Tools for Monitoring Microirrigation Performance

By Farouk A. Hassan, Ph.D

Successful operation and management of microirrigation systems require a proactive monitoring approach to maintain the desired system performance. Two devices for monitoring the performance of these irrigation systems are presented in this article.   The tensiometer for monitoring the moisture status in the rootzone for sound irrigation scheduling,   and the flowmeter for measuring the system flow rate and recording the volume of applied water.  While the use of tensiometer enables determining when to irrigate, the flowmeter provides the necessary information for evaluating the system performance, and for estimating irrigation efficiency.

Tensiometer

Tensiometer parts shown in figure 1 include the ceramic tip, the transparent plastic tube (stem) with a side port to accommodate a vacuum gauge, the vacuum gauge, and the rubber stopper attached to the service cap.   The tensiometer is usually installed in the field  with the ceramic tip (ceramic cup) placed where the soil moisture status is to be monitored, and the plastic tube is long enough so that the stopper and the gauge remain above ground (see figure 2).

Figure 1. Tensiometer with ceramic tip, plastic tube and side port

Figure 2. Tensiometer installed in the field

A.    Operation  

For the proper use of tensiometer, the porous cup should be soaked in water overnight to ensure that the pores in the wall of the cup are saturated.   The tube is then filled  with water so that no air bubbles remain inside it and the service cap is closed to secure the airtightness of the tensiometer.  The tensiometer is installed  in the field with the porous cup in glovetight contact with the soil.  The saturated pores of the porous cup connect with the soil pores and create a continuous hydraulic (water) connection between the water in  the soil pores and water inside  the porous cup.

                  After irrigation, as the soil dries it exerts tension (or suction effect) on the water in the soil pores.  The developed tension is transmitted via the established hydraulic  connection to the water inside the porous cup causing some water to be sucked out of the cup leaving vacuum above the water column in the tube.  The gauge registers  the magnitude of the developed vacuum.  Further soil  dryness causes more water to move outward from the porous cup to the soil.  This is reflected by higher gauge readings. 

                  When the field is reirrigated, suction in soil pores is reduced and the previously created vacuum above the water column in the stem of the tensiometer causes the water to be drawn back into the porous cup .  This inward  water movement reduces the vacuum and lowers the gauge reading. 

                  As soil starts to dry again, the water moves outward from the porous cup and the above pattern repeats itself.  Evidently any air leakage into the tensiometer could impair  its function as it destroys the developed vacuum.  A good maintenance routine  with regular air removal from the tensiometer stem and filling it with water could avoid such leakage. 

                  The tensiometer gauge is calibrated in units of centibar (cb).   A bar is about one atmosphere, and a centibar is 1/100   of a bar.  Though the scale of the vacuum gauge reads up to 100 cb, a tensiometer can operate from 0 to 80 cb.   Readings from 0 to 5 cb indicate a saturated soil in  which the plant roots will suffer from lack of oxygen, while a reading of 80 cb reflects the dry end of the scale.

B.    The Instrument of Choice  

High frequency of water application under microirrigation   maintains  moisture content near field capacity with a corresponding low soil-moisture tension in the root zone (well below 80 cb).  Tensiometer is most suited for monitoring  soil-moisture  depletion under this wetting pattern.  It is also relatively inexpensive instrument and easy to use.  Monitoring soil moisture depletion in the root zone is essential for determining   when to irrigate and an indispensable part of a good irrigation management program.

C.     When to Irrigate

Early in the season irrigate vegetables, row crops and newly planted trees when 20% of the available water in the active root zone is depleted.   Later in the season irrigation  frequency will change with growth stage and local conditions.  Irrigation of well established tree crops should begin when depletion approaches 25-30%.  Table 1 provides a guideline for tensiometer readings at field capacity (F.C.) and at 20-25% depletion of available water in soils of different texture.

Table 1:  Moisture characteristics of soils of different texture

D:    Monitoring  Soil Moisture for Vegetable and Row Crops      

Install tensiometer, in sets of two, in the row between plants.   The ceramic tip of the first tensiometer should be placed at about 6 inches deep in the soil for shallow-rooted  crops (e.g. lettuce, celery) and at 12 inches for deep-rooted crops (e.g. tomatoes, melons).  The second tensiometer should be installed about 12 inches deeper than the first one.

