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Storage and release functions of western U.S. traditional river valley irrigation systems may counteract early and rapid spring river runoff associated with climate variation. Along the Rio Grande in northern New Mexico, we instrumented a 20-km-long irrigated valley to measure water balance components from 2005 to 2007. Hydrologic processes of the system were incorporated into a system dynamics model to test scenarios of changed water use. Of river water diverted into an earthen irrigation canal system, some was consumed by crop evapotranspiration (7.4%), the rest returned to the fiver as surface return flow (59.3%) and shallow groundwater return flow that originated as seepage from canals (12.1%) and fields (21.2%). The modeled simulations showed that the coupled surface water irrigation system and shallow aquifer act together to store water underground and then release it to the river, effectively retransmitting river flow until later in the year. Water use conversion to nonirrigation purposes and reduced seepage from canals and fields will likely result in higher spring runoff and lower fall and winter river flow. DOI: 10.1061/(ASCE)IR.1943-4774.0000265 CE Database subject headings: Aquifers; Hydrologic models; Hydrogeology; Irrigation systems; Surface water; Groundwater; Groundwater recharge; Rio Grande. Author keywords: River-aquifer interaction; Hydrology; Hydrologic models; Hydrogeology; Irrigation systems; Surface water; Groundwater; Groundwater recharge; Rio Grande.

Recent amendments to the Safe Drinking Water Act emphasize efforts toward safeguarding our nation's water supplies against attack and contamination. Specifically, the Public Health Security and Bioterrorism Preparedness and Response Act of 2002 established requirements for each community water system serving more than 3,300 people to conduct an assessment of the vulnerability of its system to a terrorist attack or other intentional acts. Integral to evaluating system vulnerability is the threat assessment, which in the process by which the credibility of a threat is quantified. Unfortunately, full probabilistic assessment in generally not feasible, since there in insufficient experience and/or data to quantify the associated probabilities. For this reason, an alternative approach is proposed predicated on Marker latent effects (MLE) modeling, which provides a framework for quantifying imprecise subjective metrics through possibilistic or fuzzy mathematics. Here, the MLE approach is introduced and demonstrated within the context of water supply system threat assessment. DOI: 10.1061/(ASCE)0733-9496(2005)131:3(218) CE Database subject headings: Risk management: Risk analysis: Water distribution: Water quality: Potable water: Accidents.