parameterizing-v-notch-weir-equations-for-flow-monitoring-drainage

Parameterizing V-notch Weir Equations for Flow Monitoring in a Drainage Control Structure

Accurate estimation of drainage discharge (flow rate or water volume per unit time) is essential for calculating nutrient loads delivered to surface water ...

Iowa State University

The surface waters of Rathbun Lake watershed in southern Iowa are impacted by agricultural sources of sediments and nutrients, including phosphorus (P). Because stream sediments often play an important role in regulating P concentrations in stream water, we investigated sediment-water column P relationships in four creeks within the watershed and then evaluated the relationship between sediment properties and indicators of the risk of P loss. Based on Mehlich-3-extractable P (17 to 68 mg kg(-1)) and degree of P saturation (2 to 12 %), stream bank and bed sediments at the four sites were unlikely to serve as major sources of P. However, equilibrium P concentrations, which ranged from 0.02 to 0.12 mg L(-1), indicated that bed sediments could release P to the water column depending on dissolved P (DP) concentrations in the stream water and the time of year. The likelihood of P desorption from the sediments increased with increasing pH (r = 0.92, p < 0.01) and sand content (r = 0.78, p < 0.05), but decreased with clay content (r = -0.72, p < 0.05) and iron (Fe) (r = -0.93, p < 0.001) associated with organic matter. From these results, we speculate that changes in land use within the riparian areas may, at least initially, have little effect on P concentrations in the streams. Low concentrations of DP relative to total P (TP) in these streams, however, suggest that P loads to Rathbun Lake can be reduced if P inputs from eroded bank sediments are controlled.

Phosphorus source-sink relationships of stream sediments in the Rathbun Lake watershed in southern Iowa, USA.

Orthophosphorus (OP) is the form of dissolved inorganic P that is commonly measured in groundwater studies, but the spatial distribution of groundwater OP across a watershed has rarely been assessed. In this study, we characterized spatial patterns of groundwater OP concentrations and loading rates within the 5218ha Walnut Creek watershed (Iowa) over a two-year period. Using a network of 24 shallow (<6m) monitoring wells established across watershed, OP concentrations ranged from <0.01 to 0.58mg/l in all samples (n=147) and averaged 0.084±0.107mg/l. Groundwater OP concentrations were higher in floodplains and OP mass loading rates were approximately three times higher than in uplands. We estimated that approximately 1231kg of OP is present in floodplain groundwater and 2869kg is present in upland groundwater within the shallow groundwater zone (0-5m depth). Assuming no new inputs of OP to shallow groundwater, we estimated it would take approximately eight years to flush out existing OP mass present in the system. Results suggest that conservation practices focused on reducing OP loading rates in floodplain areas may have a disproportionately large water quality benefit compared to upland areas.

Distribution and mass of groundwater orthophosphorus in an agricultural watershed.

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 km 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.

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

Nitrate (NON) export from row crop agricultural systems with subsurface tile drainage continues to be a major water quality concern. Woodchip bioreactors are an effective edge-of-field practice designed to remove NON from tile drainage. The NON removal rate of woodchip bioreactors can be impacted by several factors, including hydraulic residence time (HRT). This study examined the impact of three HRTs, 2 h, 8 h, and 16 h, on NON removal in a set of nine pilot-scale woodchip bioreactors in Central Iowa. NON concentration reduction from the inlet to the outlet was significantly different for all HRTs (p < 0.05). The 16 h HRT removed the most NON by concentration (7.5 mg L) and had the highest removal efficiency at 53.8%. The 8 h HRT removed an average of 5.5 mg L NON with a removal efficiency of 32.1%. The 2 h HRT removed an average of 1.3 mg L NON with a removal efficiency of 9.0%. The 2 h HRT had the highest NON mass removal rate (MRR) at 9.0 g m day, followed by the 8 h HRT at 8.5 g m day, and the 16 h HRT at 7.4 g m day, all of which were statistically different (p < 0.05). Significant explanatory variables for removal efficiency were HRT (p < 0.001) and influent NON concentration (p < 0.001), (R = 0.80), with HRT accounting for 93% contribution. When paired with results from a companion study, the ideal HRT for the bioreactors was 8 h to achieve maximum NON removal while reducing the impact from greenhouse gas emissions.

