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Nitrate in Tile Drainage of the Semiarid Palouse Basin

^a School of Earth and Environmental Sciences, Washington State Univ., Pullman, WA 99164 (C.N. Butcher, currently: GeoInsight, Inc., 5 Lan Dr., Suite 200, Westford, MA 01605)
^b USDA-ARS, Dep. of Crop and Soil Sciences, Washington State Univ., Pullman, WA 99164
^c Dep. of Geology, Univ. at Buffalo, SUNY, 876 Natural Sciences Complex, Buffalo, NY 14260

View larger version (49K): [in this window] [in a new window]   Fig. 1. Topographic map (after Goodwin, 2006) of the Cook Agronomy Farm (CAF), showing the tile-drained study catchment, approximately 12 ha in area. Elevations are masl, contour interval is 2.5 m. NE–SW–running crop–rotation sections are delineated by light double lines. Drain line position is approximate. Bulls-eye symbol indicates location of drain outlet with nearby time domain reflectometry and soil water sampling instrumentation, installed via a backhoed trench (see text).

View larger version (50K): [in this window] [in a new window]   Fig. 2. Watershed and study site data for the 4-yr study period. (a) Event and cumulative daily precipitation by water year (October–September). Vertical arrows indicate time of cumulation of 150 mm of fall precipitation (pptn). Co-incident onset of high-flow season is evident in the discharge hydrograph for a 660-ha catchment (grayscale) containing the study site. (b) Tile drain (TD) discharge rate and 48-h mean soil volumetric water content at selected depths; (c) Tile drain nitrate concentration and nitrate-corrected electrical conductivity (EC), with solid vertical bars showing times of 150-mm precipitation accumulation from panel a. Subsequent vertical bars denote passage of 30 days time.

View larger version (26K): [in this window] [in a new window]   Fig. 3. Soil–water nitrate concentrations for the three sampling depth intervals over the course of the study. Values are means for all samples obtained at a particular depth and sample collection date. The tile drain (TD) discharge rate and nitrate concentration (Fig. 2b and 2c) are shown (grayscale) for context. Solid circles indicate magnitude (in kg N ha–1) and timing of N fertilizer applications in the cropping strip nearest Whelan Road, which coincided with the entire westernmost section (Fig. 1) in 2001–2002 and 2002–2003.

View larger version (28K): [in this window] [in a new window]   Fig. 4. Nitrate-corrected electrical conductivity (EC) in soil water vs. depth. Symbols show average concentrations for all samples obtained at a particular depth and sample collection date, within the time interval noted.

View larger version (22K): [in this window] [in a new window]   Fig. 5. Results of soil nitrate extractions on soil samples collected beneath the tile drained area before fertilization in Fall 2002 (D.R. Huggins, unpublished data, 2002). Open bar for each depth interval represents 125 to 127 samples; error bars denote 1 SD. Shaded bar shows corresponding estimated soil water concentration that would result from dissolution of the observed soil nitrate in otherwise nitrate-free water at saturated soil water content (see text).

View larger version (19K): [in this window] [in a new window]   Fig. 6. Dynamic relations among soil N processes and hydrology lead to highly variable seasonal tile drain (TD) nitrate concentrations, as delineated here in expanded form for 1 yr (data excerpted from Fig. 2). Phase 1: base flow recovers after crop senescence. Phase 2: full saturation develops, setting the stage for concurrent sharp tile-drain flow and nitrate increases, as pore water nitrate is mobilized throughout the profile. Phase 3: tile-drain nitrate concentrations and flow decline in response to plant uptake of water and nutrients in late spring and summer. EC = nitrate-corrected electrical conductivity; TDR = time domain reflectometer.

View larger version (14K): [in this window] [in a new window]   Fig. 7. Stream nitrate concentration time series for a monitoring station that includes drainage from the study site (open triangles), plotted with that station's hydrographic data (grayscale, from Fig. 2a) and tile-drain (TD) nitrate concentration (diamonds, from Fig. 2c). Note strong similarities both within and among seasons. Catchment areas differ by approximately 50 times.