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Rainfall-runoff dynamics and nitrogen processing in an arid-land watershed: where does nitrogen retention occur?
WELTER, JILL1, FISHER, STUART1, GRIMM, NANCY1, 1
ABSTRACT- In the Sycamore Creek watershed in Arizona, only ten percent of annual atmospheric nitrogen inputs to the watershed are exported in streamflow. In this arid, nitrogen-limited system, where is the missing nitrogen, and which components of the watershed (terrestrial or aquatic) are responsible for its retention? Storm size and intensity are extremely variable in the Southwestern USA and may influence where and when nitrogen retention occurs. For individual storms, we monitored rainfall and runoff chemistry and volume, as well as the spatial extent of surface runoff. Runoff was collected from upland slopes and within the network of intermittent rills and channels that hydrologically link the terrestrial landscape with perennial streams during storms. Results show that small storms (<0.5 cm) wet the desert uplands and generated overland flow, but did not hydrologically connect the terrestrial landscape with streams. As storm size increased, the extent of flow in the channel network increased; however, only the largest storms generated flow in high-order channels. During the 2000 monsoon season, only one storm (2.9 cm) resulted in flooding in Sycamore Creek (the only perennial stream in the catchment). Results suggest that storms interact with the landscape, generating "hot spots" for nitrogen retention in relation to storm size and intensity. Most of the time retention of atmospheric nitrogen is confined to terrestrial uplands. Only rare large storms transport nitrogen to perennial streams.
KEY WORDS: aquatic-terrestrial linkages, nitrogen retention, arid-land watersheds, nitrogen cycling