HR: 1340h
AN: H43C-1512    [Abstracts]
TI: Two modeling approaches for quantifying hydrologic and biologic controls on large-scale nitrogen cycling, Upper Rio Grande, NM
AU: * Oelsner, G P
EM: goelsner@hwr.arizona.edu
AF: Dept. Hydrology and Water Resources, University of Arizona, P.O. Box 210158-B, Tucson, AZ 85721-0158, United States
AU: * Oelsner, G P
EM: goelsner@hwr.arizona.edu
AF: SAHRA, University of Arizona P.O. Box 210158-B, Tucson, AZ 85721-0158, United States
AU: Brooks, P D
EM: brooks@hwr.arizona.edu
AF: Dept. Hydrology and Water Resources, University of Arizona, Harshbarger Bldg., 1133 E. James E. Rogers Way, Tucson, AZ 85721, United States
AU: Brooks, P D
EM: brooks@hwr.arizona.edu
AF: SAHRA, University of Arizona P.O. Box 210158-B, Tucson, AZ 85721-0158, United States
AU: Hogan, J F
EM: jhogan@hwr.arizona.edu
AF: Dept. Hydrology and Water Resources, University of Arizona, P.O. Box 210158-B, Tucson, AZ 85721-0158, United States
AU: Hogan, J F
EM: jhogan@hwr.arizona.edu
AF: SAHRA, University of Arizona P.O. Box 210158-B, Tucson, AZ 85721-0158, United States
AU: Meixner, T
EM: tmeixner@hwr.arizona.edu
AF: Dept. Hydrology and Water Resources, University of Arizona, Harshbarger Bldg., 1133 E. James E. Rogers Way, Tucson, AZ 85721, United States
AU: Meixner, T
EM: tmeixner@hwr.arizona.edu
AF: SAHRA, University of Arizona P.O. Box 210158-B, Tucson, AZ 85721-0158, United States
AU: Tidwell, V
EM: vctidwe@sandia.gov
AF: Sandia National Laboratories, PO Box 5800, MS 0735, Albuquerque, NM 87185-0735, United States
AU: Tidwell, V
EM: vctidwe@sandia.gov
AF: SAHRA, University of Arizona P.O. Box 210158-B, Tucson, AZ 85721-0158, United States
AU: Roach, J D
EM: jdroach@sandia.gov
AF: Sandia National Laboratories, PO Box 5800, MS 0735, Albuquerque, NM 87185-0735, United States
AU: Roach, J D
EM: jdroach@sandia.gov
AF: SAHRA, University of Arizona P.O. Box 210158-B, Tucson, AZ 85721-0158, United States
AB: Variations in nutrient concentrations can be caused by both abiotic changes in hydrology and biotic processes. Most process-level studies of nutrient cycling are conducted in small catchment systems and at points on large river systems. Relatively less understanding has been developed on how biotic and abiotic processes influence large-scale nutrient concentrations and variability in large river systems. To address this issue, we performed biannual synoptic chemical sampling along a 640 km reach of the Upper Rio Grande for five years to determine the large-scale patterns in dissolved carbon and nitrogen concentrations and then used two different and simple models to evaluate the abiotic and biotic processes that generate the observed large-scale patterns. First, we used a Cl mixing model, validated with Br to quantify the effects of evapoconcentration, tributaries, and point sources on dissolved nitrogen and carbon concentrations. Ratios of observed to predicted concentrations close to 1 suggest that abiotic hydrologic processes are the dominant controls on concentrations while ratios departing from 1 indicate that biological processes are important controls. Our conservative mixing model generally captured patterns in DOC concentrations, suggesting minimal, net biological processing. In contrast, both nitrate and TDN concentrations were altered biogeochemically in all reaches. In areas where observed and predicted values differed, the spatial variability of river characteristics was more strongly correlated to relative nutrient retention than seasonal or inter-annual discharge variability. Second, we used an integrated surface water – groundwater dynamic simulation model to evaluate the agricultural conveyance and riparian systems as potential nitrogen removal locations. Under conservative behavior, modeled nitrate concentrations were higher than observed in the groundwater, river, and conveyance channels. We calibrated the model using denitrification in the groundwater and uptake by riparian vegetation and crops to match modeled to observed concentrations. Neither uptake of nitrate by riparian vegetation and crops or denitrification alone reduced nitrate sufficiently. However, a combination of 10% denitrification in the groundwater, riparian uptake equal to 90% of ET and crop uptake equal to 50% of ET resulted in nitrate concentrations that generally matched the magnitude and seasonal variations of observed nitrate concentrations.
DE: 0402 Agricultural systems
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 0483 Riparian systems (0744, 1856)
DE: 1847 Modeling
DE: 1879 Watershed
SC: Hydrology [H]
MN: 2007 Fall Meeting