V e g e t a r i a n  N e w s l e t t e r
UF/IFAS - Horticultural Sciences Department
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 Vegetarian 04-10 grnbullet.gif (839 bytes) October 2004

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Water Conservation through Soil Moisture Sensing - Field Evaluation of Two Soil Moisture Sensors for Automatic Control of Drip Irrigation of Tomatoes in South Florida

Improving irrigation efficiency can contribute greatly to reducing production costs of tomatoes, making south Florida’s tomato industry more competitive and sustainable. Efficient and modern irrigation systems in Florida and areas with similar conditions such as low water holding capacity soils and shallow rooted crops should be based on three principles: 1) low volume-high frequency, 2) soil moisture sensor based scheduling, and 3) automatic operation (Dukes et al., 2003). Soils with water holding capacities in the 4-8% range by volume (sands, gravels) are common in south Florida and present special water management challenges (Muñoz-Carpena, 2002).

A low-volume/high frequency (LVHF) soil moisture-based drip irrigation system was tested on a 40-acre commercial tomato farm in Miami, South Florida (Fig. 1). Seven irrigation treatments were compared (Table 1).

Fig. 1. Ms. Tina Dispenza, Engineering Technician at UF/IFAS TREC, downloading water use data at the field study.

 

Table 1. Irrigation treatments.

#

Treatment

Description

Sensor

Set point

t1

T-10

Timer/Soil tension based

Switching low-tension tensiometer (model TGA-LT, Irrometer Inc., CA)

10 kPa (10 cbar).

t2

T-15

Timer/Soil tension based

Switching low-tension tensiometer (model TGA-LT, Irrometer Inc., CA)

15 kPa (15 cbar)

t3

WM-10

Timer/Soil tension based

Granular matrix sensor (Watermark WEM-II, Irrometer Inc., CA)

10 kPa (10 cbar).

t4

WM-15

Timer/Soil tension based

Granular matrix sensor (Watermark WEM-II, Irrometer Inc., CA)

15 kPa (15 cbar)

t5

Time-100%

Timer based to apply 100% of crop needs 1

--

5 irrigations/day (12 min each)

t6

Time-150%

Timer based to apply 100% of crop needs 1

--

5 irrigations/day (12 min each)

t7

Farmer (control)

Standard grower’s schedule typical in the area (high volume-low frequency).

--

2-3 times per week, 2-3 hrs per irrigation

1Based on recommended water needs for Miami (Simonne et al., 2001)

In the first six treatments, the LVHF type, the system was pressurized by means of an electrical pump and a pressure tank, and controlled by an irrigation timer (controller) set to irrigate 5 times per day. The last treatment consisted of the farm’s standard commercial practice, where a portable pump was used on a twice a week manual irrigation schedule during the winter crop season. Four of the six LVHF treatments resulted from interfacing, in a closed control loop with the irrigation controller, two types of soil moisture sensors (switching tensiometer and granular matrix sensor) set at two moisture points (wet: 10 cbar, optimal:15 cbar) (Fig. 2). Details on these types of sensors for use in South Florida can be found in Muñoz-Carpena et al. (2002) and Muñoz-Carpena (2004).

Fig 2. Sensors evaluated in the study: a) switching tensiometers; b) granular matrix sensors with control box.

The other two LVHF resulted from the same system with no sensors with two timer schedules, one to supply 100% of the maximum IFAS recommended crop water needs for Miami, and the other to supply 150% of those needs (Simonne, et al., 2001).

Total seasonal water use results (Fig. 3) show that the six LVHF treatments conserved water, and did not significantly (p<0.05) diminish tomato yields below that of the commercial field. Switching tensiometers at 15 cbar set point performed the best (up to 73% reduction in water use when compared to commercial farm, 50% with respect to the 100% recommended crop water needs treatment) (Fig. 3).

Fig. 3. Water use for each irrigation treatment during the tomato season.

Routine maintenance was critical for reliable operation of the switching tensiometers (tensiometers had to be recharged at least once a week for normal operation). Granular matrix sensors behaved erratically, and did not improve water savings compared to the 100% recommended crop water needs treatment. These sensors did not react fast enough to frequent wet and drying cycles typical of LVHF (5 times a day), especially during the rewetting cycles. This lack of sensor response is possibly exacerbated by the fact that their lower working range (7 cbars) is very close to the values used as irrigation set points (15-25 cbar) in the coarse shallow soil typical of South Florida.

References

Dukes, M.D., E.H. Simonne, W.E. Davis, D.W. Studstill, and R. Hochmuth. 2003.  Effect of sensor-based high frequency irrigation on bell pepper yield and water use.  Proceedings of 2nd International Conference on Irrigation and Drainage, May 12-15, Phoenix, AZ.  pp. 665-674.

Muñoz-Carpena, R., Y. Li. and T. Olczyk. 2002. Alternatives for low cost soil moisture monitoring devices for vegetable production in the south Miami-Dade County agricultural area. Fact Sheet ABE 333 of the Dept. of Agr. and Bio. Engineering, University of Florida. http://edis.ifas.ufl.edu/AE230

Muñoz-Carpena, R. 2004. Field devices for monitoring soil water status. Extension Bul. 343  of the Dept. of Agr. and Bio. Engineering, University of Florida. http://edis.ifas.ufl.edu/AE266

Simonne, E.H., M.D. Dukes and D.Z. Haman. 2001. Principles and practices of irrigation management for vegetables. In: D.N. Maynard and M. Olson (eds.). Vegetable Production Guide for Florida. Chapter 8. Gainesville: Citrus and Vegetable Magazine and University of Florida Extension-IFAS.

(Rafael Muñoz-Carpena, assistant professor, TREC-Homestead, Michael D.  Dukes, assistant professor, Agricultural and Biological Engineering Dept.  - Vegetarian 04-10)