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Spatial and temporal dynamics of coupled groundwater and nitrogen fluxes through a streambed in an agricultural watershed
Author(s) -
Kennedy Casey D.,
Genereux David P.,
Corbett D. Reide,
Mitasova Helena
Publication year - 2009
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2008wr007397
Subject(s) - watershed , spatial variability , flux (metallurgy) , hydrology (agriculture) , environmental science , groundwater , hydraulic conductivity , nitrogen , spatial distribution , streams , soil science , soil water , geography , geology , chemistry , mathematics , remote sensing , computer network , statistics , geotechnical engineering , organic chemistry , machine learning , computer science
This paper presents results on the spatiotemporal dynamics of the coupled water flux ( v ) and nitrogen fluxes ( f N = v [N], where [N] is the concentration of a dissolved N species) through a streambed in an agricultural watershed in North Carolina. Physical and chemical variables were measured at numerous points in the streambed of a 0.26‐km reach: hydraulic conductivity ( K ) and head gradient ( J ) and the concentrations of NO 3 − and other N species in streambed groundwater, from which water flux ( v = KJ ) and N fluxes (e.g., f NO3 = v [NO 3 − ]) through the streambed were computed, mapped, and integrated in space. The result was a novel set of streambed maps of the linked variables ( K , J , v , and N concentrations and fluxes), showing their spatial variability and how it changed over a year (on the basis of seven bimonthly sets of maps). Mean f NO3 during the study year was 154 mmol m −2 d −1 ; this NO 3 − flux, together with that of dissolved organic nitrogen ( f DON = 17 mmol m −2 d −1 ), accounted for >99% of the total dissolved N flux through the streambed. Repeat measurements at the same locations on the streambed show significant temporal variability in f NO3 , controlled largely by changes in v rather than changes in [NO 3 − ]. One of the clearest and most persistent aspects of spatial variability was lateral variability across the channel from bank to bank. K and v values were greater in the center of the channel; this distribution of K (ultimately a reflection of sediment dynamics in the channel) apparently focuses groundwater discharge toward the center of the channel. The opposite pattern (low values in the center) was found for J , [NO 3 − ], and (to a lesser extent) f NO3 . Overall, f NO3 was characterized by localized zones of high and low values that changed in size and shape over time but remained in basically the same locations (the same was true of K , J , and [NO 3 − ], though less so for v ), with 70% of NO 3 − flux occurring through about 38% of the streambed area. Lateral distributions of the physical hydrologic attributes ( K , J , and v ) were highly symmetrical across the channel, while those of [NO 3 − ] and f NO3 showed higher values on the left than on the right, likely a reflection of different N use on opposite sides of the stream. The streambed‐based approach taken here offers insights concerning the spatial and temporal dynamics of linked water and N fluxes through a streambed and their controls.