Washington State University

Irrigation Engineering

Troy Peters and students in hop yard

Dr. Peters with student research assistants Prossie Nakawuka and Romulus Okwany in project hop yard.

Establishing Abiotic Stress

Water is a crucial and often expensive input to any successful agricultural crop. In times of water shortage, whether caused by drought, climate change, politics, or economic barriers, the impacts on productivity can be severe. Yet some crops (wine grapes being a well-known example) benefit from judicious water deprivation. One objective of our project was to determine the impacts of varying water levels on the quality and quantity of the subject crops, hops and mint, as well as the impacts on the pest (disease, weed, arthropod) complex. The cornerstone to this project was the development of the irrigations systems that would create these water-shortage conditions.

The Science of Drought Simulation

Dr. R. Troy Peters, Irrigation Engineer with the WSU Department of Biosystems Engineering and co-PD on this project, designed custom irrigation systems to limit water and simulate drought by applying water at measurable rates varying from a minimal amount to 100% of standard production practices. Once the systems were deployed in the hop yards and mint fields, measurements were taken to assess the amount of irrigation and evaluate the efficacy of the system. In-field catch cans were utilized to measure the efficacy of the sprinkler irrigation systems while in-soil probes were utilized for drip systems. Our data verified that the intended amounts of water, ranging from full irrigation to approximately 15% of full irrigation, were being delivered.

Mint Visibly Taller Adjacent to Irrigation Source

Mid-July mint regrowth following first cutting. Mint is visibly greener at the higher irrigation levels near the line source, becoming increasingly brown at greater distances/ lower irrigation levels.

Step by Step,
Drop by Drop

In the first year of the project, Dr. Peters' team completed the critical establishment task of constructing deficit irrigation systems in both mint and hops. In the first production year, 2010, the mint system was sprinkler-based. At the conclusion of the season, consultation with industry determined that this preliminary system was not optimal, so a drip system was developed for the 2011 growing season. Drip irrigation was established in hops by renovating an older system, installing a system to control irrigation levels, and installing probes for moisture measurement. In the 2011 production season, blocked and replicated treatments were conducted in both crops under drip irrigation. These fields have been irrigated at varying amounts and the yields and quality effects are being tested.

As of August 2011, one harvest has been made from a drip-irrigated mint plot. The results look promising and the data are under analysis. Harvest from the first hop yard under our deficit irrigation regimen is forthcoming in September 2011.

Initial Results

Data from our first season indicated that deficit irrigation decreased oil and hay yields in peppermint, but increased the harvest index (mint oil concentration) in native spearmint. 


March 2011 Article in Irrigation Science: Impact of sustained deficit irrigation on spearmint (Mentha spicata L.) biomass production, oil yield, and oil quality

Washington State IPM Coordinator, WSU Irrigated Agriculture Research & Extension Center, Prosser WA 99350, 509-786-9287