water wastage and supports sustainable agriculture practices.

In summary, consumptive use calculation provides a quantitative measure of the water requirement of crops like wheat, facilitating informed irrigation decisions and optimizing crop yields in agricultural settings.