44.2. Irrigation Requirement (IR)

Irrigation Requirement is the amount of water that must be supplied to the crop field through irrigation to meet the crop water requirement, after accounting for effective rainfall and other natural water sources.

The Irrigation Requirement (IR) is a crucial concept that quantifies how much water farmers need to add to their fields. This figure is derived by first determining the total water needed for the crops to thrive, known as the Crop Water Requirement (CWR). However, not all water needs to be supplied by irrigation; some of it comes from rain and other sources. Therefore, IR subtracts effective rainfall and contributions from groundwater and soil water from the total CWR to find out the amount of water that needs to be supplied via irrigation.

Imagine you are baking a cake that requires 3 cups of flour, but you already have 1 cup from last week's shopping. You would subtract that from the total needed to know how much more you need to buy – in this case, 2 cups. In irrigation, the focus is on how much water 'missing' after accounting for the rainfall and natural sources.

IR = CWR − ER − GW − SW Where:

• IR = Irrigation Requirement
• CWR = Crop Water Requirement
• ER = Effective Rainfall
• GW = Contribution from Groundwater
• SW = Contribution from Soil Water (Initial moisture content)

The equation for calculating the Irrigation Requirement (IR) is fundamental to understanding how irrigation works effectively. It simply states that the IR is equal to the Crop Water Requirement (CWR) minus the contribution from effective rainfall (ER), groundwater (GW), and soil water (SW). This equation helps farmers and agronomists calculate the precise amount of water to irrigate crops based on various input values. By plugging in the numbers for CWR, ER, GW, and SW, one can determine exactly how much more water is needed to meet the plants' needs.

Think of this equation as keeping track of your spending. If you have a monthly budget (CWR), money coming in (ER, GW, SW) reduces what you need from your salary (IR). If you need $2000 but get$300 from side jobs, $200 from savings, and$100 from gifts, you’ll only need $1400 from your salary. Similarly, the equation allows farmers to know exactly how much water to add.

• Soil Moisture Deficit Method
• Pan Evaporation Method
• Crop Coefficient Method (Kc Method) ET = K × ET_0 Where:
• ET_c = Crop Evapotranspiration
• ET_0 = Reference Evapotranspiration
• K_c = Crop Coefficient

There are several methods farmers use to determine how much water to add through irrigation. The 'Soil Moisture Deficit Method' assesses how much water is missing from the soil. The 'Pan Evaporation Method' involves measuring how much water evaporates from a pan over time and helps estimate water loss for crops. Lastly, the 'Crop Coefficient Method' is a calculation that uses a ratio (the crop coefficient) to relate the crop’s water needs to reference conditions, multiplied by known evaporation rates. Each method provides useful information based on local conditions, ensuring the right volume of water is applied.

You can think of these methods like ways to track your car’s fuel efficiency under different conditions. Just as you'd check your mileage when driving uphill versus on a flat road, farmers need to assess their crops’ water needs differently based on soil type, weather, and crop type to understand how much water they should add.

• Net Irrigation Requirement (NIR): Actual amount of water required at the root zone.
• Gross Irrigation Requirement (GIR): Water applied at the field inlet, accounting for losses. NIR = GIR / Efficiency

It is essential to differentiate between Net Irrigation Requirement (NIR) and Gross Irrigation Requirement (GIR). The NIR is the actual amount of water that reaches the plants' roots, while the GIR is the total water applied to the field. The GIR accounts for losses due to evaporation, runoff, or percolation, meaning not all the water applied is effectively used by the crops. Farmers need to consider efficiency to determine how much water they should apply, ensuring they are watering the crops adequately without waste.

Think of this like filling a bucket with water. The water you put into the bucket is akin to GIR; however, if there are holes in the bucket, the water that actually sits in it (NIR) will be less than what you poured in. To maximize water efficiency, understanding these differences is key.

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