2 Basic Principles of Irrigation
2.1 THE SOIL-WATER BALANCE
Irrigation systems are used to manage the amount of water stored in the soil.
Evapotranspiration (ET) or consumptive use, which is the combination of water evaporated from the soil surface and the water transpired by plants, causes soil moisture to decrease over time. If rainfall or irrigation does not replace this water, soil moisture falls to the extent that plants reduce growth and ultimately wilt and die.
In the absence of rainfall, irrigation is used to maintain soil moisture at a level that plant growth generally is not limited.
When irrigation is applied, the rate at which water moves through the soil depends on the soil type. In clay soils, this rate is quite slow, while in sandy soils, the rate is usually quite rapid. After irrigation or significant rainfall, the upper layers of the soil may become saturated, depending on the depth applied. Water applied in excess of what the soil can retain will drain into the deeper layers, usually the groundwater system. The excess water drains out under the force of gravity, and after a few hours in light free draining soils, or perhaps several days in tighter soils, the soil is said to be at field capacity or at the full point. Plant growth generally does not take place in saturated soils, so excess watering may have a detrimental effect on plant growth.
As the soil dries out, the rate of evaporation from the surface falls fairly quickly until the soil surface is dry. The remaining soil moisture, which is usually the majority of the available water, is used by plants for transpiration and growth.
The rate at which plants absorb water reduces as the soil dries out, because it becomes progressively harder for the plants to suck the water from the soil. Initially plants are easily able to draw water from the soil, and growth is not affected. After some time the soil becomes dry enough to slow plant growth. This soil moisture level at this stage is known as the critical deficit, or the stress point. The rate at which the atmosphere can suck water from the plants exceeds the rate at which plants can absorb moisture, and temporary wilting may occur. This soil moisture level is often considered to occur at a soil suction (soil tension) of about 1 bar, although this varies with different crop types, and is the point at which irrigation should be applied to return plants to full growth.
If the crop is not irrigated or water is not received from other sources, the soil will eventually dry out to the point where permanent wilting occurs, and the plants will die.
The amount of water in the soil between the stress point and field capacity is called the readily available soil moisture. Theoretically, the most efficient irrigation is achieved when soils reach the stress point and enough water is applied to return soil moisture to a level just below field capacity. In practice, this is not strictly true, as will be explained later.
In summary, irrigation is about keeping soil moistures between the stress point and field capacity, so that growth is not limited.
2.2 MEASURING CROP WATER USE
Answering the basic question of how much water crops need and when depends on understanding crop water use. Without knowing or being able to estimate what the current moisture status of the soil is, or whether the crop is under stress or likely to be under stress in the near future, it is impossible to irrigate efficiently.
There are three fundamental ways of measuring crop water use, namely;
- soil moisture measurements;
- plant water measurements; and
- evapotranspiration measurements.
2.2.1 Soil Moisture Measurements
Measuring soil moisture is a well-recognised method of estimating crop water use and is highly recommended. It allows you to determine how much water to apply, and with some knowledge of stress points for a crop, when to apply that water.
There are a number of methods available to farmers for measuring soil moisture.
Hire a consultant
One of the easiest ways is to employ a specialist to measure soil moisture readings for you. The specialist can also tell you when to next irrigate and how much water to apply.
However, some farmers prefer to do things themselves, and other options are available.
Observe and feel
This is the most basic method of using a spade to dig a hole to check subsurface soil moisture visually and by feel. It is quick, easy and low cost. However, it is not precise and it takes time for farmers to gain the experience to interpret the results.
Oven drying
This involves taking a sample of soil from the field (25-50 g), weighing it and drying it in an oven (usually a microwave oven), and then weighing it.
The gravimetric soil moisture is calculated as follows:
Soil moisture = [(Mw - Mt)/(Md - Mt)] - 1
where Mw = weight of wet soil in the container
Mt = weight of the empty container
Md = weight of dry soil in the container
Multiplying by 100 will give the percentage of water in the soil.
Although this method is accurate, it is time consuming, and requires a soil sample to be taken from the field each time. It also doesn’t tell you how much of the water in the soil was available for plant use, as plants cannot extract water to the point of making the soil oven dry.
Indirect methods of soil moisture measurement
In these methods, a meter or a device is used to measure a physical property of the soil that can be related to the soil moisture content.
Some of the methods in common use in New Zealand are:
- Tensiometers;
- Neutron Probes;
- Aquaflex Cables;
- TDR;
- Electrical Resistance Meters;
- Capacitance Meters.
