Part 13, Soil Moisture Measuring Methods and Efficiency
By Ed Butts, PE, CPI
This month will conclude our series on irrigation fundamentals and practices. The 14 articles match the single-topic series I have put together. It was somewhat challenging at times to decide what to write and how much of each method to include.
It is my sincere hope I have been able to convey the basic principles of the major methods of irrigation in an easy-to-understand format along with a little of the science that goes with it as essential background information and as a springboard to those who wish to learn more.
This month, as a hopefully fitting finale, we will provide a brief review of a few remaining related topics: the basic methods of measuring soil moisture and effective irrigation efficiency evaluation techniques.
Soil Moisture Measurement Techniques
Maintaining the proper volumetric balance of moisture within the soil versus the open pore space (i.e., air) is crucial to the crop’s proper growth and sustainability, and is so important it warrants a brief discussion.
It’s a known fact that too much water in the soil will cause the plant’s roots to literally drown while too little water will eventually place the plant into a water-deficient stress, potentially causing it to wilt and die.
This condition is often made worse by other uncontrollable factors such as deep or lateral soil percolation losses, soil horizon and root zone depths, and overland runoff.
The secret is knowing how to balance the soil’s moisture level between these two extreme values with the applicable stage of crop growth. In many cases this is conducted by using a simple and time-honored, hand-held method where a soil sample is squeezed between the fingers or hands to ascertain the degree of moisture. However, instruments specifically designed to measure or weigh the soil’s moisture content for irrigation scheduling purposes are preferred.
There are several technologies used for determining the soil moisture level, but they typically fall into two main categories:
- Volumetric measurement: measuring the percentage of water, by volume, within a given amount of soil
- Tension measurement: measuring the physical force that retains water in the soil; measured as soil water tension or stress to the plant.
1. Tensiometers are sealed water-filled tubes with a porous ceramic tip placed at the bottom end and a vacuum gauge set at the top (Figures 1 and 2) that measure the tension in soil.
A tensiometer measures water potential or tension as vacuum in units of either centibars or kilopascals (kPa) in metric units.
The positive value of pressure potential is called tension. Thus, if one measures a pressure potential value of –15 kPa in a given soil, the reciprocal of this value (or tension) of that soil is said to be 15 kPa.
If the soil is very moist, its tension is low, which might mean that there is too much water for optimal growth of most plants. Thus, tensiometers are widely used to monitor the availability of water in soils for plant use. They range in standard lengths from 6 inches to 72 inches and are generally inserted tightly into the soil down to the plant’s respective root zone depth, but are often used to read shallow root zone moisture (Figure 3a) and deep root zone moisture (Figure 3b), as shown on a corn plant.
Water moves vertically between the tensiometer tip and surrounding soil until an equilibrium is reached. This is the point where the tension registers on the gauge at the top of the unit. They are available in standard tension units between 0 to 100 kPa or low-tension values for use on coarse soils that rarely generate tension values above 30 centibars.
Tensiometers operate best at soil moisture tensions near field capacity, typically around 10 to 20 kPa (refer to Figure 1c) for most soils. However, the specific readings must be tailored to the precise soil type and tensiometer/root zone depth.
As water is drawn away from the tip and the soil dries, additional vacuum is pulled on the tensiometer until a midspan reading between 60 to 40 kPa is indicated. This is generally when irrigation should be conducted to refill the soil. For most soils, maintaining a gauge reading between 20 to 60 kPa is regarded as optimum, but the exact range will depend on the length of the tensiometer and soil type.
Tensiometers need to be serviced (primed) before initial use and reprimed if the tips are allowed to dry out.
This method of measurement is a popular method because it is directly related to the plant’s ability to extract water from the soil, can be applied to different soil types, is reliable and accurate, and relatively low in cost. Observing proper installation practices and maintaining a tight seal against the tip is critical, though, as exposure to the atmosphere can easily distort readings.
Irrigators often use tensiometers for irrigation scheduling because they provide direct and rapid measurements of soil moisture status and are easily managed and verifiable when using multiple units. In addition, tensiometers can be automated to control irrigation water applications when the soil water potential decreases to a predetermined value using an on-off type of vacuum switch (Figure 1b) or an analog
current or voltage transmitter for variable readings to an automatic controller.
