Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and nutrient budgets.

Floodplain storage commonly represents one of the largest sediment fluxes within sediment budgets. In watersheds responding to large scale disturbance, floodplain-channel lateral connectivity may change over time with progression of channel evolution and associated changes in channel geometry. In this study we investigated the effects of channel geometry change on floodplain inundation frequency and flux of suspended sediment (SS) and total phosphorus (TP) to floodplain storage within the 52.2 km2 Walnut Creek watershed (Iowa, USA) through a combination of 25 in-field channel cross section transects, hydraulic modeling (HEC-RAS), and stream gauging station-derived water quality and quantity data. Cross sectional area of the 25 in-field channel cross sections increased by a mean of 17% over the 16 year study period (1998-2014), and field data indicate a general trend of degradation and widening to be present along Walnut Creek's main stem. Estimated stream discharge required to generate lateral overbank flow increased 15%, and floodplain inundation volume decreased by 37% over study duration. Estimated annual fluxes of SS and TP to floodplain storage decreased by 61 and 62% over study duration, respectively. The estimated reductions in flux to floodplain storage have potential to increase watershed export of SS and TP by 9 and 18%, respectively. Increased contributions to SS and TP export may continue as channel evolution progresses and floodplain storage opportunities continue to decline. In addition to loss of storage, higher discharges confined to the channel may have greater stream power, resulting in further enhancement of SS and TP export through accelerated bed and bank erosion. These increased contributions to watershed loads may mask SS and TP reductions achieved through edge of field practices, thus making it critical that stage and progression of channel evolution be taken into consideration when addressing sediment and phosphorus loading at the watershed scale.

Livestock manure driving stream nitrate.

Growth and consolidation in the livestock industry in the past 30 years have resulted in more total farm animals being raised on fewer Iowa farms. The effects of this on stream water quality at the landscape scale have largely gone unexplored. The main objective of this work was to quantify the effects on stream nitrate levels of livestock concentration in two western Iowa watersheds relative to seven other nearby watersheds. To achieve this objective, we used data on high-frequency nitrate concentration and stream discharge, commercial nitrogen fertilizer use, and manure-generated nitrogen in each watershed. Our analysis shows much higher stream nitrate in the two watersheds where livestock concentration has been greatest, and little difference in commercial fertilizer inputs with the widespread availability of manure N. Reducing N inputs and better management of manure N, including analysis of crop N availability in soil and manure, can reduce uncertainty regarding fertilization while improving water quality.

Agricultural conservation practices in Iowa watersheds: comparing actual implementation with practice potential.

As part of the solution to reduce the size of the Gulf of Mexico hypoxia, the state of Iowa has created the Iowa Nutrient Reduction Strategy (INRS) to reduce total nitrogen and phosphorous loads by 45% by 2035. A major component of the strategy is implementation of conservation practices to reduce loads of non-point source pollution from agricultural lands. To identify potential locations for conservation practices in Iowa watersheds, the Agricultural Conservation Planning Framework (ACPF) is being used. In addition, the location of existing implemented practices are being identified by the Iowa Best Management Practices Mapping Project (IBMP). From these two products, a methodology was developed to compare the differences between actual implementation and practice placement potential. The compared conservation practices are grassed waterways, wetlands and ponds, and water and sediment control basins (WASCOBs). The comparison is performed in three hydrologic unit code 12 (HUC-12) watersheds in three distinct landform regions of Iowa. Analyses show that grassed waterways are widely implemented (at least 78% of the potential) in the three watersheds. For ponds and wetlands, the majority of the existing structures were smaller than the ACPF potential wetlands (average drainage area between 7 and 20 ha compared to between 89 and 109 ha). WASCOB implementation was only present in one watershed, most likely due to regional differences in conservation preferences. Coupled together the IBMP and ACPF will be important for stakeholders of watersheds in planning future investment and advancing towards a more systems-based approach to conservation.