The shallow tensiometer monitors the moisture status of the active root zone.   Irrigation should begin when the shallow tensiometer readings are in the ranges of 20 to 25 centibar (cb) in sandy soils,  25 to 30 cb in clay loam soils and 35 to 40 cb in heavy clay soils.  The deeper tensiometer should read about 10 cb between irrigations.  Much higher readings show insufficient irrigation, while lower readings may indicate too heavy or too frequent irrigations or poor drainage.

E:      Monitoring  Soil Moisture for Orchards and Vineyards            

In new orchards and vineyards the ceramic tip of the tensiometer should be placed in the root ball.   After several weeks, install the tensiometer near the drip line of the growing tree.  Additional tensiometers should be added to accommodate changes in root distribution over time as the plants grow.  Tensiometer should be placed on the southwest side of the tree in the northern hemisphere, since this side receives the hot afternoon sun and tends to dry more quickly.  Tensiometers are installed in the row,  between trees or vines,  about 12 inches from the emission point in sandy soils and about 18 inches from the emission point in clay soils,  close to tree drip line.

                  For most mature orchards and vineyards tensiometers are installed in sets of three at 12, 24 and 36 inches deep.  For deeper rooted trees (e.g. almond, walnut) a fourth tensiometer may be installed at a depth of 48 inches.  The top tensiometer is in the part of the root zone that dries out first.  Under good management this tensiometer should read about 25 and 10 centibar (cb) in  sandy soils before and after irrigation respectively  and about 35 and 25 cb in clay soils before and after irrigation  respectively.

      Deeper tensiometer should go down to about 10 and 25 cb range in sandy and clay soils respectively after irrigation.   If the readings of the deep tensiometer remain significantly   higher than these values, this means that the volume of applied water, irrigation  time,  or frequency of application should be increased.  On the other hand, if the readings of the deeper tensiometer do not come up to the 10-25 cb range between irrigations it means that application is too heavy, too frequent, or drainage is restricted.

F:    Points of Consideration  for Tensiometer Utilization                      

1. Deeper tensiometer  provide information on  whether water is flowing upward in the root zone or being lost to deep percolation.  A downward movement of water may be desirable if  salt leaching is intended.

2. Tensiometer placement must be in the zone wetted by the downward and outward movement of water in the soil.  If the tip is outside the wetted zone, readings will indicate  low soil moisture content resulting in over-irrigation.  On the other hand, if the tip is too close to the emitter,   the high moisture content indicated by the instrument may lead to under-irrigation.

3. Correction  of the gauge reading is usually required  to account for the depth of placement of the tensiometer.  The depth of tensiometers is determined as the distance from the middle of the ceramic tip to the gauge.  Three centibar should be subtracted from the reading for every foot of depth.  A 4-ft tensiometer should require that 12 cb (= 4 feet x 3 cb per foot) be subtracted from the gauge reading.  A gauge reading of 40 centibar on a 4-ft  tensiometer actually indicates 28 cb (= 40 -12) of moisture tension.

4.  The upper limit of gauge reading at elevations up to 1000 feet above sea level is 80 cb.  At higher elevations, this limit should be reduced, due to the decrease in  atmospheric pressure,  by 3 cb per 1000 feet increase in elevation; i.e., at 3500 feet above sea level the upper limit of gauge reading should be 72 cb.

5.  Daily  readings of tensiometer should be done at the same time of the day, preferably  early  in the morning.  At each reading add water if the water level in the tensiometer falls more than 1 to 2 inches below the stopper.

6.  Recording and plotting of readings on a chart indicating date, depth of reading, and date of irrigation are necessary to achieve the full benefit of the tensiometer use for monitoring  water applications and for irrigation  management.  These records can also be useful for future planning.

7.  Irrigation  management decisions should be based on the readings of more than one set of tensiometers.  No definite number of tensiometer sets per number of acres is recommended.  However, at least two sets should be installed in each management unit of the field that differs in crop, soil texture, profile depth and stratification, cover crop,   other cultural practices and the desired degree of characterization.

Flowmeter

Monitoring the flow of irrigation  water is an essential aspect of any efficient  irrigation  management.  The meter reading may indicate the flow rate, the total flow volume,  or both.  Most flow rate indicators report in gallons per minute (gpm) or in cubic feet per second (cfs), while total flow indicators (totalizers) report in gallons, acre-feet, or cubic feet.  Some indicators report in  metric units.

