1. Crop Water Requirement (ET crop)

• Definition: The crop water requirement is the total amount of water needed to meet the water loss due to evapotranspiration (ET), which includes both evaporation and transpiration.
• Key Factors Influencing Crop Water Requirement:
  • Climate: In sunny and hot climates, crops need more water per day compared to cloudy or cool climates. Temperature, humidity, and solar radiation are major climatic factors affecting crop water needs.
  • Crop Type: Different crops have varying water requirements. For instance, crops like maize and sugarcane are high water-demanding crops, while crops like millet or sorghum need less water.
  • Growth Stage: Water needs change throughout the growth cycle. Young crops or crops in the early stages require less water, whereas fully grown crops or crops in the reproductive stages need more water.

2. Evapotranspiration (ET)

• Definition: Evapotranspiration is the combined loss of water from the soil (evaporation) and from plant surfaces (transpiration). It is expressed as the depth of water lost per unit time, e.g., mm/day.
  • Evaporation: The process where water from soil or vegetation surface changes to vapor and escapes into the atmosphere. It occurs from:
    • Bare soil
    • Intercepted water on plant leaves (dew, rain, etc.)
    • Water bodies (lakes, ponds)
  • Transpiration: Water loss from plants through small pores (stomata) on their leaves. Water moves from the soil, through the plant, and eventually evaporates through the stomata.
• Importance: Quantifying evapotranspiration is essential for:
  • Managing agricultural water resources
  • Designing irrigation systems
  • Estimating the water requirements for different crops
  • Helping with crop scheduling and planning irrigation

3. Consumptive Use (CU)

• Definition: Consumptive use refers to the total volume of water used by the crop in both evaporation and transpiration, as well as a small fraction used in metabolic processes (which is generally negligible, i.e., around 1% of the total ET).
• Types of Consumptive Use:
  • Daily Consumptive Use: The total amount of water used by a crop on a daily basis (expressed in mm/day or cm/day).
  • Seasonal Consumptive Use: The total water used by the crop during its entire growing season.
  • Peak Period Consumptive Use: The highest water consumption during the crop’s growth period, which typically happens during the flowering or reproductive stages when crop growth and evapotranspiration rates are high.

4. Potential Evapotranspiration (PET)

• Definition: PET refers to the maximum rate of evapotranspiration that can occur from a hypothetical well-watered, actively growing crop with an abundant water supply. It is a theoretical value used for calculating crop water requirements under optimal conditions.
• Historical Context: The concept of potential evapotranspiration was introduced in the 1940s and 1950s by researchers like Penman. It represents the maximum rate of evapotranspiration that could occur if the soil water supply was not limiting.
• Use: PET serves as a standard reference for estimating crop water needs and for understanding how much water a specific crop could potentially use under ideal conditions.

5. Reference Crop Evapotranspiration (ET₀)

• Definition: ET₀ refers to the evapotranspiration from a reference crop, usually grass (about 8-15 cm tall), under standard conditions. It serves as a benchmark for estimating evapotranspiration from other crops.
• Calculation Methods: There are several ways to estimate ET₀, including methods like:
  • Blaney-Criddle Method
  • Modified Penman Method
  • Radiation Method
  • Pan Evaporation Method
• Use: ET₀ is widely used to determine irrigation needs for crops and to calculate crop-specific evapotranspiration using crop coefficients (Kc).

6. Actual Crop Evapotranspiration (ET crop)

• Definition: This refers to the actual rate of evapotranspiration from a specific crop under given soil water and atmospheric conditions.
• Formula: ETcrop = Kc × ET₀

Where Kc is the crop coefficient (which adjusts reference evapotranspiration to the specific crop) and ET₀ is the reference crop evapotranspiration.

• Influencing Factors:
  • Soil moisture: The amount of available water in the soil impacts transpiration.
  • Weather conditions: Temperature, wind speed, and solar radiation directly affect the rate of evapotranspiration.
  • Crop type: Crops with deep roots or high biomass typically have higher transpiration rates than shallow-rooted crops.

7. Water Requirement

• Definition: Water requirement is the total amount of water a crop needs to grow and mature, regardless of its source (e.g., rainfall, irrigation, groundwater). It is the sum of:
  • Evapotranspiration: Water lost through evaporation and transpiration.
  • Unavoidable losses: Water lost during irrigation applications (e.g., runoff, percolation).
  • Special needs: Water needed for specific crop activities like land preparation, leaching salts, or frost control.
• Formula: WR = ET + Application Losses + Special Needs
• Water Requirement in Practice: If all water is supplied through irrigation, water requirement equals irrigation requirement.