Iowa Stream Nitrate, Discharge and Precipitation: 30-Year Perspective.

We evaluated Iowa Department of Natural Resources nitrate (NO3-N) and US Geological Survey hydrological data from 1987 to 2016 in nine agricultural watersheds to assess how transport of this pollutant has changed in the US state of Iowa. When the first 15 years of the 30-year water-quality record is compared to the second 15 years (1987-2001 and 2002-2016), three different metrics used to quantify NO3-N transport all indicate levels of this pollutant are increasing. Yield of NO3-N (kg ha-1) averaged 18% higher in the second 15 years, while flow-weighted average concentrations (mg L-1) were 12% higher. We also introduced the new metric of NO3-N yield (g ha-1) per mm precipitation to assess differences between years and watersheds, which averaged 21 g NO3-N ha-1 per 1 mm of precipitation across all watersheds and was 13% higher during the second half of the record. These increases of NO3-N occurred within a backdrop of increasing wetness across Iowa, with precipitation and discharge levels 8 and 16% higher in the last half of the record, indicating how NO3-N transport is amplified by increasing precipitation levels. The implications of this are that in future climate scenarios where rainfall is more abundant, detaining water and increasing evapotranspiration within the cropping system will be necessary to control NO3-N losses. Land use changes that include use of cover crops, living mulches, and perennial plants should be expanded to improve water quality and affect the water balance within agricultural basins.

Orthophosphorus Contributions to Total Phosphorus Concentrations and Loads in Iowa Agricultural Watersheds.

Phosphorus (P) is delivered to streams as episodic particulate P and more continuous soluble P (orthophosphorus [OP]), and it is important to determine the proportion of each P form in river water to more effectively design remedial measures. In this study, we evaluated the annual mean ratios of OP to total P (TP) concentrations and loads in 12 Iowa rivers and found systematic variation in the ratios. The OP/TP ratios were >60% in two tile-drained watersheds of the Des Moines Lobe and in a shallow fractured bedrock watershed in northeast Iowa, whereas in southern and western Iowa, OP contributions to TP were <30%. Higher OP/TP ratios were associated with greater row crop intensity in the watershed and a greater proportion of baseflow in the river. Orthophosphorus contributions from croplands would be greater in watersheds characterized by widespread tile drainage and well-drained soils, whereas cropland TP export would be dominated by particulate P in dissected till plains with poorly drained soils. Understanding the dominant form and transport pathway of P from agricultural areas in a watershed is seen as an important first step in determining appropriate conservation practices to reduce P loads.

Tile Drainage Density Reduces Groundwater Travel Times and Compromises Riparian Buffer Effectiveness.

Strategies to reduce nitrate-nitrogen (nitrate) pollution delivered to streams often seek to increase groundwater residence time to achieve measureable results, yet the effects of tile drainage on residence time have not been well documented. In this study, we used a geographic information system groundwater travel time model to quantify the effects of artificial subsurface drainage on groundwater travel times in the 7443-ha Bear Creek watershed in north-central Iowa. Our objectives were to evaluate how mean groundwater travel times changed with increasing drainage intensity and to assess how tile drainage density reduces groundwater contributions to riparian buffers. Results indicate that mean groundwater travel times are reduced with increasing degrees of tile drainage. Mean groundwater travel times decreased from 5.6 to 1.1 yr, with drainage densities ranging from 0.005 m (7.6 mi) to 0.04 m (62 mi), respectively. Model simulations indicate that mean travel times with tile drainage are more than 150 times faster than those that existed before settlement. With intensive drainage, less than 2% of the groundwater in the basin appears to flow through a perennial stream buffer, thereby reducing the effectiveness of this practice to reduce stream nitrate loads. Hence, strategies, such as reconnecting tile drainage to buffers, are promising because they increase groundwater residence times in tile-drained watersheds.

Agro-hydrologic landscapes in the Upper Mississippi and Ohio River basins.

