Soil Moisture Status & Instrumentation 1

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Irrigation Guide

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ment Plan

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SOIL MOISTURE STATUS & INSTRUMENTATION

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METHODS OF MEASURING SOIL MOISTURE

There are three main ways of measuring soil moisture volumetrically, that is, in terms of inches/foot. One well-known method is the neutron probe. The neutron probe consists of a small cylinder, usually about 2" x 8" that is connected to a control box by 6-8 feet of electrical cable. The radioactive material (the source of the neutrons) is contained in the cylinder. When not in use the cylinder is stored within the control box, which is lined with paraffin wax and lead for safety.

Access holes are drilled and PVC or aluminum pipe inserted at appropriate places in a field. To measure soil water the neutron probe cylinder is lowered into the hole to pre-determined depths (dependent on the crop and soil). When turned on, the source within the cylinder will emit fast, low-energy neutron particles in about a 6" ball around it. When these fast-moving particles strike a hydrogen molecule (present in water, H2O) they are slowed down. The instrument detects and counts these slowed particles. The total number of slowed particles is calibrated to read out in terms of total soil moisture.

The neutron probe is an expensive instrument (although costs, size, and weight are coming down) and requires special training, licensing, and storage procedures. Also, the required access tubes and time to take a reading, realistically restrict its use to one or two spots in a field. Usually it is used by scientists, consultants, and larger farms. Please click the following link for photos and additional information.

http://www.sowacs.com/sensors/index.html

Another way to measure soil moisture is to take a soil sample of a known volume, weigh it, dry it thoroughly, then weigh it again. The difference in weight between the wet and dry soil is the total water content. This method, known as the "gravimetric" method, is slow and not used in production agriculture except to calibrate other methods.

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Measuring Soil Moisture By Hand Probing, The "feel" Method

One of the easiest, cheapest, and most flexible methods is by "feel". That is, using a soil sampler or soil auger to take samples of soil from varying depths of the root zone and judging the water content by the feel and appearance of the sample. It is easy because anyone can do it. It is cheap because all it takes is an inexpensive soil sampler (see the Additional Resources section of the Appendix for places to obtain soil samplers). And it is flexible because you can go any place in the field, at any time.

This flexibility is one of its great strengths. With neutron probes, tensiometers, and gypsum blocks, you can only measure soil water at the point of installation of the access tube or instrument. If using tools like leaf pressure chambers or infrared thermometers, you are restricted as to time of day (and in the case of the infrared "gun", whether it is cloudy or not, and windy or not).

With practice, it is easy to become fairly accurate. The following two links help relate the feel and appearance of soil samples to the water content, the "feel method." The first has some good color photos and presents the depletion in percentage. The second presents a table that relates appearance to the depletion in inches.

http://www.ianr.unl.edu/pubs/irrigation/g690.htm
http://cati.csufresno.edu/cit/rese/90/900607/

Additionally, NRCS has similar information.

Following is an example of estimating the SMD for a preirrigation using inch values from the table on the second link.

The purpose of the irrigation is to take a 6 foot profile to field capacity. Effective root zone = 6 feet.
Soil samples are withdrawn from .5 feet, 1.5 feet, 2.5 feet, 3.5 feet, 4.5 feet, and 5.5 feet.
The field is layered so that samples 1 through 3 are classified as medium loams and samples 4 through 6 are heavy loams. Noting the headings on the four columns of descriptions, you want to match the feel of each sample to a description in the appropriate column. Then read the indicated soil moisture deficit to the right or left.
Sample 1 (a medium loam from .5 feet) is dark, will slick, but will not ribbon. This indicates a SMD for the top foot of .4-.5 inches. Sample 2 (from 1.5 feet) is dark and forms a ball but will not slick or ribbon, indicating a SMD for the second foot of .7 inches. The remaining samples are then examined.
The total of the SMDs for the 6 samples is 3.5 inches. This gives you an idea of the net irrigation water to apply. Remember, effective, efficient irrigations are the result of knowing WHEN, HOW MUCH, and HOW to irrigate. The soil moisture depletion is the HOW MUCH to irrigate.
Note that if you need to satisfy a leaching requirement (explained in both the Irrigation Scheduling and Salts and Drainage chapters), the leaching depth should be added to the estimate of the soil moisture depletion to determine the total to irrigate.

Probing is cheap enough that it can be used both before and after an irrigation. Use it before an irrigation to both time the irrigation and estimate the correct amount of water to apply. Sample the soil after an irrigation to make sure that you got enough water into the root zone. If the after-irrigation sampling indicates too much or too little water in, adjust the next irrigation's management as required.