They are also widely used in SCADA radio-based systems for monitoring of soil moisture using datalogging or IoT devices.
Because the porous ceramic tip is permeable to both water and dissolved salts, tensiometers may not accurately record the water potential due to the presence of dissolved salts (i.e., osmotic potential). If soils are saline or a poor quality of water is being used for irrigation, the osmotic potential will be a large portion of the total potential. In these cases, the osmotic potential should be alternatively measured using soil salinity sensors.
Average cost for a single tensiometer is generally between $75-$125 depending on the length and type. There is also an additional one-time cost of the installation and service kit that can be used on all units, which is $100-$150. Typical accuracy, when installed correctly, is +/–1% to 2% of the full range.
2. Electrical Conductivity or Resistivity: This method, commonly known in the industry as a gypsum block, measures soil water tension based on changes in the soil electrical conductivity from variations in the soil’s moisture content.
It features two electrodes embedded in a block of semi-porous material, usually gypsum. The electrodes are connected to lead wires that extend to the soil surface for reading by a portable meter or switching interface if used on an automatic system.
As water moves in or out of the porous block in equilibrium with the surrounding soil, changes in the electrical resistance between the two electrodes occur. The resistance meter readings are then converted to water tension using a calibrated curve matched to the specific soil type.
Gypsum block electrodes work best in very fine soils, such as loamy clays. Coarser and more sand-like soils make it more difficult for the gypsum block to achieve equilibrium with the surrounding soil.
Gypsum blocks operate over a wider range of soil moisture tensions than tensiometers but tend to deteriorate over time, often seal up when used on or within saline soils or exposure to saline water, and may need yearly service or replacement.
As with tensiometers, resistance probes (Figure 4a) are often used in multiple sets and depths with varying depths to match the crop’s root zone depth and field capacity range (Figure 4b). Resistance or conductivity readings are often applied between the deepest and shallowest block. This provides more accurate readings and confirmation of readings along with a greater working range.
Individual blocks can cost as little as $2 to $5 each with meters costing $400.
Granular matrix sensors are newer styles of electrical conductivity devices that are similar in concept to gypsum blocks but use a solid-state electrical resistance sensing device that is used to measure the soil water tension. As the tension changes with water content, the resistance changes as well. That resistance can be measured using a portable meter or sent to a remote operating interface via a radio signal (as shown in Figure 5).
They are less susceptible to soil degradation and erosion than directly buried gypsum blocks since they are placed into a protective PVC shield to interact with the soil. The sensors are more expensive than gypsum blocks, generally around the $30-$40 range each, but can be read by a variety of devices including a handheld meter, eight-channel datalogger, and wireless mesh networks. Accuracy varies with the soil type and resistance properties but is generally between 95% to 98%.
3. Time Domain Reflectometry (TDR): This is a newer type of irrigation scheduling tool that sends an electrical signal through and between steel rods imbedded in the soil (Figure 6a). Similar to underwater sonar, this device measures the signal’s return time to estimate the soil water content since wet soil returns (i.e., reflects) the signal more slowly than dry soil.
This type of sensor, when set up as a portable unit (Figure 6b), provides fast and accurate readings of soil water content for spot checks and requires little to no maintenance. However, it does require an interface device and more work in interpreting the data and may also require special calibration depending on soil characteristics.
This method is commonly applied with rain sensors used with landscape controllers to manage daily irrigation cycles. The cost ranges from $100 to $600. The accuracy also varies with the application but is typically between 97% to 98%.
4. Capacitive Sensors: Capacitive sensors use a pair of parallel stainless-steel rods (wave guides) or a single-piece insert with dual plates (Figure 7). They can also be fully contained within a PVC pipe and installed vertically into a soil borehole.
Systems using a PVC pipe design typically have multiple sensors mounted along the length of pipe, thus allowing simultaneous soil moisture measurements at several different depths.
Tubes come in different lengths and diameters. For most agricultural applications the length will be between 2 to 5 feet, with the diameter between 1 to 4 inches. Successful installation has been done using several different methods, but the final result should be a snug fit to the soil with no annular air gaps.
Capacitive reactance occurs because the two copper plates within the sensor form the same two plates of a common capacitor. These two plates generally face one another with an empty space placed in between them. Wet soil placed between the plates creates the capacitive charge, in microfarads (MFD), which is a measurement of how much of the charge on the plates for a given voltage will change over time. This material is called the dielectric, and how much the capacitance changes for a given material is called the dielectric constant of the material.
