FAQ
Useful information for Irrigators
Basic Principles:
Visual Soil Moisture Sensor
When to irrigate: When 50% of the water in the soil has been removed. This can vary depending on crop type and other factors for example crops may need to be subjected to mild water stress to change from vegetative to reproductive states or to improve crop quality.
How much water to apply: Enough to bring soil moisture back to field capacity (See definitions). Actual amount depends on the amount of water used, soil type, crop etc.
How much do crops use: Different crops use different amounts of water to make calculations easier the Evapotranspiration from a full grass cover is given a value of 1, called a crop factor (See below) . Bare ground loses about 30% the amount of moisture that full grass cover looses so has a crop factor of 0.3. Developing crops will use intermediate amounts of water (See table at end of page).
Soil Moisture How to measure: Observing soil texture can give an indication but is not very accurate. Using purpose built monitoring tools will significantly reduce water waste and help increase production.
Measuring how much you apply: The simplest way to measure the amount of water your irrigator is applying is to place a number of parallel sided jars (glass preserving jars are ideal) before irrigation, measure the depth of water in the jars with a ruler after irrigation.
Evapotranspiration: Potential evaporation across the Central Canterbury Plains is typically between 700 to 1000 mm per year
Typical evapotranspiration by Month.
- Sep 57mm
- Oct 91mm
- Nov 111mm
- Dec 132mm
- Jan138mm
- Feb 105mm
- Mar 81mm
- Apr 43mm
- May 25mm
- Year Total 851
* Data 1970 – 2000
Water balance
Horticultural Soil Moisture Probe
A water balance can be calculated from monthly rainfall minus Potential Evapotranspiration (PE). In Canterbury, this shows a deficit of rainfall over PE typically from October to March on heavy soils with November – February being highest (45 – 70 mm per month). Light soils require a longer irrigation season potentially from September to April, or even early May in some years
Irrigation Terms – Definition of terms:
- Soil Moisture: The water content of the soil. Acts as a carrier to supply plants with nutrients.
- Saturation: Soil will only hold a maximum amount of water.The amount varies for different soil types but is typically from twenty to fifty percent of the soil by volume. In a soil at saturation excess water will drain through the soil, being wasted and taking valuable plant nutrients with it. Atsaturation there is little air in the soil. This can causedamage to roots, prevent uptake of oxygen to the plant and encourage the development of soil born diseases.
- Evapotranspiration (ET): Soil moisture is lost from the top of the soil by evaporation from the surface of the soil and by transpiration from the leaves of plants growing in the soil. Evapotranspiration is the measure of the total loss of moisture by the plant and soil system.
- Refill Point: The minimum soil moisture level at which irrigation is applied. The refill point is likely to vary in irrigation systems depending on such factors as water availability, soil type and crop being grown. Often the refill point is set at 50% of the field capacity.
- Wilting Point: The soil moisture level at which plants are unable to extract sufficient water to maintain normal moisture levels in the plant tissues. The reduced moisture in the plant cells causes the cells to become “limp” and the plant to wilt. Wilting point typically occurs at 10 to 30% of field capacity.
- Field Capacity: The maximum amount of water a soil can hold without significant amounts being lost by drainage. Sufficient water is normally applied during irrigation to bring the soil to field capacity.
Typical Crop Factors
- Bare ground 0.3
- 1/4 canopy 0.4
- 1/2 canopy 0.6
- 3/4 canopy 0.75
- Full canopy 0.90
- Maturing crop 0.65
- Grass 1.0
- Grapes (up to) 0.80
information: http://www.dairyinsight.co.nz/reports/20176-4.pdf
With the compliments of :