U of I researcher finds overestimation in potato farmers’ water consumption valuations

Idaho potato fields consume far less water during peak irrigation season than models available to farmers and water managers would suggest, according to a University of Idaho scientist’s recent findings.

Meetpal Kukal, an assistant professor of hydrologic science and water management in the Department of Soil and Water Systems, has finished the first of several years of field monitoring intended to improve the accuracy of consumptive water use estimates that are important to agriculture. Water deemed to be consumed is removed from the system, either by being stored in plant tissue or being emitted as vapor from soil or foliage, through a process known as evapotranspiration (ET).

In a report he presented to farmers attending the 58th annual Idaho Potato Conference, hosted Jan. 21-22 in Pocatello, Kukal’s observations from a southwest Idaho field show that current models used by Idahoans are overestimating water consumption in potato fields during peak irrigation season by as much as 40%.

“ET on irrigated crop land is the largest outflux in the system, but ET is also the most uncertain one. Even a little bit of uncertainty in ET results in a lot of unaccounted water,” Kukal said. “We are severely overestimating water needs in the peak season, and that’s the time when our irrigation systems are the most challenged.”

Early in the irrigation season, however, models currently used by water managers appear to be slightly underestimating water consumption in potato fields. Regional irrigators have told Kukal his observations mirror their own experiences.

Kukal studies ET using a network of monitoring stations, called eddy covariance towers, located in fields from Wilder through Declo. He drew potato data from a Clearwater Russet field in Wilder. One of Kukal’s graduate students intends to make additional calculations specific to other crops.

Idaho law administers water rights to users based on the volume of water they divert into canals or pump from wells, granting priority to older water rights. However, much of that water seeps back into the aquifer, where it can be reused downstream. Water managers may track consumptive use in addition to diversions for a fuller picture of how much water is available in the system at any given time.

The Idaho Department of Water Resources (IDWR) predicts how various scenarios might affect the water supply, and it calculates water consumption based on crop coefficients — or ET estimates specific to each crop. These crop coefficients were established in the 1970s using lysimeters, which weigh soil to measure changes in moisture levels. But those coefficients don’t represent differences in potato varieties, soil type and crop management, and it is not well understood how these factors may influence consumption.

Kukal’s approach indirectly calculates ET by tracking radiant and wind energy in the field and determining the precise volume of water that energy would evaporate from plant tissue and soil.

“To evaporate one gram of water, we know exactly how much energy is being used,” Kukal said.

Sensors in the eddy covariance towers measure water vapor concentrations and wind speeds in three directions 10 times per second.

“The crop coefficients have their place,” Kukal said. “They’re a useful resource that we have had historically and will continue to serve us, but it’s important to start revisiting and finetuning those to include newer crop varieties and cultivars, and the best technique we have available to us today is the eddy covariance measurements.”

Placing towers in every field would be impractical. Rather, Kukal hopes to use data from his network of monitoring stations to refine and calibrate ET estimates from satellites operated by NASA’s Landsat program. Landsat satellites pass by a given location every eight days and record the temperature and crop canopy progression. Kukal also envisions using his monitoring-station data to create updated crop coefficients covering many more production and management scenarios.

Kukal plans to continue collecting ET data from eddy covariance stations for several more years. He anticipates resulting modeling improvements will help IDWR with management decisions and farmers with better irrigation scheduling.

“These measurements need to be long term, because there are so many combinations of all of these factors that ideally you need to keep improving our understanding of ET,” Kukal said.