Water Conservation and Irrigation Workshop

UH CTAHR Molokai Extension News Release

There aren’t too many things in Hawaii we measure in the billions.  The size of the state’s economy is about $67 billion, the volcano at the Hawaii Volcano National Park produces about 6.4 billion cubic feet of lava per day and the 100-acre Molokai Irrigation System reservoir has a storage capacity of 1.2 billion gallons.  But if we want to see 50 percent of Molokai that is dry almost all year round to green up, it will require 389.6 billion gallons of water per year.  That is because Molokai has the highest recorded annual average pan evaporation rate in the state, at 118 inches per year according to historic data in DNLR reference “Pan Evaporation: State of Hawaii 1894-1983.”  Following Molokai, there are sites on Hawaii Island with 108 inches, Maui with 99 inches, Oahu and Kauai with 98 inches per year.

The evaporation rate represents effect of all the combinations of climatic stimulants including humidity, wind, radiant energy that cause water to evaporate.  It is also a value that can be used to determine the transpiration rate of plants because the same climatic conditions influence plant transpiration.  When you irrigate your plants, you are trying to replace the moisture the plant lost through transpiration.

Experts have determined that the irrigation rate, expressed as value Kc for most plants will fall between 60 to 100 percent of the evaporation rate.  However, pineapple, a plant that has the capacity to efficiently retain moisture in their leaves, requires only a Kc value of 0.2 of evaporation rate.  Thus, in practicing water conservation and irrigation management, the objective is to irrigate your plants at Kc value.  Water waste will occur if irrigation application rate is greater than the plant transpiration rate, and applying less than transpiration rate, moisture deficient condition will occur in plants resulting in subpar production performance.

In areas on Molokai the annual evaporation rate is 118 inches per year and rainfall around 20 inches.  The deficiency between the two figures explains why vegetation is parched dry in these areas and will require billions of gallons of water from rainfall to keep it green all year round.

In 2012, Molokai Agriculture Working Group of the County of Maui Workforce Development Board (WFB) prioritized water conservation and irrigation management as an education and training need.   Last October, a workshop was held to better understanding irrigation water resource on Molokai and the Molokai Irrigation System.
To conclude the WFB education and training activity, a Field Day and Workshop titled “Water Conservation and Irrigation Management in Crop Production: Seeking Value Kc” will be held this Thursday, Oct. 24, from 1:30 to 3 p.m. at the Molokai Applied Research and Demonstration Farm located at UH-MCC Farm.  At the field day, you will have the opportunity to observe a field demonstration and learn about evaporation rates, Kc value, and effects of various cultural practices to conserve our scarce water resource.

If you have questions or plan to attend, please call the Molokai CTAHR-Cooperative Extension Service Office at 808-553-3315.