Impact of hydraulic residence time on nitrate removal in pilot-scale woodchip bioreactors.

Riparian buffers are a conservation practice that increases vegetation diversity on the agricultural landscape while providing environmental benefits. This study specifically focused on the ability of riparian buffers to remove nitrate from shallow groundwater. There are many studies that assessed nitrate removal within buffers, but not many have a long-term, continuous data set that can analyze for variation in nitrate removal rates over time. Here we report on 21 years of nitrate well data, from 1996 through 2017, for three buffers in the Bear Creek watershed in central Iowa. These buffers are named using abbreviations to help keep landowners anonymous (e.g. RN, RS, and ST). Studied buffers RS and ST showed greater nitrate reduction (or removal) after 10 and 6 years of its establishment, respectively. Buffer RN did not experience a significant nitrate removal increase with time, but instead had higher nitrate removal rates when compared to buffers RS and ST of 10.3 g NO-N m day from the start of this study. From this data, we suggest that past land management played a major role in the responses observed. RN had previously been established in cool-season grasses for grazing before being converted to a buffer, while RS and ST had been managed in a corn and soybean rotation. RN was thought to have higher denitrification immediately with increased labile soil carbon input and enhanced soil aggregation due to the grassland perennials, while buffer vegetation establishment increased soil carbon inputs and soil aggregation over time for RS and ST. These nitrate removal trends would not have been observed without access to long-term, continuous data. This study highlighted the importance of long-term data sets and the need to assess conservation practices over time to determine their longevity and efficiency with time.

Long-term nitrate removal in three riparian buffers: 21 years of data from the Bear Creek watershed in central Iowa, USA.

Denitrification bioreactors are an effective edge-of-field conservation practice for nitrate (NO) reduction from subsurface drainage. However, these systems may produce other pollutants and greenhouse gases during NO removal. Here a dual-chamber woodchip bioreactor system experiencing extreme low-flow conditions was monitored for its spatiotemporal NO and total organic carbon dynamics in the drainage water. Near complete removal of NO was observed in both bioreactor chambers in the first two years of monitoring (2019-2020) and in the third year of monitoring in chamber A, with significant (p < 0.01) reduction of the NO-N each year in both chambers with 8.6-11.4 mg NO-N L removed on average. Based on the NO removal observed, spatial monitoring of sulfate (SO), dissolved methane (CH), and dissolved nitrous oxide (NO) gases was added in the third year of monitoring (2021). In 2021, chambers A and B had median hydraulic residence times (HRTs) of 64 h and 39 h, respectively, due to varying elevations of the chambers, with drought conditions making the differences more pronounced. In 2021, significant production of dissolved CH was observed at rates of 0.54 g CH-C m d and 0.07 g CH-C m d in chambers A and B, respectively. In chamber A, significant removal (p < 0.01) of SO (0.23 g SO m d) and dissolved NO (0.21 mg NO-N m d) were observed, whereas chamber B produced NO (0.36 mg NO-N m d). Considering the carbon dioxide equivalents (COe) on an annual basis, chamber A had loads (~12,000 kg COe ha y) greater than comparable poorly drained agricultural soils; however, the landscape-scale impact was small (<1 % change in COe) when expressed over the drainage area treated by the bioreactor. Under low-flow conditions, pollution swapping in woodchip bioreactors can be reduced at HRTs <50 h and NO concentrations >2 mg N L.

Article	Year
Phosphorus source-sink relationships of stream sediments in the Rathbun Lake watershed in southern Iowa, USA. Environmental monitoring and assessment\| PubMed ID: 27393193	2016
Distribution and mass of groundwater orthophosphorus in an agricultural watershed. The Science of the total environment\| PubMed ID: 29996430	2018
Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and nutrient budgets. The Science of the total environment\| PubMed ID: 30743899	2019
Impact of hydraulic residence time on nitrate removal in pilot-scale woodchip bioreactors. Journal of environmental management\| PubMed ID: 30822646	2019
Long-term nitrate removal in three riparian buffers: 21 years of data from the Bear Creek watershed in central Iowa, USA. The Science of the total environment\| PubMed ID: 32563878	2020
Extreme low-flow conditions in a dual-chamber denitrification bioreactor contribute to pollution swapping with low landscape-scale impact. The Science of the total environment\| PubMed ID: 36924958	2023

Thomas M. Isenhart

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