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Moisture
Monitoring |
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Purpose The moisture level in the soil profile is monitored for a variety of purposes using a variety of different instruments. The moisture level is monitored for real-time use, and collected and stored for later evaluation. Measurements are made periodically, by visiting a site and taking and recording a reading manually. Measurements are made using electronic sensors and dataloggers, which take and store readings automatically and continuously throughout the season.
The benefits of moisture monitoring and the uses of moisture data are many, and include • maintaining optimal moisture conditions for optimal plant growth • examining plant water use (water used and from which depths in the profile) • determining when to irrigate • determining how much irrigation water to apply • avoiding over-irrigation, which can lead to - waterlogging of the soil - damage to plant roots - reduction in growth and fruit quality - waste of water - washing of fertilizers and other chemicals below the root zone, potentially into groundwater - waste of energy, additional pumping costs, and wear on equipment • avoiding under-irrigation, which can cause - unintentional stress and reductions in vegetative and/or fruit growth - reduction in fruit production and/or quality monitoring • monitoring deficit irrigation and intentional moisture stress - for more efficient irrigations - to control or effect vegetative growth or fruit quality.
• determine if an irrigation is needed • determine how much to apply (or how long to run the system) based on how much is needed.
By irrigating based on monitoring the moisture level, the irrigator can • avoid over-irrigation and waterlogging of the soil • avoid unnecessary wear on equipment • avoid waste and added expense of energy • avoid leaching of fertilizers and chemicals below the active root zone.
• examine the water-use patterns of the plants • estimate depth of active roots throughout the season • evaluate the performance of the irrigator or irrigation system • determine if the plant-water needs are being satisfied • examine periods of moisture stress • correlate moisture/water use with vegetative growth • examine effects of moisture conditions or irrigation strategy on fruit quality • detect instances and the extent of over-irrigations and deep drainage below the root zone.
Data collected with the moisture sensors are analyzed by graphing the measurements. A series of readings taken over time are plotted to show changes in moisture level and trends in water use throughout the profile.
Data collected periodically, with a dial-gauge tensiometer for example, might look like the following graph when plotted. Visiting the tensiometer station every few days and reading the dial gauges would result in data points spaced a few days apart. Moisture conditions at other times are not available, and these unknown conditions are assumed by joining the points with straight lines.
Data collected periodically (symbols) at two depths, with data points connected by straight lines.
Data collected continuously would show a more complete record of moisture conditions. In the following graph, the lines show data collected continuously, at regular time intervals, over the same time period as above. The periodically collected data from above are shown also. Trends in moisture level are similar in both graphs, but more data collected automatically gives more detailed information.
Data collected continuously, periodically collected data points shown
Data Moisture levels can be expressed in different units, and are most often measured in units of tension or water-content. Tension measurements are often easier to obtain, and can be made using inexpensive and reliable instruments such as tensiometers and electrical resistance sensors. Tension is a measure of the energy status of the soil-water. It tells how tightly the water is being held by the soil, and how hard the plant roots must work to extract the water from the soil). A wet soil has a tension near 0: the water is being held with little energy and is free to move or be extracted by roots. As the soil dries, the remaining water is held more tightly by the soil particles, making it harder for the roots to extract water and take in nutrients. If insufficient water is available in the soil, or if the roots have to work too hard to extract water, plant growth will be affected. Volumetric water content measurements indicate the quantity of water present in a given volume of soil. This is often expressed on a surface-area basis, and simplified to depth units as a depth of water in a given depth of soil. Water content measurements are usually made electronically. A sensor inserted in the soil and measurements of electrical characteristics of the soil-water are made. A calibration equation correlates the electrical measurements to water content, and converts to give a water content value.
The sensor is often connected to a datalogger, which makes and stores readings continuously. The sensor measures electrical properties of the soil. Since all soils are different, made up of different components with differing electrical properties, most sensors require a site-specific calibration in order to obtain best results. Calibration is often straightforward, and usually consists of collecting a number of soil samples and analyzing them gravimetrically for water content. These data are combined with the sensor’s electrical measurements to develop a site-specific calibration equation.
Moisture
characteristics of soil
The water retention curve describes the relationship between soil-water tension (or matric potential) and volumetric water content. This tells how much water is held in the soil at any given tension, or looked at the other way, how tightly a given amount of water is held by the soil.
Soil
sampling and moisture analysis • bulk density: for use in examining soil porosity, compaction • gravimetric-water content: which provides an accurate “reference” measurement for calibrating moisture sensors • water-retention, or characteristic, curve.
Soil samples are also collected and analyzed to develop a sensor calibration equation or as a check on an existing calibration. Sensor readings are made at the same time as a soil sample is taken, and the samples are analyzed for water content. The concurrent sensor readings are correlated or compared to these “standard” values.
A water retention curve is developed by applying a sequence of known pressures to a soil sample, and measuring the water content at each pressure. Samples are placed in pressure chambers, or extractors, and a high-pressure compressor/manifold unit accurately monitors and regulates the supply of pressurized air.
Compressor/manifold unit and pressure chambers, equipment used to determine water retention curves
The resulting curve shows the relationship between tension and water content, which allows water contents to be estimated based on tension measurements, and for tensions to be estimated from water-content measurements.
- simple, reliable, accurate, inexpensive - manually read dial gauge for periodic measurements
- designed to collect data automatically and continuously - displays data and graphs on built-in real-time LCD display - stores readings automatically to maintain a complete record of moisture and water use
- measures volumetric water content - easily connects to an automated weather station to complete water-balance assessments
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Earth Systems Solutions Santa Barbara, CA 93111 USA Phone:1+805-967-2726 |
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