Some of these devices are used to take readings manually, while others record soil moistures on a continuous basis. Low cost devices such as tensiometers are normally used by farmers, while more expensive devices such neutron probes and TDR are normally used by irrigation consultants as part of a soil moisture scheduling service, although some farmers do use these methods. Each has distinct advantages and disadvantages, and these should be discussed with an irrigation expert.
As with oven drying, most soil moisture measurement devices can give you the water content of the soil, but they do not tell you how much of that water is available to the plant. Tensiometers give an indication of how freely water is available to plants, but do not directly tell you how much water is available. However, guidelines are available to tell you how dry soils can get before plants come under moisture stress. Consult an irrigation expert or advisor for this information.
2.2.2 Plant Measurements and Methods
The general idea behind plant measurements is to measure the water status of the plant rather than the soil. The plant is the link between the soil and the atmosphere, and its water status can give an indication of when to irrigate.
Some of these methods are destructive, that is they require a sample of the plant to be removed for testing. Other methods involve fixing sensors to plants. Because individual plants are measured, these methods are more commonly used for orchards and tree crops. More broad-band methods such as remote sensing measure plant water status over a larger area of crop, and are more suited to pasture or field crops.
Examples of plant measurement methods are:
- Leaf Water Potential (Pressure chamber);
- Heat Pulse Method;
- Stem Diameter Measurement;
- Stomatal Resistance Methods;
- Microwave Measurements;
- Infrared Thermometry (Crop Water Stress Index).
Some of these methods are not commonly available commercially, and tend to be used more for scientific experiments. However, there may be farms or orchards where one of these methods is appropriate.
These methods give the water status of plants and therefore give you a guide as to when you should irrigate. They do not tell you how much water needs to be applied to replenish the soil water. If these methods are used, they need to be used in conjunction with some knowledge of soil water content and soil water holding capacity.
2.2.3 Evapotranspiration Measurements and Methods
Evapotranspiration (ET) is the total water lost by direct evaporation from plants and the ground surface and water transpired from plants. It is a way of estimating how much water has been taken from the soil, and therefore how much irrigation is required. Water budgets are used to calculate soil moistures indirectly by adding rainfall and irrigation (the amount of water applied to the soil) and subtracting ET (the amount of water taken out of the soil) to determine soil moisture status.
The most common methods of determining ET are:
- Weather Stations;
- Evaporation Pans;
- Lysimeters.
In New Zealand, ET figures calculated from weather station data are commonly presented in local newspapers during the irrigation season. Farmers may also purchase their own self-contained climate stations and calculate ET on their farm.
Evaporation pans (e.g. Class A Pans) are not widely used in New Zealand by farmers, although some irrigation resource consents now specify that pans be installed and used to schedule irrigation.
Lysimeters are more commonly used in scientific studies because they can measure ET directly.
2.3 WATER METERS
Knowing how much water an irrigation system uses is an important precursor to effective irrigation management, and is vital to using the recommended indicators in a meaningful way. Many of the indicators have a water quantity or flow rate component, (e.g. production per unit of water used, profit per unit of water used), and unless water use is measured, it will not be possible to calculate these indicators. In addition, measured flow rates or volumes may also be required to satisfy resource consent conditions.
The recommended method for measuring irrigation flow rates or volumes is to use a water meter. The types and quality of meters varies between brands, so a meter that meets or exceeds BS, AWWA, ISO and DIN standards is recommended. An accuracy of 5 percent should be easily attainable for water meters operating in pressurised systems, while an accuracy of 10 percent is probably more realistic for surface water systems.
For any pumped system, an alternative to measuring flow using a water meter is to relate the quantity of water pumped to the units of electricity used. This method requires volume of water used to be measured by a water meter and noting the electricity used for a given length of time. Volumetric water use can then be directly calculated from units of electricity used. Flow rates can also be calculated by dividing the volumes of water used by the run time of the system. An hour meter on the pump switch-gear is necessary for this.
Using electricity use to calculate water use is accurate only if the operating conditions of the pump or irrigation system do not change significantly.
For systems pumping from a bore, a seasonal lowering of water levels will often cause inaccurate readings. Pump wear will tend to lower efficiency and change the readings. Having the irrigators in different positions, or running blocks at different flow rates will also render the values inaccurate.
A water meter should be fitted in preference to using electricity readings.
Where irrigation systems are applying water at more than one location of the farm at the same time, e.g. for multiple irrigator systems, a water meter on each irrigator is recommended for optimum management.
Contact for Enquiries
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