Solute and Nutrient Export and Redistribution During Summer Floods in the San Pedro River, Arizona Mixing model Anne K. Huth1, Martha H. Conklin1,2 and Roger C. Bales1,2 7/18/02 7/22/02 7/27/02 7/29/02 8/6/02 7/17/01 2002 2001 1University of Arizona, Tucson, AZ 2University of California, Merced, CA Introduction Flood nitrate 17 July 2001 (Wet winter) Hydrological and biogeochemical fluxes are influenced by climate and antecedent moisture conditions in semi-arid catchments. Widespread winter frontal precipitation and local episodic summer rains induce surface water flow. Summer monsoonal rainfall produces a large volume of surface water flow in a short period and causes significant nutrient redistribution in and among riparian stream compartments. A stream reach in the basin floor of a semi-arid watershed in southeastern Arizona was investigated during 2001 (a wet year; 25 cm winter rain) and 2002 (a dry year; 5 cm winter rain). 22 July 2002 (Dry winter) Cl- mixing model revealed higher soil water input in 2001 than 2002. Largest flood of 2002 season (27 July) had lowest soil water input due to large volume of dry soils that were flushed. Study Area N Arizona US Mexico Implications Wetter 2001 antecedent soil moisture resulted in larger summer floods, despite more monsoon precipitation in 2002. Flood nitrate concentrations in 2001 were lower than concentrations observed in 2002 floods. Before leafout (i.e. winter and early spring months), soil solutes and nutrients accumulate in riparian zone soils. This accumulation may continue through long term drought periods, in which antecedent soil moisture remains excessively low for several consecutive years. During extended drought periods in the southwestern US, a major shift in solute pool sizes in stream system compartments is expected to occur, and will have major implications for solute and nutrient accumulation, transport, and loss in semi-arid riparian ecosystems. Disturbance due to fire may also become more frequent in semi-arid and arid basins during extended drought periods, increasing solute and nutrient export in watersheds that experience significant vegetation removal and subsequent soil erosion.                                        Research Questions                                                                                                           Nitrate mass flux Cumulative precipitation 2001 2002 How do wet and dry antecedent conditions affect hydrologic flowpaths during summer floods? What are the linkages between hydrologic and nutrient pathways during episodic high flow events? Early season 2001 flood reveals a lower mass flux of nitrate moved compared with 2002 flood. Each of these early season floods occurred under similar antecedent moisture conditions from the start of the monsoon, for better comparison between years. Nitrogen Cycling in the Bank OM NH4+ NO3- N2 N20 A rapid rise in stage flushes out nitrate that accumulated during dry periods Streamflow WY 2002 WY 2001 Soil water nitrate 2001 2002 7/25/01 8/21/01 7/25/02 7/29/02 8/13/02 Higher 2002 soil water nitrate concentrations resulted from the gradual accumulation of soil solutes during the preceding dry winter and spring months. Nitrate reduction during 2002 due to physical transport, denitrification, and plant and microbial uptake. Acknowledgements Thanks to P. Brooks and N. Grimm for intellectual input on this project. Thanks also to P. Haas, J. Petti, and J. Villinski for help with field work. This material is based upon work supported by the Center for the Sustainability of semi-Arid Hydrology and Riparian Areas (SAHRA) under grant EAR-9876800 from the National Science Foundation (NSF), and by an NSF GK-12 grant DGE-9979670.