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Iowa, Iowa, United States

Schilling K.E.,Iowa Geological and Water Survey | Zhang Y.-K.,University of Iowa | Zhang Y.-K.,Nanjing University
Ground Water | Year: 2012

Temporal scaling in stream discharge and hydraulic heads in riparian wells was evaluated to determine the feasibility of using spectral analysis to identify potential surface and groundwater interaction. In floodplains where groundwater levels respond rapidly to precipitation recharge, potential interaction is established if the hydraulic head (h) spectrum of riparian groundwater has a power spectral density similar to stream discharge (Q), exhibiting a characteristic breakpoint between high and low frequencies. At a field site in Walnut Creek watershed in central Iowa, spectral analysis of h in wells located 1 m from the channel edge showed a breakpoint in scaling very similar to the spectrum of Q (~20 h), whereas h in wells located 20 and 40 m from the channel showed temporal scaling from 1 to 10,000 h without a well-defined breakpoint. The spectral exponent (β) in the riparian zone decreased systematically from the channel into the floodplain as groundwater levels were increasingly dominated by white noise groundwater recharge. The scaling pattern of hydraulic head was not affected by land cover type, although the number of analyses was limited and site conditions were variable among sites. Spectral analysis would not replace quantitative tracer or modeling studies, but the method may provide a simple means of confirming potential interaction at some sites. © 2011, The Author(s). Ground Water © 2011, National Ground Water Association.

Schilling K.E.,Iowa Geological and Water Survey | Chan K.-S.,University of Iowa | Liu H.,University of Iowa | Zhang Y.-K.,University of Iowa
Journal of Hydrology | Year: 2010

There is convincing evidence that land use/land cover (LULC) change has contributed to increasing discharge in the Upper Mississippi River Basin (UMRB) but key details remain unresolved. In this study, we extend our previous work (Zhang and Schilling, 2006) to quantify how much of the increasing discharge was due to LULC change. We examined daily streamflow for the 1890-2003 period from the US Geological Survey stream gage at Keokuk, Iowa and compiled county agricultural statistics for soybean production in the watershed above the gage to quantify how much of the change in the relation of discharge to precipitation was due to increased soybean cultivation. By allowing the slope of the discharge-precipitation relationship to be a function of the area of the UMRB planted in soybean, we determined that increasing soybean acreage increased the slope of qt- Pt by 32%. With row crop expansion anticipated from ethanol production, increasing agricultural production is expected to result in increased water yield and nutrient export. Results provide important benchmarks for assessing the significance of LULC change on the regional water and climate patterns in the UMRB. © 2010 Elsevier B.V.

Stenback G.A.,Iowa State University | Crumpton W.G.,Iowa State University | Schilling K.E.,Iowa Geological and Water Survey | Helmers M.J.,Iowa State University
Journal of Hydrology | Year: 2011

Accurate estimation of nutrient loads in rivers and streams is critical for many applications including determination of sources of nutrient loads in watersheds, evaluating long-term trends in loads, and estimating loading to downstream waterbodies. Since in many cases nutrient concentrations are measured on a weekly or monthly frequency, there is a need to estimate concentration and loads during periods when no data is available. The objectives of this study were to: (i) document the performance of a multiple regression model to predict loads of nitrate and total phosphorus (TP) in Iowa rivers and streams; (ii) determine whether there is any systematic bias in the load prediction estimates for nitrate and TP; and (iii) evaluate streamflow and concentration factors that could affect the load prediction efficiency. A commonly cited rating curve regression is utilized to estimate riverine nitrate and TP loads for rivers in Iowa with watershed areas ranging from 17.4 to over 34,600km 2. Forty-nine nitrate and 44 TP datasets each comprising 5-22years of approximately weekly to monthly concentrations were examined. Three nitrate data sets had sample collection frequencies averaging about three samples per week. The accuracy and precision of annual and long term riverine load prediction was assessed by direct comparison of rating curve load predictions with observed daily loads. Significant positive bias of annual and long term nitrate loads was detected. Long term rating curve nitrate load predictions exceeded observed loads by 25% or more at 33% of the 49 measurement sites. No bias was found for TP load prediction although 15% of the 44 cases either underestimated or overestimate observed long-term loads by more than 25%. The rating curve was found to poorly characterize nitrate and phosphorus variation in some rivers. © 2010 .

Jones C.S.,Iowa Soybean Association | Schilling K.E.,Iowa Geological and Water Survey
Journal of Environmental Quality | Year: 2011

Fluvial sediment is a ubiquitous pollutant that negatively aff ects surface water quality and municipal water supply treatment. As part of its routine water supply monitoring, the Des Moines Water Works (DMWW) has been measuring turbidity daily in the Raccoon River since 1916. For this study, we calibrated daily turbidity readings to modern total suspended solid (TSS) concentrations to develop an estimation of daily sediment concentrations in the river from 1916 to 2009. Our objectives were to evaluate longterm TSS patterns and trends, and relate these to changes in climate, land use, and agricultural practices that occurred during the 93-yr monitoring period. Results showed that while TSS concentrations and estimated sediment loads varied greatly from year to year, TSS concentrations were much greater in the early 20th century despite drier conditions and less discharge, and declined throughout the century. Against a backdrop of increasing discharge in the Raccoon River and widespread agricultural adaptations by farmers, sediment loads increased and peaked in the early 1970s, and then have slowly declined or remained steady throughout the 1980s to present. With annual sediment load concentrated during extreme events in the spring and early summer, continued sediment reductions in the Raccoon River watershed should be focused on conservation practices to reduce rainfall impacts and sediment mobilization. Overall, results from this study suggest that eff orts to reduce sediment load from the watershed appear to be working. © 2011 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.

Schilling K.E.,Iowa Geological and Water Survey | Jindal P.,Iowa State University | Basu N.B.,University of Iowa | Helmers M.J.,Iowa State University
Hydrological Processes | Year: 2012

Pollutant delivery through artificial subsurface drainage networks to streams is an important transport mechanism, yet the impact of drainage tiles on groundwater hydrology at the watershed scale has not been well documented. In this study, we developed a two-dimensional, steady-state groundwater flow model for a representative Iowa agricultural watershed to simulate the impact of tile drainage density and incision depth on groundwater travel times and proportion of baseflow contributed by tile drains. Varying tile drainage density from 0 to 0.0038m -1, while maintaining a constant tile incision depth at 1.2m, resulted in the mean groundwater travel time to decrease exponentially from 40years to 19years and increased the tile contribution to baseflow from 0% to an upper bound of 37%. In contrast, varying tile depths from 0.3 to 2.7m, while maintaining a constant tile drainage density of 0.0038m -1, caused mean travel times to decrease linearly from 22 to 18years and increased the tile contribution to baseflow from 30% to 54% in a near-linear manner. The decrease in the mean travel time was attributed to decrease in the saturated thickness of the aquifer with increasing drainage density and incision depth. Study results indicate that tile drainage affects fundamental watershed characteristics and should be taken into consideration when evaluating water and nitrate export from agricultural regions. © 2011 John Wiley & Sons, Ltd.

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