42.10. Irrigation Scheduling Based on Root Zone Water Balance

Using the water balance approach: ∆S = P + I − ET − D − R Where: • ∆S = change in soil water storage • P = precipitation • I = irrigation • ET = evapotranspiration • D = deep percolation loss • R = surface runoff

The water balance equation is a fundamental tool in understanding how water moves in and out of the soil. In this equation, ∆S represents the change in soil moisture content, which can increase or decrease based on various factors.

• P (Precipitation): This includes all forms of water that fall from the sky, contributing to soil moisture when it rains or snows.
• I (Irrigation): This is the water applied to the soil by human efforts to assist plant growth.
• ET (Evapotranspiration): This includes water lost to the atmosphere through evaporation from soil and transpiration from plants. It is crucial because it reduces the amount of water available in the soil.
• D (Deep Percolation Loss): This refers to water that moves deep into the soil beyond the root zone, where it may not be available to plants.
• R (Surface Runoff): This is the water that flows over the ground surface instead of being absorbed by the soil. Excess water can lead to erosion or nutrient loss. Overall, the equation helps us account for all water entering and leaving the soil, allowing for better irrigation scheduling.

Think of the water balance equation like a bank account. Just as money enters and leaves your account, water enters and exits the soil. Precipitation and irrigation are like deposits that add money to your account, while evapotranspiration, deep percolation losses, and surface runoff are like withdrawals that take money out. Managing this balance ensures you have enough 'water savings' in your soil to support your plants, much like ensuring you have enough funds to cover your expenses.

Maintaining soil moisture within the AW range ensures optimal plant growth and efficient irrigation.

The term 'AW range' refers to the 'available water' that is present in the soil, specifically the amount of water between the field capacity (the maximum water that soil can hold) and the permanent wilting point (where plants cannot extract water). Keeping the soil moisture within this range means that plants have access to enough water for healthy growth and prevents them from suffering from drought stress. Proper irrigation scheduling based on water balance helps to ensure that plants receive the right amount of water they need at different growth stages, which is crucial for agricultural productivity.

Imagine you are cooking a meal and need just the right amount of water for pasta. If you use too little, the pasta becomes hard and undercooked, but if you use too much, it becomes soggy and unpalatable. Similarly, plants require the right amount of water to thrive, and maintaining moisture within the available water range ensures they are neither too dry nor too overwatered.

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