8. Water Requirement of Different Crops

• Different crops have vastly varying water needs depending on their growth habits and climatic requirements. Below is a table illustrating the typical water requirements (in mm) for several common crops:

Note: These values depend on various factors, including local climate, soil type, irrigation practices, and crop management.

9. Effective Precipitation (EP)

• Definition: Effective precipitation refers to the portion of rainfall that is stored in the soil and is available for plant uptake. It does not include water lost due to surface runoff or deep percolation.
• Formula: EP=P−(R+DP)
• Where:
  • P = Precipitation
  • R = Runoff
  • DP = Deep percolation (water moving below the root zone)
• Importance: Effective precipitation is important in determining how much supplemental irrigation is needed, as it provides an estimate of available water for crops from rainfall.

10. Crop Coefficients (Kc)

• Definition: The crop coefficient (Kc) is a factor used to adjust the reference evapotranspiration (ET₀) to the actual evapotranspiration of a specific crop (ET crop). Kc varies by crop type, growth stage, and climatic conditions.
• How Kc is Used: Kc allows the conversion of reference crop evapotranspiration (ET₀) to crop-specific evapotranspiration (ET crop). By multiplying the reference ET₀ by Kc, we get the water requirements of a particular crop:
•  
• ETcrop = Kc × ET₀
• Growth Stages and Kc: The Kc varies throughout the crop’s growth cycle:
  • Initial Stage: In the early growth stage (when the crop is small), Kc is low since the crop cover is sparse and the evapotranspiration is mainly from the soil surface.
  • Development Stage: During this period, the crop starts growing rapidly, and Kc increases as the plant canopy expands.
  • Mid-Season Stage: This is the period of peak water demand when the crop is fully developed. Kc reaches its maximum, and evapotranspiration is primarily from the plant’s transpiration.
  • Late Season Stage: As the crop matures and begins to dry, Kc decreases, and transpiration rates fall.

Example of Kc Values:

• For wheat, Kc might range from 0.4 (early growth) to 1.2 (mid-season) and drop back to around 0.7 at maturity.
• For maize, Kc could range from 0.6 to 1.3, peaking during its reproductive phase.

Sources of Kc values:

• The Kc values are often derived from experimental data or empirical models. The FAO Irrigation and Drainage Paper No. 56 provides standard Kc values for a wide range of crops based on climatic conditions.

11. Water Use Efficiency (WUE)

• Definition: Water use efficiency is the ratio of the amount of crop yield produced per unit of water used. It is a measure of how effectively a crop uses water to produce biomass or yield. Higher WUE indicates better efficiency in utilizing available water resources.
• Formula: WUE = Crop Yield (kg or tons) / Water Used (m³ or liters)

•  Importance of WUE:
  • Environmental Sustainability: By increasing WUE, farmers can reduce water consumption while maintaining or improving crop productivity, which is critical in areas facing water scarcity.
  • Agronomic Practices: Optimizing irrigation schedules, selecting drought-tolerant crop varieties, improving soil management (such as adding organic matter to enhance water retention), and minimizing water losses during irrigation all contribute to better WUE.
• Factors Affecting WUE:
  • Crop Variety: Some crops are naturally more water-efficient than others. For instance, crops like millets or sorghum have higher WUE compared to rice or sugarcane.
  • Irrigation Practices: Efficient irrigation methods, such as drip irrigation or sprinkler systems, reduce water losses compared to surface irrigation.
  • Soil Health: Healthy, well-drained soils with high organic content retain water better and reduce the water requirement for crops, thus improving WUE.
  • Climate: In regions with hot and dry conditions, WUE can be improved by providing supplemental irrigation and using drought-tolerant crops.

12. Irrigation Requirement (IR)

• Definition: Irrigation requirement is the amount of water that needs to be added to the crop through irrigation to meet its water needs, considering rainfall and effective precipitation.
• Calculation: IR=ETcrop – P effective

• Where:
  • ETcrop = Crop evapotranspiration (calculated using Kc × ET₀)
  • Peffective = Effective precipitation (the amount of rainfall available for the crop)

Irrigation Scheduling: To avoid under or over-irrigating crops, it is crucial to regularly monitor soil moisture levels and adjust irrigation schedules. Irrigation should be done when soil moisture falls below a certain threshold to maintain optimal crop growth.

Irrigation Methods:

• • Surface Irrigation: Water flows over the soil surface and is used for crops like rice. It is a less water-efficient method, as water may evaporate or be lost through runoff.
  • Drip Irrigation: Water is delivered directly to the root zone in small amounts. This method is highly efficient and reduces water loss.
  • Sprinkler Irrigation: Water is sprayed over the crop like rainfall. This is an efficient method for crops in arid and semi-arid regions.
  • Subsurface Drip Irrigation: Water is applied below the soil surface directly to the root zone, reducing evaporation losses.