Sources and transport of organic carbon were studied in two large, Sonoran desert watersheds (Salt and Verde rivers) and a two-reservoir system on the Verde River. Total organic carbon (TOC) concentrations in the unregulated rivers above the reservoirs did not follow a simple relationship with flow. TOC concentrations usually declined during spring runoff and reached low concentrations (1-3 mg L-1) by early summer. In most years, distinct peaks (10-30 mg L-1) occurred in late summer, coincident with small but abrupt increases in flow associated with the first monsoon rains. A two-reservoir mass balance showed that 72% of the particulate organic carbon (POC) input was retained, probably due largely to simple sedimentation. Production of dissolved organic carbon (DOC) within the reservoir was 41% of the inflow loading, even though the reservoirs accounted for only 0.14% of the watershed area. Reservoir DOC production comprised a large fraction of total watershed production because upstream DOC production was extremely low (0.2 g C m-2 yr-1), the reservoirs were moderately productive, and water residence times were fairly long during most of the year. We postulate that reservoirs are major contributors to total watershed DOC production in arid regions.
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Acknowledgements--This project was supported by the Arizona Water Resources Research Center (P403983) and ASU's Faculty Grant-in-Aid Program (F-019-94). We would like to thank Leslie Farnsworth-Lee for her assistance with sampling and analysis; the City of Phoenix Water Services Department for running TOC and DOC analyses in the early part of the project; Gregg Elliott, Slavco Jovanovic, and Dallas Reigel from the Salt River Project for providing water budget data, maps, and insights; and Henry Sanger and Larry Young from the USGS for assistance in collecting samples from the upstream Verde River site.
- Dissolved organic carbon
- Natural organic matter
- Total organic carbon