Watershed water balance changes as furrow irrigation is converted to sprinkler irrigation in an arid region

Authors: Bjorneberg, David - Dave; King, Bradley - Brad; Koehn, Anita

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2019
Publication Date: 5/12/2020
Citation: Bjorneberg, D.L., King, B.A., Koehn, A.C. 2020. Watershed water balance changes as furrow irrigation is converted to sprinkler irrigation in an arid region. Journal of Soil and Water Conservation. 75(3):254-262. https://doi.org/10.2489/jswc.75.3.254.
DOI: https://doi.org/10.2489/jswc.75.3.254

Interpretive Summary: Irrigation is the largest water use in the western U.S. The Upper Snake Rock Conservation Effects Assessment Project in southern Idaho began in 2005 to measure the impacts of conservation practices that have been implemented in this irrigated watershed. This study documents the changes in watershed-scale water balances from 2006 to 2016. During this time period, farmers converted furrow irrigated fields to sprinkler irrigation so the amount of sprinkler irrigation increased from 46% of the cropland to 59%. More than 75% of annual watershed inflow was irrigation water diverted into the watershed from the Snake River and distributed through canals to 82,000 ha of cropland, while annual precipitation was only 10 to 23% of the annual inflow. Approximately 30% of the annual watershed inflow flowed back to the Snake River as irrigation return flow. Water balances indicate that irrigation exceeded evapotranspiration in the spring and fall. Irrigation scheduling based on daily ET or soil moisture measurement would help irrigation application match crop water needs. Annual irrigation project efficiency, defined as evapotranspiration divided by the amount of diverted irrigation water, varied from 61 to 73%, but project efficiency did not increase as the amount of sprinkler irrigation increased. The only significant trends indicating that increasing sprinkler irrigation impacted the water balances were increasing irrigation project efficiency in July and increasing irrigation return flow during the irrigation season. Farmers may be applying less Irrigation water with sprinkler irrigation compared to furrow irrigation, which could have caused return flow to increase since irrigation diversion did not change. The irrigation project was designed with a supply-based allocation scheme to uniformly distribute the natural flow water right from the Snake River to farms. Therefore, irrigation diversion depended more on the available water supply than crop irrigation requirements. This study demonstrated that water rights and water policy can have a bigger impact on irrigation project efficiency than practices implemented on farms.

Technical Abstract: Irrigation is the largest water use in the western U.S. The Upper Snake Rock Conservation Effects Assessment Project in southern Idaho began in 2005 to quantify the impacts of conservation practices in this irrigated watershed. The objective of this study was to determine the changes in the watershed water balances as farmers converted furrow irrigated fields to sprinkler irrigation from 2006 to 2016. More than 75% of annual watershed inflow was irrigation water diverted into the watershed from the Snake River and distributed through canals to 82,000 ha of cropland, while annual precipitation was only 10 to 23% of the annual inflow. Approximately 30% of the annual watershed inflow flowed back to the Snake River as irrigation return flow. Water balances indicate that irrigation exceeded evapotranspiration in the spring and fall. Irrigation scheduling based on daily ET or soil moisture measurement would help irrigation application match crop water needs. Annual irrigation project efficiency, defined as evapotranspiration divided by the amount of diverted irrigation water, varied from 61 to 73%, but project efficiency did not increase as the amount of sprinkler irrigation increased from 46% in 2006 to 59% in 2016. The only significant trends indicating that increasing sprinkler irrigation impacted the water balances were increasing irrigation project efficiency in July and increasing irrigation return flow during the irrigation season. Farmers may be applying less Irrigation water with sprinkler irrigation compared to furrow irrigation, which could have caused return flow to increase since irrigation diversion did not change. The irrigation project was designed with a supply-based allocation scheme to uniformly distribute the natural flow water right from the Snake River to farms. Therefore, irrigation diversion depended more on the available water supply than crop irrigation requirements. This study demonstrated that water rights and water policy can have a bigger impact on irrigation project efficiency than practices implemented on farms.