43.2. Consumptive Use

Consumptive use refers to the amount of water used by plants and evaporated from surrounding soil and water surfaces in a given area and time. It is not returned to the immediate water source.

It includes:

• Evapotranspiration (ET): Total water lost by evaporation and transpiration
• Interception losses: Water retained on plant leaves that evaporates without reaching the ground
• Water incorporated into plant tissues

Consumptive use is essentially the total amount of water that is utilized by plants and not returned to the water source, comprising three key components:

1. Evapotranspiration (ET): The total water lost from the soil and plants, combining both evaporation (from the ground) and transpiration (from plants).
2. Interception losses: This is the water that is caught on plant leaves and evaporates directly into the atmosphere before it reaches the ground.
3. Water incorporated into plant tissues: Some water is taken up by plants and becomes part of their structure, which is another way that water is 'consumed'.

Think of consumptive use like a sponge soaking up water. Just as a sponge holds water and doesn’t release it back to where it came from until it's squeezed, plants absorb water from the soil, use it to grow, and release some of it back into the air without returning all of it to the soil.

The components of consumptive use include:

• Evaporation (E): From soil and water surfaces
• Transpiration (T): Water absorbed and transpired by plants
• Evapotranspiration (ET) = E + T

Consumptive use can be broken down into its main components:

1. Evaporation (E): This is the process where water is converted from liquid to vapor and enters the atmosphere from surfaces such as soil and open water bodies.
2. Transpiration (T): This is a specific process concerning plants, where they absorb water through their roots and release it as vapor through tiny pores in their leaves. The total water loss through both processes is often referred to as Evapotranspiration (ET), which is simply the sum of evaporation and transpiration.

Imagine how a hot day feels — the water from the pavement evaporates, and you might notice that plants seem to be wilting because they are losing water too. That combined effect of the ground drying out and plants losing moisture is similar to how evaporation and transpiration work together in consumptive use.

Several factors influence consumptive use, including:

• Type of crop
• Stage of crop growth
• Climatic conditions (temperature, humidity, wind, solar radiation)
• Soil characteristics and fertility
• Water availability
• Cultural practices and irrigation method

Consumptive use is influenced by various factors:

1. Type of crop: Different plants have different water needs. For example, rice requires more water compared to a drought-resistant plant like cactus.
2. Stage of crop growth: Young plants generally require less water than fully grown plants.
3. Climatic conditions: Temperature, humidity, wind speed, and sunlight influence how much water evaporates from plants and soil.
4. Soil characteristics: Different soils retain water differently; sandy soils drain quickly while clay soils hold water longer.
5. Water availability: In times of plenty, crops will consume more water, but in drought conditions, usage may decrease.
6. Cultural practices and irrigation method: Methods of watering crops (like drip irrigation vs. flood irrigation) also determine how efficiently water is used.

Consider a diverse garden with a variety of plants. Some require more water due to their type, while others might not be thirsty at all. If it’s a hot, sunny day, the thirsty plants will utilize even more water, demonstrating how environmental factors can change the water needs of the plants.

Methods for measuring and estimating consumptive use include:

Direct Methods:

• Lysimeter Method:
  • Controlled environment
  • Measures percolation and evapotranspiration precisely
• Soil Moisture Depletion Method:
  • Change in soil moisture before and after crop cycle
  • Requires multiple soil samples

Indirect/Empirical Methods:

• Blaney-Criddle Method: CU=K⋅P⋅(0.46T+8) Where:
  • CU = Consumptive use (mm)
  • K = Crop coefficient
  • P = % of annual daytime hours for the period
  • T = Mean monthly temperature (°C)
• Thornthwaite Method:
  • Uses air temperature and latitude
  • Good for preliminary planning
• Penman Method:
  • Considers radiation, temperature, humidity, wind
  • One of the most accurate
• Modified Penman-Monteith Method:
  • Adopted as FAO standard for ET estimation.

There are several methods to measure and estimate consumptive use:

1. Direct Methods:

   • Lysimeter Method: This utilizes a controlled environment to precisely measure both percolation and evapotranspiration.
   • Soil Moisture Depletion Method: Here, the moisture content of the soil is measured before and after the crop cycle, requiring multiple samples to see how much water was used.

2. Indirect/Empirical Methods: These use formulas based on variables like temperature and type of crop.

   • Blaney-Criddle Method provides an equation to estimate water use based on crop factors and temperatures.
   • Thornthwaite Method estimates water use using basic climate information.
   • Penman Method combines various weather factors for more accurate calculations.
   • Modified Penman-Monteith Method is a standard method recommended by the FAO for estimating evapotranspiration.

Think of measuring consumptive use like tracking your water consumption at home. You might check your water bill (like using the lysimeter) or estimate how much you use by the number of showers and dishes you wash (similar to indirect methods). Each approach gives you insight into how much water you actually use.

Consumptive Use is the water actually consumed.

Water Requirement includes:

• Consumptive use
• Percolation losses
• Leaching requirement
• Other unavoidable losses

It's important to differentiate between consumptive use and water requirement:

• Consumptive Use: This refers specifically to the amount of water that is absorbed and utilized by plants or evaporated from the soil.
• Water Requirement: This is a broader concept that includes:
  1. Consumptive use (the amount used)
  2. Percolation loss: Water that seeps below the root zone and is not helpful to crops.
  3. Leaching requirement: Additional water needed to wash away salts that might accumulate in the soil.
  4. Other unavoidable losses: Water lost for other reasons.

Imagine a glass of water. The amount you actually drink is like consumptive use, but the total amount you poured in might also consider spills or overflows — that's your water requirement. Knowing both helps you plan better for your hydration needs.

The part of rainfall that is available to meet the consumptive use of the crop. It excludes:

• Deep percolation
• Surface runoff

Effective rainfall is the portion of rainfall that actually contributes to meeting the water needs of crops. This means it accounts only for the rain that does not escape from the root zone:

• Deep percolation: Rainfall that seeps too deep into the ground, beyond the reach of plant roots, hence not beneficial to the crops.
• Surface runoff: Water that flows away from the location of the crop and is lost to drains or fields, which plants cannot utilize.

Think of effective rainfall like the water that drains into a sponge. When you pour water on a countertop (like rainfall), most of it might just run off, but the sponge only absorbs what it can hold. The sponge’s absorption reflects what is effectively available to the plant.

IR=W R−ER Where:

• IR = Irrigation Requirement
• WR = Water Requirement
• ER = Effective Rainfall

The irrigation requirement (IR) is calculated using the formula that considers both the overall water need (water requirement, WR) and what has been provided by effective rainfall (ER). The idea is to determine how much additional water is needed through irrigation to fulfill the crop's needs:

• IR: The amount of water that needs to be supplied through irrigation.
• WR: Includes all necessary water for crop growth.
• ER: The water provided naturally through rainfall that is usable. The formula shows that if effective rainfall can meet some needs, then less irrigation is required.

Imagine you are budgeting for a meal. If you know how much money you have in savings (like effective rainfall) and how much you need for the meal (water requirement), the difference tells you how much more you need to spend (irrigation requirement). This helps you plan better so you don't overspend!

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