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When Plants Sweat

Understanding the concept of irrigation

We all do it. In mammals, the loss of fluid from the pores of the skin is called perspiration. In plants, it’s known as transpiration. Even the land circulates and loses moisture — a process called evaporation. Most people don’t think twice about how much plants sweat — but for farmers, especially those on Molokai where water is scarce — understanding plant transpiration can make all the difference.

Molokai has the highest recorded rate of evaporation in the state at 118 inches per year, according to Alton Arakaki, a University of Hawaii College of Tropical Agriculture and Humans Resources (CTAHR) Molokai extension agent. Evaporation is caused by a variety of environmental stimulants, such as humidity, wind and sun. It is also closely tied to plant transpiration rates.

Evapotranspiration is a combination of the environment’s evaporation rate and the amount of water lost by plants through transpiration from their leaves. In agriculture, it’s important to understand the balance between the evapotranspiration rate and the amount of water needed for irrigation.

That balance, according to Arakaki, is best managed through a strategy of applying the same amount of water to a crop as is lost through evapotranspiration. Irrigate any more that and water will be wasted, while less will cause plants to not produce at their peak. To help farmers understand this process, Arakaki held a water conservation and irrigation management workshop last month at the UH Molokai Applied Research and Demonstration Farm. There, Arakaki shared a variety of field tests aimed to show the effects of various irrigation methods and amounts.

“What we have is what we have, so we have to use it to the best of our ability,” said Arakakai of the need for water conservation. “The state’s kuleana is delivering water to the meter… Once it [arrives there], it’s the farmer’s responsibility.”

Evaporation rates for a particular location can be determined by using a calibrated evaporation pan. Arakaki showed workshop attendees how to construct the pan and use a PVC pipe with a floating water measure inside to take accurate evaporation measurements. Farmers can use those measurements to determine how much water is needed per acre to irrigate various crops.

This value is expressed as Kc, or the percentage of pan evaporation rate that will equal the evapotranspiration of the crop. Each crop has a different Kc value — pineapple, for example, requires a Kc of only about 20 percent, whereas many other plants will fall between 60 and 100 percent of the evaporation rate, said Arakaki.

He said farmers have found that water has more influence on crop yield than fertilizer. In field tests, Arakaki showed the effects of different irrigation types such as drip and sprinkler. Drip irrigation generally offers a more consistent yield than sprinklers, he said.

Windbreak treatments can also affect crop success. Correct windbreak angle — 135 degrees in relation to the wind direction — and careful variety choices (Sorghum Sudan grass is used at the Demonstration Farm) can affect the evaporation rate from the wind. Arakaki said the general rule for windbreak is every foot of vertical windbreak gives 10 feet of protection for adjacent crops.

Arakaki also showed workshop attendees a test plot exploring the use of mulch on crop yield and the amount of water needed. Using oats, three demonstration plots with wood chips, plastic mulch and no mulch all receive the same amount irrigation and showed that the use of mulch doesn’t necessarily lead to success. Wood chips actually absorb water, leaving less available for the plants, explained Arakaki. In the test site, the oats growing without any covering grew best because of the water-holding capacity of the soil itself.

“For some farmers, mulch is for weed control… [or to] protect the soil from wind erosion,” said Arakaki, adding that while mulch can be beneficial in some circumstances, using wood chip mulch may require extra irrigation.

The workshop resulted from conference held last year to determine agriculture needs in the County of Maui, according to Roland Prieto of the county’s Office of Economic Development. The goal was to bring immediate benefit to those in the ag community, and water was identified as a priority for Molokai. This was the second workshop is a series to help farmers and community members gain a better understanding of irrigation water resources on the island.


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