Capacitive sensors require a datalogger or display meter as an operating interface, which costs about $400-$600. In-soil single sensor units cost about $100, while multisensor and portable units can cost $1000 and more. Capacitive sensors display percentages of volumetric soil moisture with an accuracy of +/– 3% to 5% of the volumetric moisture.
5. Neutron Probes: One of the more accurate but expensive devices available for measuring the amount of moisture in the soil is the neutron moisture meter, also known as a neutron probe (Figure 8a).
As the neutrons emitted from the probe collide with hydrogen ions in the water, they are slowed and deflected with some of the slowed neutrons deflected back to the probe where a counter measures the slowed neutrons as the only ones counted. Therefore, the greater the number of slowed neutrons that return, the greater the water or moisture content of the soil.
As with the other methods, neutron probes can be used as portable units or to operate automatic irrigation systems using on-off switching, radio, or analog signals (shown in Figure 8b) sent to remote sites.
Neutron probes are expensive with a typical cost between $3500 and $5000, but when properly applied and used the accuracy is usually 98% and greater.
A sixth method, known as gravimetric measurement, is the classical procedure used as the verifier or proof for all other methods and the preferred method for total accuracy. It is generally performed in a soils laboratory and requires calibrated instrumentation and established measurement procedures. A specific volume of soil is sampled, placed into a container, weighed in the sampled (moist) condition, ovendried, and weighed again after drying.
Drying is conducted at 221°-230°F to a constant weight, with the difference in weight determined to be the moisture content. The mass water content is the decimal value that equals the weight of water divided by the weight of the ovendried soil.
Although more accurate, this method is more laborious, takes longer to conduct than the other methods, is not easily convertible or adaptable to field use, and is more expensive to perform, given the need to purchase the laboratory equipment and higher cost of laboratory personnel.
Irrigation Project Efficiency and Three Areas to Maintain It
Throughout this series I have continually touted irrigation system efficiency. Irrigation efficiency (E’) is defined by many sources as:
The ratio, usually expressed as a percent, of the volume of the irrigation water transpired by plants, plus that evaporated from the soil, plus that necessary to regulate by leaching, the salt concentration in the soil solution, and that used by the plant in building plant tissue, as opposed to the total volume of water diverted, stored, or pumped for irrigation purposes.
Whether it is application, distribution, or pumping plant efficiency, each one is a key component toward the overall goal of providing the proper amount of water to sustain a crop or plant without incurring runoff or excessive percolation loss, while at the lowest cost and use of power, labor, and water.
In principle, each stated form of efficiency stands on its own as a separate element of the overall or project irrigation system efficiency. However, the overall project efficiency can be broken down into three primary classifications. This relationship creates a cradle-to-grave approach or the point from where the water originates to the crop where it is applied. Therefore, the overall irrigation efficiency becomes the product of the following three separate components: the irrigation system efficiency, the water pumping and conveyance efficiency, and the field irrigation efficiency.
If a reservoir is used for bulk storage of water or gravity is used for the transmission and conveyance of water, the losses or gains associated with these components would also need to be included as factors (more on this later).
The efficiency of a specific irrigation project can vary widely from site to site and region to region. As all irrigation projects will include at least one variance, even with seemingly identical sites, subtle differences in design, construction, equipment used, or system operation are often the only distinguishing factors that can be used for a direct comparison.
Accordingly, evaluators of irrigation project efficiency must use reasonable and equitable analysis methods and uniform evaluation criteria to provide a fair and complete comparison. Although project irrigation efficiency may vary considerably during the irrigation season, it is commonly considered as a seasonal value and evaluated as such.
Each of the primary areas of system comparison are outlined individually.
1. Irrigation System Efficiency
The efficiency of a specific irrigation system will largely depend on how well the system performs and is operated in applying the correct amount of water to a uniform coverage and depth to the crop while combating the separate elements of environmental, static, and frictional head losses, and distribution inadequacies.
Generally, the system with the highest efficiency will apply the needed water directly to the root zone or crop canopy, as appropriate, without incurring excessive water loss or runoff. This is what makes drip irrigation so attractive and more efficient than other less efficient methods such as sprinkler or flood irrigation.