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Last updated September 2000


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Cost-Share Programs Irrigation Guide USBR Water Manage- ment Plan Water Management Handbook Water Quality Data	SOIL MOISTURE STATUS & INSTRUMENTATION Previous Next
	METHODS OF MEASURING SOIL MOISTURE There are three main ways of measuring soil moisture volumetrically, that is, in terms of inches/foot. One well-known method is the neutron probe. The neutron probe consists of a small cylinder, usually about 2" x 8" that is connected to a control box by 6-8 feet of electrical cable. The radioactive material (the source of the neutrons) is contained in the cylinder. When not in use the cylinder is stored within the control box, which is lined with paraffin wax and lead for safety. Access holes are drilled and PVC or aluminum pipe inserted at appropriate places in a field. To measure soil water the neutron probe cylinder is lowered into the hole to pre-determined depths (dependent on the crop and soil). When turned on, the source within the cylinder will emit fast, low-energy neutron particles in about a 6" ball around it. When these fast-moving particles strike a hydrogen molecule (present in water, H2O) they are slowed down. The instrument detects and counts these slowed particles. The total number of slowed particles is calibrated to read out in terms of total soil moisture. The neutron probe is an expensive instrument (although costs, size, and weight are coming down) and requires special training, licensing, and storage procedures. Also, the required access tubes and time to take a reading, realistically restrict its use to one or two spots in a field. Usually it is used by scientists, consultants, and larger farms. Please click the following link for photos and additional information. http://www.sowacs.com/sensors/index.html Another way to measure soil moisture is to take a soil sample of a known volume, weigh it, dry it thoroughly, then weigh it again. The difference in weight between the wet and dry soil is the total water content. This method, known as the "gravimetric" method, is slow and not used in production agriculture except to calibrate other methods. Top of Page	Measuring Soil Moisture By Hand Probing, The "feel" Method One of the easiest, cheapest, and most flexible methods is by "feel". That is, using a soil sampler or soil auger to take samples of soil from varying depths of the root zone and judging the water content by the feel and appearance of the sample. It is easy because anyone can do it. It is cheap because all it takes is an inexpensive soil sampler (see the Additional Resources section of the Appendix for places to obtain soil samplers). And it is flexible because you can go any place in the field, at any time. This flexibility is one of its great strengths. With neutron probes, tensiometers, and gypsum blocks, you can only measure soil water at the point of installation of the access tube or instrument. If using tools like leaf pressure chambers or infrared thermometers, you are restricted as to time of day (and in the case of the infrared "gun", whether it is cloudy or not, and windy or not). With practice, it is easy to become fairly accurate. The following two links help relate the feel and appearance of soil samples to the water content, the "feel method." The first has some good color photos and presents the depletion in percentage. The second presents a table that relates appearance to the depletion in inches. http://www.ianr.unl.edu/pubs/irrigation/g690.htm http://cati.csufresno.edu/cit/rese/90/900607/ Additionally, NRCS has similar information. Following is an example of estimating the SMD for a preirrigation using inch values from the table on the second link. The purpose of the irrigation is to take a 6 foot profile to field capacity. Effective root zone = 6 feet. Soil samples are withdrawn from .5 feet, 1.5 feet, 2.5 feet, 3.5 feet, 4.5 feet, and 5.5 feet. The field is layered so that samples 1 through 3 are classified as medium loams and samples 4 through 6 are heavy loams. Noting the headings on the four columns of descriptions, you want to match the feel of each sample to a description in the appropriate column. Then read the indicated soil moisture deficit to the right or left. Sample 1 (a medium loam from .5 feet) is dark, will slick, but will not ribbon. This indicates a SMD for the top foot of .4-.5 inches. Sample 2 (from 1.5 feet) is dark and forms a ball but will not slick or ribbon, indicating a SMD for the second foot of .7 inches. The remaining samples are then examined. The total of the SMDs for the 6 samples is 3.5 inches. This gives you an idea of the net irrigation water to apply. Remember, effective, efficient irrigations are the result of knowing WHEN, HOW MUCH, and HOW to irrigate. The soil moisture depletion is the HOW MUCH to irrigate. Note that if you need to satisfy a leaching requirement (explained in both the Irrigation Scheduling and Salts and Drainage chapters), the leaching depth should be added to the estimate of the soil moisture depletion to determine the total to irrigate. Probing is cheap enough that it can be used both before and after an irrigation. Use it before an irrigation to both time the irrigation and estimate the correct amount of water to apply. Sample the soil after an irrigation to make sure that you got enough water into the root zone. If the after-irrigation sampling indicates too much or too little water in, adjust the next irrigation's management as required. Previous Next Top of Page
	Last updated September 2000