Why use this calculator
Not all rainfall becomes available water for plants. Understanding how much of a rain event actually reaches and is held in the root zone helps you make better irrigation decisions, assess drought risk, and plan planting schedules. Different soil types absorb and store water very differently, making this calculation essential for site-specific water management.
How to use it
Enter the rainfall amount in millimetres, select your soil type, and enter the crop root depth in centimetres. The calculator determines effective rainfall after losses, the resulting soil moisture increase, and how many days of plant water supply the rainfall provides.
The formula
Effective Rainfall = Rainfall x Absorption Factor (by soil type). Soil Moisture Increase = Effective Rainfall / Total PAWC x 100. Days Supply = Effective Rainfall / Daily ET (5mm). PAWC = Plant Available Water Capacity per cm depth x Root Depth.
Worked examples
25mm rainfall on loam soil with 30cm root depth
18.8mm effective rainfall, 41.7% moisture increase, 3.8 days water supply
40mm rainfall on clay soil with 20cm root depth
16.0mm effective rainfall, 57.1% moisture increase, 3.2 days water supply
When people use it
- Deciding whether to irrigate after a rainfall event
- Assessing drought risk during dry periods between rain events
- Comparing water-holding capacity of different soil types across the farm
- Planning crop selection based on available soil moisture
Tips
- Light frequent rains are less effective than fewer heavier events due to surface evaporation
- Mulch and stubble retention significantly improve rainfall absorption
- Monitor soil moisture with probes rather than relying solely on calculations
- Clay soils absorb water slowly so intense rainfall may run off before infiltrating
Questions people ask
- Why do different soils absorb different amounts of rain?
- Sandy soils allow water to pass through quickly, losing much to deep drainage below the root zone. Clay soils have low infiltration rates, causing surface runoff during heavy rain. Loam soils provide the best balance of infiltration and water retention for plant use.
- What is Plant Available Water Capacity?
- PAWC is the amount of water held in the soil between field capacity (when drainage stops) and permanent wilting point (when plants can no longer extract water). It varies by soil type and determines how much water the soil can store for plant use.
- Why use 5mm daily evapotranspiration?
- 5mm per day is an average ET rate for actively growing crops in temperate to warm conditions. Actual ET varies with temperature, humidity, wind, crop type, and growth stage. In hot dry conditions, ET can exceed 8-10mm per day, while in cool cloudy conditions it may be only 2-3mm.