Sprinkler irrigation systems are inherently less efficient due to the higher pressure needed; a typical lack of uniform coverage between sprinkler heads; and distortion and drift losses from wind, evaporation, and other environmental factors. In addition, if sand or abrasives are present from the water source, the scouring action onto nozzles can create severe inequities of flow between heads, generating unbalanced water distribution.
Conversely, although sprinkler irrigation is not as inherently efficient as drip irrigation, applying the water closer to the crop and at lower pressure, such as using LEPA irrigation with drop tubes, can provide efficiencies that rival drip or micro methods. Lastly, although the system’s friction loss is an element of the pumping plant head, and thus the efficiency, the head losses associated with each irrigation method should be considered as a factor of the system’s efficiency.
Although the values stated earlier are mostly representative of actual systems, an exact determination of the distribution uniformity (DU) would require extensive field testing, such as catch-can tests and careful monitoring of crop uniform growth and production. Therefore, the judicial use of estimates based on established protocol or past experience are often applied.
2. Pumping Plant and Conveyance Efficiency
This is an obvious element of system efficiency, not only for irrigation systems but for all pumping and conveyance systems used for domestic, municipal, and industrial water systems.
The process for determining the pumping plant efficiency is well-documented and includes the four variable primary elements of flow rate, total dynamic head (TDH), and pump and driver efficiencies.
The first component of horsepower, the flow rate, is largely dictated by the method of irrigation and can typically vary from a low of 2 to 3 GPM per acre for drip irrigation to 10 GPM per acre for sprinkler irrigation, and up to 50 GPM per acre for flood irrigation.
The second component, head, is the sum of the pumping lift, static head, frictional head losses, and operating pressure. As with the flow rate, the operating pressure can also greatly affect the required horsepower and vary widely between methods. They can range from a low of 10 psi for drip irrigation up to 125-150 psi for Big Gun systems. In addition, horsepower requirements needed to deliver water from a deep well is often 10 times or more than that required for pumping the same flow rate from a river or lake.
Finally, the efficiency of different pump types and their driver will also vary. Vertical turbine and centrifugal pumps will often display a new plant efficiency of 75%-80%, while the same capacity and head-rated submersible pump and motor combination may yield a plant efficiency of 60%-65%.
These factors create a pumping plant or wire-to-water efficiency that become the first of two elements in this category. The second factor are the conveyance losses, although these are generally not a significant factor with an enclosed pipeline.
Conveyance losses can become substantial with long or undersized pipeline runs or open channel flows such as leaking canals. Conveyance losses are regarded as the relationship between water-in versus water-out, or the volume of water entering the system against the volume of water exiting the system. This is generally the volume that exits the sprinklers, over the same time span.
Although conveyance losses, when factored as a part of irrigation efficiency, are generally regarded as a net water loss that is compared to the volume of water required for crop growth, these losses can also be equated to the added pumping costs associated with pumping the water and the head required to deliver this lost water.
3. Field (Farm) Efficiency
The efficiency associated with the application and use of water after it exits the irrigation system comprises the field or “farm” efficiency. This includes the elements of scheduling, managing the ratio of irrigation-applied water versus rainfall to maintain the proper balance of water to air in the soil profile, and maintaining water quality through adequate leaching of salts from the soil.
Irrigation scheduling is one of the most important factors. Three distinct methods of scheduling the delivery of irrigation water are commonly recognized: demand, rotation, and continuous flow.
The demand method is most often used with automatic controller systems, such as landscape irrigation, that are able to deliver water in response to the prior period’s consumptive use and is often applied to replace the water consumed through a single day of evapotranspiration.
The rotation method is most commonly used with agricultural irrigation applications and is based on a predicted consumptive use of water over a specific time period, generally seven to 14 days. This method relies heavily on the use of retained moisture in the soil profile, with irrigation replenishing the moisture lost to evapotranspiration.
The final method, the continuous method, is the least common and is used to match the application of water with the crop’s uptake value. It is risky to use as it heavily relies on daily calculations of the crop’s current uptake value, local evaporation, and losses.
The method of water delivery has a pronounced effect on project irrigation efficiency. Application of irrigation water must be closely adjusted, both to the daily requirements of the crop, the leaching requirement, and to the available waterholding capacity of the soil root zone, if satisfactory production is to be obtained.
