S MALL -S CALE R AINFALL V ARIABILITY IN W ESTERN P UERTO R ICO AND ITS I MPLICATIONS ON A GRICULTURAL W ATER M ANAGEMENT Eric Harmsen, Professor Dept. of Agricultural and Biosystems Engineering, University of Puerto Rico, Mayagüez Jarek Cintron Department of Industrial Engineering, UPRM, Edvier Cabassa Department of Electrical and Computer Engineering, UPRM,

S ITUATION Rain gauges are the most common means for farmers to determine whether their crops have received sufficient water, and whether supplemental irrigation is needed. Farmers commonly rely on rainfall information from a single rain gauge on their farm or from a rain gauge located off the farm. Similarly, university researchers commonly rely on rainfall information from the single rain gauge located on the experimental station.

But how reliable is rainfall information from a single gauge located, in some cases, several kilometers from a field? To help answer this question, a rainfall variability study was conducted near the University of Puerto Rico-Mayagüez Campus in Western Puerto Rico.

A PPROACH Twenty-eight rain gauges were installed within a 4 km x 4 km area. Rainfall amounts were measured every 5 minutes between August 2006 and August 2007.

I NSTALLING THE RAIN GAUGES

S TUDY AREA

S TUDY AREA – RAIN GAUGE LOCATIONS

E QUIPMENT Watchdog Rain GaugeWatchdog Shuttle

S TUDY AREA – FINAL RAIN GAUGE LOCATIONS

R ESULTS For the 62 storms evaluated, the average storm rainfall, standard deviation, and minimum and maximum rainfall were mm, mm, mm and 4.53 mm, respectively. For any single storm the statistics can be more striking, for example, May 24 th, 2007, the average storm rainfall, standard deviation, and minimum and maximum rainfall were 55.4 mm, mm, mm and 28.2 mm, respectively.

RAINFALL ANIMATION FROM RAIN GAUGES NETWORK JUNE 27, 2007

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mm Total Storm Rainfall

R ESULTS These results have implications for scheduling irrigation.

I MPLICATIONS OF R AINFALL R ESULTS In the May 24 th example previously mentioned, the rainfall gradient was 43.4 mm/km; which means that if a farmer’s field was located 1 km from the rain gauge, the rainfall value would have been in error by 43.4 mm (1.7 inches)! This result could have lead to a serious over or under estimation of supplemental irrigation.

R ECOMMENDATION Every farm should have at least one rain gauge. Investigators should not assume that the experiment station rain gauge will provide accurate values of rainfall at their experimental plots. $10> $1,000

Is a $1,000+ weather station worth buying?

T HE C OST OF O VER -A PPLYING I RRIGATION W ATER Assume the following: Small 10-acre farm grows pumpkin Estimated consumptive use (CU) for season = 500 mm Actual potential CU for season = 400 mm Overall cost of water = $30/acre-ft (considering only: cost of water and electricity) Assume the normalized yield vs. CU curve in the next slide is applicable. Value of a typical Calibaza crop = $3,600/acre.

E XAMPLE CONTINUED Results: Excess water applied = 100 mm = 1.07million gallons = 3 acre-ft (lost to groundwater) Normalized CU = 1.25, therefore normalized yield = 0.9 (or 0.1 loss) Potential $ LOST = cost of water + lost yield = 3 ac-ft x $30/ac-ft + [0.1*$3600/ac] x 10 ac = $3690 Agr. Chemicals are leached to groundwater (cost was not included in calcluation). Groundwater was potentially contaminated

C OST OF U NDER -A PPLYING I RRIGATION W ATER Assume the following: Same pumpkin farm (10-acres) Estimated CU for season = 300 mm Actual potential CU for season = 400 mm Assume the normalized yield vs. CU curve is applicable. Value of a typical Calibaza crop = $3,600/acre.

E XAMPLE CONTINUED Results Water deficit = 100 mm With a normalized CU of 0.75, the normalized yield = 0.85 (or 0.15 loss) Potential $ LOST = lost yield = [0.15*$3600/ac] x 10 ac = $5,400

C ONCLUSIONS FROM E XAMPLES The potential value of the crop may be significantly reduced by over or under- application irrigation water. When water is over-applied, in addition to the reducing the potential value of the crop, certain costs are also wasted (water, fuel, chemicals, etc.) Over application of water can lead to degradation of ground and surface waters.

Yes! In order to reduce losses related to yield, water, energy and chemicals. Is a $1,000+ weather station worth buying?

A CKNOWLEDGEMENTS Financial support from: USDA-TSTAR NOAA CREST NASA EPSCoR NSF-CASA