Mastering the Soil Water Balance: Your Irrigation Blueprint

In our previous article, "Unlocking Your Soil's Water Potential: Understanding Plant-Available Water," we established that soil acts like a bucket with a limited capacity for holding water, known as its soil water-holding capacity. Adding too much water, exceeding field capacity (FC), results in overflow—water lost to deep percolation.

The soil water balance is essentially the calculation that tracks how much water (from irrigation or rainfall) enters and exits your soil. This calculation is your blueprint for effective irrigation scheduling. By monitoring daily evapotranspiration (ET), which represents crop water use (withdrawals from the "bucket"), and rainfall or irrigation (deposits), you can determine the current water content of your soil.

The goal is to irrigate when the soil water content falls below a predetermined minimum level, called the Management Allowable Deficit (MAD).

Figure 1. Soil water balance is the calculation that tracks how much water (irrigation or rainfall) enters and exits your soil.

Key Terms:

• Field Capacity (FC): The maximum amount of water a soil can hold against gravity indefinitely (% of volume).

  
  

• Permanent Wilting Point (PWP): The amount of water remaining in the soil when plants can no longer extract it, leading to wilting and death.

  
  

• Available Water (AW): The difference between field capacity and permanent wilting point (AW = FC – PWP), representing the water accessible to plants.

  
  

• Management Allowable Deficit (MAD): The percentage of available water that you, as a grower, are willing to allow to be depleted before irrigating.

In essence, mastering the soil water balance empowers growers to move beyond reactive irrigation towards a proactive, data-driven approach.

By accurately tracking inputs (rainfall, irrigation) and outputs (evapotranspiration), and by understanding critical soil parameters like FC, PWP, and MAD, you can ensure your crops receive the precise amount of water they need, when they need it.

This not only optimizes yield and quality but also conserves water resources, contributing to sustainable agricultural practices. Implementing these principles will transform your irrigation scheduling from a guessing game to a science, leading to healthier crops and a more efficient operation.