Losses of both soil and water on the farm must be held to a minimum, while the needs of the farm are also served. Attainment of these conditions calls for flexibility of water deliveries throughout the project distribution system and is expressed as the following (applied in consistent values):
Estimated farm or field irrigation efficiency (FE’-in %) =
ET = Estimated volume of evapotranspiration
Wl = Volume of water required for leaching
PR = Volume of effective precipitation (rainfall)
Wd = Volume of irrigation water delivered
Whenever possible, field irrigation efficiency should be based on the measured value of evapotranspiration, or the estimated consumptive use should be adjusted for actual soil water levels maintained, the actual duration of growing seasons, and adequacy of the irrigation sets.
Other Potential Efficiency Losses or Gains
The loss and waste that occur in irrigation water storage reservoirs, as well as from the conveyance and distribution systems, generally occur as seepage, evaporation, consumptive use, and operational losses and waste and vary with the type and design of the irrigation project.
For example, high levels of seepage from deep storage reservoirs will reduce the reservoir storage efficiency while the same volume within a shallow reservoir may not produce the same loss. This difference is likely a direct result of the greater head placed on the water in the deeper reservoir.
In the selection of a reservoir site, the permeability of the soil or mantle covering the reservoir area is evaluated. In some instances where high permeabilities are uncovered, either greater compaction of the soil or applying a compacted, overlying blanket of a bentonite or grout layer may effectively decrease the seepage losses. A polyethylene or vinyl film liner has also been used to cover smaller reservoirs, but is currently considered to be too expensive for most larger reservoirs.
When a reservoir is filled, some of the water is absorbed by and into the soil of the reservoir. This often combines with soil particles to form a quasi-barrier to exfiltration.
Conversely, when the reservoir water level is lowered, water can drain from the bank back into the reservoir. This water is referred to as bank storage and may amount to a sizeable volume in a large reservoir.
Many irrigation systems deliver water using enclosed pipes from gravity-supplied sources for surface or drip irrigation, or in certain cases adequate pressure may even be provided for higher pressure sprinkler irrigation. Gravity-supplied water sources do not include the obvious capital and operating costs and inefficiencies associated with an electric- or engine-powered pumping plant, and can therefore provide innumerable benefits as well as efficiency gains over pumping plants of comparable capacity and head.
Conveyance and transmission losses, however, can still present a significant drop in efficiency, particularly since many gravity-supplied sources transmit water over substantial distances in some cases. This can amount to dozens of miles through old and leaking canals or pipelines.
To be fair to the analysis, the volumetric water losses with a gravity-supplied water source must be factored into the conveyance system efficiency. There is almost always a correlation between an abundance and the cost of water and project irrigation efficiency. Where water is scarce or high in cost, the conveyance and storage efficiencies are normally higher due to more careful use and application of the limited water. Conversely, where water is abundant or low in cost, the efficiencies are lower from the perception of greater allowed waste.
Thus, in a sense, economics play a major role in determining existing and new project irrigation system efficiencies. Project management as well as farm management involves balancing the immediate cost of water against the higher labor and investment costs required to use the water more efficiently. In many cases, the true costs of using excessive amounts of water are not recognized immediately, but may be reflected in reduced yields due to unnecessary leaching of plant nutrients, reduced yields caused by accumulation of soluble salts, or exchangeable sodium, or in extra drainage installations which may be required to control rising groundwater levels.
Uniformity, in both definition and measurement of efficiencies, still makes rigid comparisons of the capabilities or potential efficiencies of similar systems difficult, mainly because of the human element involved in the operation of the system. Variations in operational procedures can cause marked differences in irrigation efficiencies of virtually identical systems. Reliable evaluation of basic differences in system performances, such as between surface and sprinkler systems, can only be made when systems are operated to provide the same adequacy of irrigation over the same percentage of the field, and when both are operated as designed.
Although irrigation design is often performed using a seatof- the-pants approach, the proper use of the practice actually involves a carefully crafted technology that often makes use of 21st century science in order to reduce physical waste, loss of water, and degradation of water quality; protect the soil and environment; optimize irrigation scheduling, system performance, and crop growth; and control the potential for enduring excessive labor or operational costs.
Next month we will return to a more predictable format of discussing a single topic, so as always and until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at email@example.com.