A:   The Meter of Choice 

Propeller flowmeters are the most commonly used for agriculture.    Figure 3 shows an example of propeller meter.  Though other metering technologies are in use propeller meters are the meters of choice for irrigation because they are accurate (? 2.0%),  of relatively low cost, require no external power, and withstand harsh environmental  conditions.

Figure 3. Propeller meter.

B:   Points of Consideration for Propeller Flowmeter Utilization  

1.  The meter should be installed downstream from a straight unobstructed section of the pipe of eight to ten diameter in length and a straight section two pipe diameter long immediately downstream from the meter to minimize turbulence caused by various  fittings and valves. Erratic behavior of the rate indicator may also be due to the presence of air or gas in the water.   Straitening   vanes (six-vane straightener) can be placed just ahead of the flowmeter to break up most swirls and ensure accurate measurements.

2.  Inaccuracy of the propeller meter is most likely  due to mechanical problems.  

A propeller meter with mechanical problems  will have an unsteady flowrate reading

(a bouncing needle) or the rotating propeller may create noise and vibration.  Mechanical problems are usually caused by a jam inside the meter.  Once the body that is causing the jam is removed the meter will  work properly again.

3.  A flowmeter is most accurate and the pressure loss caused by meter is minimal (less than 1 psi) when used within its flow range.  The range is very   large and is expressed as the "turndown" of the meter.  The turndown is the ratio of the maximum flow rate to the minimum  flow rate and is often 15 :1.  That means that the meter would remain accurate up to 15 times its minimum flow rate.  For example,  a meter with a minimum flow rate of 100 gpm and a turndown of 15 :1 would remain accurate up to a flow of 1,500 gpm.

C:   Benefits of Using Flowmeter

The use of flowmeter makes it possible to identify changes in flow rate (measured at the same pressure) during the season.   Excessively higher than usual flow rates or significantly lower rates may provide early alert to some potential operational problems as indicated below.  Other benefits of water metering include ensuring that the scheduled amount of water is actually applied and to avoid over-irrigation and leaching of NO3-N   below the root zone and contamination of groundwater.  Flowmeter record keeping is useful for planning,  management and water budgeting, assessing the performance of the system, and estimating irrigation efficiency as described below.

D:   Water Metering and System Performance

The flowmeter records together with measuring irrigation time can be used for monitoring  system performance.   Actual irrigation time is the elapsed time between the beginning of the irrigation run and until the reading of the flow meter indicates that the total water volume scheduled for irrigation is applied.  For monitoring  system performance:

1. Obtain an estimate of irrigation time in hours using the following formulae for different  types of emitter lines:

        a.  For hose line (in-line / on-line emitters)   

        Estimated Irrigation Time (hours ) =   [irrigation requirements (inches)  x  acres x 452.5]          / gpm

         where : acres = .............. number of acres per irrigated set or field

                       gpm = ................irrigation system flow rate in gallons per minute, at average            operating pressure

        Example

           The irrigation requirements for a 20 acres field is 0.3 inch.   The capacity of the surface              drip system used to irrigate this field is 500 gpm.  What should the irrigation time?

            Estimated Irrigation Time  = (0.3 x 20 x 452.5) / 500 = 5.5 hours

        b. For drip tape

        Estimated Irrigation Time (hours) =   1.04 x tape spacing (feet) x irrigation requirements           (inch) / tape flow rate (gallon/minute/100 feet)

        Example

           The irrigation requirements of a tomato field is 0.3 inch.  The field is irrigated with           subsurface drip  with tape spaced at 5 feet.  The tape flow rate is 0.4  gallon per            minute per 100 feet.  What should the irrigation  time be for this field?.

          Estimated irrigation  time  =1.04 x [5(feet) x 0.3 (inch)]/ [0.4 (gallon/minute/100 feet)]             = 4.0 hours

          To convert the depth of irrigation  requirements in acre-inch to volume of water in           gallons or cubic feet (flowmeter units), use the following  formula:

         One acre-inch  = 27152 US gallons = 3630 cubic feet

         c. For jets  and microsprinklers (for trees)

          Estimated Irrigation Time (hours) = irrigation  requirements (gallons/tree) / emitter             discharge (gph)

          where: gph = gallons per hour

         Example

         A 20 acres orchard is irrigated  with microsprinkler system, one microsprinkler of 20           gph per tree.  The orchard is   irrigated  daily  with irrigation   requirements of 80 gallons           per tree per day.  The tree spacing is 20 ft x 20 ft.  Estimate the irrigation  time for this           orchard,  the volume of water per irrigation, and the inches of applied water.                