A critical part of increasing conservation effectiveness is targeting the "right practice" to the "right place" where it can intercept pollutant flowpaths. Conceptually, these flowpaths can be inferred from soil and slope characteristics, and in this study, we developed an agro-hydrologic classification to identify N and P loss pathways and priority conservation practices in small watersheds in the U.S. Midwest. We developed a GIS framework to classify 11,010 small watersheds in the Upper Mississippi and Ohio River basins based on soil permeability and slope characteristics of agricultural cropland areas in each watershed. The amount of cropland in any given watershed varied from <10 to >60 %. Cropland areas were classified into five main categories, with slope classes of <2, 2-5, and >5 %, and soil drainage classes of poorly and well drained. Watersheds in the Upper Mississippi River basin (UMRB) were dominated by cropland areas in low slopes and poorly drained soils, whereas less-intensively cropped watersheds in Wisconsin and Minnesota (in the UMRB) and throughout the Ohio River basin were overwhelmingly well drained. Hydrologic differences in cropped systems indicate that a one-size-fits-all approach to conservation selection will not work. Consulting the classification scheme proposed herein may be an appropriate first-step in identifying those conservation practices that might be most appropriate for small watersheds in the basin.

Assessment of total maximum daily load implementation strategies for nitrate impairment of the Raccoon River, Iowa.

The state of Iowa requires developing total maximum daily loads (TMDLs) for over 400 water bodies that are listed on the 303(d) list of the impaired waters. The Raccoon River watershed, which covers approximately 9400 km2 of prime agriculture land and represents a typical Midwestern corn-belt region in west-central Iowa, was found to have three stream segments impaired by nitrate-N. The Soil and Water Assessment Tool (SWAT) was applied to this watershed to facilitate the development of a TMDL. The modeling framework integrates SWAT with supporting software and databases on topography, land use and management, soil, and weather information. Annual and monthly simulated and measured streamflow and nitrate loads were strongly correlated. The watershed response was evaluated for a suite of watershed management scenarios where land use and management changes were made uniformly across the watershed. A scenario of changing the entire land to row crop resulted in an increased nitrate load of about 12% over the baseline condition at the watershed outlet. Results from the 15 nitrate load reduction strategies were found to reduce nitrate from < 1% to about 85%, with the greatest potential reduction associated with changing the row crops to grassland. This research demonstrated the use of a modeling system to facilitate the analyses of TMDL implementation strategies, including the ability to target the most efficient allocation of alternative practices on a subwatershed basis.

Modeling nitrate-nitrogen load reduction strategies for the Des Moines River, Iowa using SWAT.

The Des Moines River that drains a watershed of 16,175 km(2) in portions of Iowa and Minnesota is impaired for nitrate-nitrogen (nitrate) due to concentrations that exceed regulatory limits for public water supplies. The Soil Water Assessment Tool (SWAT) model was used to model streamflow and nitrate loads and evaluate a suite of basin-wide changes and targeting configurations to potentially reduce nitrate loads in the river. The SWAT model comprised 173 subbasins and 2,516 hydrologic response units and included point and nonpoint nitrogen sources. The model was calibrated for an 11-year period and three basin-wide and four targeting strategies were evaluated. Results indicated that nonpoint sources accounted for 95% of the total nitrate export. Reduction in fertilizer applications from 170 to 50 kg/ha achieved the 38% reduction in nitrate loads, exceeding the 34% reduction required. In terms of targeting, the most efficient load reductions occurred when fertilizer applications were reduced in subbasins nearest the watershed outlet. The greatest load reduction for the area of land treated was associated with reducing loads from 55 subbasins with the highest nitrate loads, achieving a 14% reduction in nitrate loads achieved by reducing applications on 30% of the land area. SWAT model results provide much needed guidance on how to begin implementing load reduction strategies most efficiently in the Des Moines River watershed.