Specialty crop retention reservoir performance and design considerations to secure quality water and mitigate non-point source runoff

Abstract Specialty crop production requires large volumes of water for irrigation and faces four interconnected water resource concerns: (1) water availability and security, (2) need for high water quality (3) management of irrigation return flow (IRF) and operational water (OW) quality from production areas, and (4) increasing consumer and regulatory pressure to mitigate non-point source runoff. Retention reservoirs (RRs) are an effective best management practice for extending water resources and reducing an adverse effect on the environment. RRs are recommended for specialty crop producers to (1) capture stormwater and OW to conserve increasingly limited and costly water resources, (2) reduce reliance on surface- and ground-water, and (3) mitigate contaminants in OW before discharge into receiving waters. Despite the importance of RRs for water security, concerns remain about their capacity to mitigate contaminants prior to use of captured IRF and stormwater. Specialty crop producers perceive potential risks of reapplying contaminants to economically important crops. Consolidated information is needed on the risks associated with IRF and stormwater reuse, design of RRs for adequate storage for capturing both IRF and stormwater, and the effects of RRs on operational and discharged water quality. The main goals of this critical assessment were to (1) understand the effect of RRs on contaminant mitigation; (2) collate information about quality of water in RRs reused for irrigation; and (3) develop guidelines for designing RRs used in specialty crop production. Proper design or augmentation of new and existing water RRs can enable specialty crop producers to collect, reuse, or discharge freshwater that meets quality metrics. Three RR design methods were compared. Current RRs design recommendations to capture a 24-h rainfall with 25-year frequency may be inadequate to capture OW or a portion of rainfall for many parts of the USA. Modifying RR design specifications to capture a 24-h rainfall with 50-year frequency will increase operational climate resilience, water security, and reduce non-point source runoff. Additionally, use of a forebay, increasing depth, long flow path (high length/width ratio), long hydraulic retention time, and aeration or mixing are discussed to optimize water treatment within RRs.