       Estimated Irrigation  Time = 80 (gallons/tree) / 20 (GPH)= 4 hrs

       Number of trees in the field  = [43560 (sqr. ft/Ac) x 20 (Ac)] / 400 (sqr. ft/tree) = 2178         trees   

       Volume of water to be applied per irrigation = (80 gallons/tree) x (2178 trees)    =             174,240  gallons per irrigation

       Volume of water to be applied per acre = 174,240 / 20 = 8712 gallon/acre

       Inches of applied water = 8712 (gal/acre) / 27152 (gal/acre-inch) = 0.32 inches

2.  Monitor the flowmeter and the determine the actual time needed to apply the volume of irrigation requirements in gallons or cubic feet.

3. If the actual irrigation time is greater than the estimated irrigation time by more than 15%, this may be an indication of a clogging problem in progress, especially if pressure  buildup and noticeable reduction in system flow rate are observed.  Check emitters flow rate, filters differential pressure and performance and take necessary maintenance measures. 

  A significant drop in the flow rate and discharge pressure could also be the result of excessive drawdown or the need for pump adjustment or repair.  These possibilities  should be examined.   Unnoticed reduction in the system discharge rate would lead to under-irrigation of the field if flowmeter readings are ignored and irrigation run  is terminated after a preset time.  This can compromise the benefits of microirrigation.  

On the other hand, If actual irrigation time is significantly less than estimated irrigation  time with apparent difficulty in maintaining  system pressure, check for leaks and make needed repair.

E.  Estimation  of Irrigation  Efficiency 

An approximate but adequate estimate of irrigation efficiency is the ratio of crop water requirements to the actual depth of irrigation  water applied to the field to bring up the average moisture content of the rootzone to field capacity.  This estimate can be obtained as follows:

1. Total volume of water applied to the field in gallons or cubic feet (cf) = (totalized meter reading at the end of irrigation) - (totalized meter reading at beginning of irrigation)

2.  Total volume of applied water per acre (gal/Ac or cf/Ac) = Total volume of water applied to the field / number of acres irrigated

3.  Depth of irrigation water applied in inches (in)  = Total volume of applied water per acre (gal/Ac) / 27152 = Total volume of applied water per acre  (cf/Ac) / 3630

4.  Irrigation efficiency (decimal)   = water requirements (in) / depth of irrigation  water applied (in)

5.  Irrigation efficiency (%) = Irrigation efficiency (decimal) x 100

Example

The estimated water requirement of a crop is 0.25 inches and the assumed efficiency  of the microirrigation system used to irrigate this 20-acre field is 85%.  The readings of the flowmeter totalizer were 101, 000 gallons and 300, 240 gallons before and after irrigation respectively.  Calculate the scheduled irrigation application depth based on the assumed irrigation efficiency, and the actual irrigation efficiency based on the given flowmeter readings.

 Scheduled irrigation application  depth = 0.25 / 0.85 = 0.30 inches

 Total volume of water applied to the field (gallons) = 300,240 - 101000 = 199, 240  gallons

Total volume of applied water per acre (gallons/acre)  = 199,240 / 20 = 9962  gallons/acre

Actual depth of irrigation  water applied (inches) = 9962 / 27152 = 0.40 inches

 Actual irrigation efficiency (decimal)= 0.25 / 0.40 = 0.63

Actual irrigation efficiency (%) = 0.63 x 100 = 63%

If irrigation efficiency is consistently less than 80%, then necessary measures should be taken to improve efficiency (examine the possibility of clogging, check filter condition, ... etc.).

The method outlined above for estimating irrigation efficiency is not a substitute for professional irrigation evaluation service,  but it can be used when such service is not readily available.

In conclusion, performance monitoring tools are indispensable for a proactive approach of managing microirrigation.   Such an approach saves money, conserves resources, protects the environment and is a prerequisite for a successful operation.  On the other hand, lack of monitoring is a characteristic of a reactive approach which costs money and may not be fast enough to avert unexpected problems at critical times.

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Farouk A. Hassan is an Irrigation & Soils Consultant with Agro Industrial Management, P.O.Box 5632, Fresno, California 93755, Phone: (559)224-1618, Fax: (559)348-0721, E-mail: fahassan@aol.com

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