37. Water Requirement of Crops – Crops and Crop Seasons in India

Water is one of the most essential inputs for agricultural production. In a country like India, where agriculture forms the backbone of the economy, understanding the water requirement of crops is vital for efficient irrigation planning, design of irrigation structures, and sustainable water resource management. This chapter deals with the concepts related to water needs of various crops, classification of crops based on seasons, and the cropping patterns observed in India. It also covers the terminologies and calculations used in determining irrigation needs and crop consumptive use.

This section introduces the significance of water in agriculture, especially in India, which heavily relies on agriculture for its economy. Understanding how much water crops need helps farmers manage irrigation better, plan water resources wisely, and increase crop yields sustainably. The chapter will explain various aspects of crop water requirement like different crop seasons, classification of crops, and how to measure the water needs of crops efficiently.

Imagine you're a chef preparing a complex meal. Each ingredient has a specific quantity required for the dish to turn out perfectly. Similarly, understanding the exact water needs of different crops ensures they grow healthily and produce optimal yields, just like getting the right ingredients in cooking.

Crop Water Requirement (WR) is defined as the total quantity of water required by a crop during its entire growth period.

Crop Water Requirement (WR) represents the total water that crops need from planting to harvest. This includes all forms of water, whether from rainfall, irrigation, or soil moisture. Knowing WR helps in planning how much water to provide through irrigation, essential for the growth of crops across various regions and seasons.

Think of a plant as a person on a long journey needing a specific amount of water to stay hydrated. Just like you’d need to carry enough water for the entire journey to maintain your energy, crops also need a specific amount of water from sowing to harvest to grow successfully.

• Climatic factors: Temperature, humidity, wind velocity, sunshine hours, and rainfall.
• Crop factors: Type of crop, root depth, crop duration, growth stage.
• Soil factors: Soil texture, infiltration rate, water-holding capacity.
• Agronomic practices: Irrigation method, mulching, planting density, etc.

Several factors impact the water needs of crops. Climatic factors such as temperature and rainfall can increase or decrease the amount of water crops require. Similarly, different crops have varying water needs based on their type, root depth, and stage of growth. Soil characteristics also play a crucial role; for instance, sandy soils drain faster than clay soils. Additionally, farming practices like the method of irrigation and the density at which crops are planted can further affect water requirements.

Consider a sponge absorbing water: a thick sponge holds water better than a thin one. Similarly, crops that grow in dense, moisture-retaining soils need less water compared to those in quick-draining soils. Just like how you adjust your water intake based on the weather (hot vs. cool days), agricultural practices need to adapt based on these various factors to optimize water usage.

Water requirement includes the following components:

1. Consumptive Use (Cu) – Includes evapotranspiration (ET).
2. Percolation losses – Especially in paddy fields.
3. Application losses – Due to irrigation inefficiencies.
4. Leaching requirement – For salinity control in soils.
5. Special needs – Water used for seedbed preparation, transplanting, and other pre-sowing operations.

Water requirement isn’t just about the amount of water crops need; it encompasses various components. Consumptive use is the water used directly by the plant through processes like evapotranspiration. Percolation losses occur when water seeps too deep into the soil, particularly in rice paddies which require standing water. Application losses represent water lost during irrigation due to inefficiencies. Furthermore, crops may have special water needs for preparing seedbeds and transplanting. Knowing these components helps in better water planning.

Think of water usage as budgeting your monthly expenses. You need to account for essentials (like rent and food) as well as occasional costs (like dining out or entertainment). Just like budgeting, effectively managing crop water requirements involves being aware of regular needs (like water consumed by the plant) and unexpected costs (like losses during irrigation).

Evapotranspiration is the sum of:

• Evaporation (E) from soil surface and water bodies.
• Transpiration (T) by the plant leaves.

Types of ET:

• Potential Evapotranspiration (PET) – When water supply is unlimited.
• Actual Evapotranspiration (AET) – Under actual field conditions with limited water.

Evapotranspiration (ET) combines evaporation and transpiration. Evaporation is the process of water turning to vapor from soil and water surfaces, while transpiration refers to the release of water vapor from plant leaves. There are two types of ET: Potential Evapotranspiration (PET) occurs when water availability is not a concern, while Actual Evapotranspiration (AET) considers current water limitations, which affects how much water plants can use.

Imagine a person sweating while exercising. The sweat evaporating off their skin represents evaporation, while the water released through their breath relates to transpiration. If they have plenty of water (PET), they sweat freely; but if they’re dehydrated (AET), their body retains water, just like a plant under stress when water is limited.

1. Blaney-Criddle Method
   • Empirical method using mean monthly temperature and percentage of daytime hours.
   • WR = K × (P/100) × (T × 1.8 + 32) Where: K = Crop coefficient T = Mean monthly temperature (°C) P = Percentage of annual daytime hours
2. Penman-Monteith Method
   • A more accurate and physically-based method.
   • Considers net radiation, wind speed, temperature, and humidity.
3. FAO Modified Penman Method
   • Widely used for international irrigation planning.
   • Adopted by FAO for crop water requirement studies.

To manage water effectively, various methods are employed to estimate the water requirement of crops. The Blaney-Criddle Method is a basic empirical approach that uses mean temperatures and daylight hours to derive water needs. The Penman-Monteith Method is more advanced, considering multiple climatic factors like humidity and wind speed. Lastly, the FAO Modified Penman Method is utilized globally for irrigation planning, following guidelines set by the Food and Agriculture Organization.

Estimating crop water needs is like calibrating a thermostat for your home. If you only rely on one factor like outside temperature, your home might be too hot or cold. Similar to this, effective methods blend different data such as temperature and humidity to fine-tune water requirements for crops, ensuring they receive just the right amount.

--