Plants Transpire Most of the Water They Use!
Editor’s note: California Ag Today interviewed Allan Fulton, an Irrigation and Water Resources Advisor, UC Cooperative Extension Tehama County, in Redbluff CA, to comment on the debate about the agricultural industry’s use of water and to focus on a critical but disregarded process—that all plants transpire, even plants cultivated for the crops we eat.
CalAgToday: We hear in the media that our crops are using too much water. And while all plants need water to grow food, we also know that a high percentage of water taken up by all plants actually transpires back into the atmosphere, to form clouds and precipitation, right?
Fulton: Yes, when plants transpire, the water just returns to the local hydrologic cycle, leaving the harvested crop that we distribute elsewhere in the US or in the world actually very low in water content.
CalAgToday: When we think about transpiration, are the plants actually “borrowing” the water?
Fulton: Yes. We get a lot of questions about why we irrigate our crops so much, and it comes from the general public not being as close to farming every day. The truth is, plant transpiration is a necessary biological process. The water cools the tree so it stays healthy and exits the leaves through special cells called stomata. While the stomates are open to allow water to transpire, carbon dioxide enters and is used in photosynthesis, making sugars and carbohydrates for the plant to create the fruits and nuts that we eat. So, an inadequately watered plant cannot take in enough carbon dioxide during transpiration, resulting in defective fruits and nuts that are smaller, shriveled, cracked—all the things the typical consumer does not want to buy.
Plants cannot gain carbon dioxide without simultaneously losing water vapor.
CalAgToday: Can we say 95 or 99% of the water that is taken up by the plant gets transpired and definitely not wasted?
Fulton: Definitely. We converted to pressurized irrigation systems, micro-sprinklers, and mini sprinklers, so we have a lot more control over how much water we apply at any one time. We do not put water out in acre-feet or depths of 4-6 inches at a time anymore. So, much like when rainfall occurs, we can measure it in tenths, or 1 or 2 inches at most. As a result, the water doesn’t penetrate the soil very deeply, maybe only 1 or 2 feet each irrigation.
We are very efficient with the water, but because we deliver it in small doses, we have to irrigate very frequently. That is why we see irrigation systems running a lot, but they are systems that efficiently stretch our water supply and do not waste it.
CalAgToday: But again, the vast majority of the water that the tree is taking up is being transpired, right?
Fulton: Yes, most of the time, at least 90% of the water that we apply is taken up through the tree and transpired so that photosynthesis can happen.
CalAgToday: And transpiration increases on a hot day?
Fulton: Yes, we do get a little bit of loss from surface evaporation from wet soil, but we try to control that with smaller wetting patterns—drip-confined wetting patterns. When you think about it, the heat of the day is in the afternoon when many irrigation systems don’t run because of higher energy costs. There are incentives not to pump in the middle of the afternoon, but those who do try to confine the wetted area to limit evaporation. And the hot hours of the day make up about 4 hours of a 24-hour cycle, so we irrigate mostly during the night time and early morning hours to lesson evaporative loss.
CalAgToday: Growers are doing everything they can to conserve water. If the trees and vines are all transpiring most of their irrigated water, why is using water to grow food a problem?
Fulton: I think the emphasis throughout the United States has always been to provide a secure food supply. That security has many benefits, economically and politically; and in the end, we are trying to provide the general public with good quality, safe food at the best price possible.
 Debbie Swarthout and C.Michael Hogan. 2010. Stomata. Encyclopedia of Earth. National Council for Science and the Environment, Washington DC.
The California Irrigation Management Information System (CIMIS) is a program unit in the Water Use and Efficiency Branch, Division of Statewide Integrated Water Management, California Department of Water Resources (DWR) that manages a network of over 145 automated weather stations in California. CIMIS was developed in 1982 by DWR and the University of California, Davis (UC Davis). It was designed to assist irrigators in managing their water resources more efficiently. Efficient use of water resources benefits Californians by saving water, energy, and money.
The CIMIS user base has expanded over the years. Currently, there are over 40,000 registered CIMIS data users, including landscapers, local water agencies, firefighters, air control board, pest control managers, university researchers, school teachers, students, construction engineers, consultants, hydrologists, government agencies, utilities, lawyers, weather agencies, and many more.