Groundwater Management Area Research Articles

Impact of split nitrogen applications on nitrate leaching and maize yield in irrigated loamy sand soils of Northeast Nebraska

AbstractLittle information is available on optimizing the number of nitrogen (N) splits based on nitrate (NO3‐N) leaching and maize yield in sandy soils. To address this gap, we evaluated the impact of multiple N splits (2‐, 3‐, 4‐, and 5‐N splits) on NO3‐N leaching and maize (Zea mays L.) grain yield in irrigated loamy sand soil at a producer site in the Bazile Groundwater Management Area of Northeast Nebraska. Porous suction cup lysimeters were installed at a depth of 120 cm to collect pore water samples from 23 leaching events in 2021, a dry year. Increasing the number of N‐splits did not affect the pore‐water NO3‐N concentration; however, it was 169%, 152%, 150%, and 129% higher in 2‐, 3‐, 4‐, and 5‐N split treatments compared to control, that is, without N application. Though the 2‐, 3‐, 4‐, and 5‐N splits had 110%, 71%, 120%, and 91% higher area‐based NO3‐N leaching than the control, less deep percolation and more evapotranspiration in control led to no significant differences in area‐based NO3‐N leaching among all treatments. All N‐splits resulted in higher maize yield, nitrogen use efficiency, plant N uptake, harvest index, and aboveground biomass than control; however, the number of N‐splits did not affect these parameters. The inclusion of environmental cost reduced the return to nitrogen by 92–143 $ ha−1 across all N‐split treatments but did not significantly affect the differences among the splits. Overall, the results indicate that increasing the number of N‐splits does not provide agronomic, economic, and environmental benefits in irrigated maize fields during a dry year.

Baseflow nitrate dynamics within nested watersheds of an agricultural stream in Nebraska, USA

There is a need to evaluate high surface nitrate concentrations across agricultural watersheds, both spatially and temporally, to increase understanding of source and timing of nitrogen loads in streams and rivers. Bazile Creek is a high-nitrate stream originating in the agriculturally intensive Bazile Groundwater Management Area of Eastern Nebraska, USA. It is a gaining stream that receives groundwater with high nitrate concentrations originating from nonpoint sources. The objective of this study was to determine spatial and temporal variability of baseflow nitrate concentrations in Bazile Creek and its tributaries and to relate this variability to watershed characteristics. Surface-water nitrate samples were collected monthly from July 2018 through September 2019 from nine sites in the watershed and were analyzed for nitrate concentration. Average surface water nitrate-N concentrations within the watershed ranged from 2.7 to 15 mg L−1 and were significantly different between the sites (p < 0.05). Surface water nitrate-N concentrations varied seasonally in the main channel, recording the highest concentrations in winter (December-February, average = 14.4 mg L−1) when the discharge was minimum. High nitrate-N concentrations were observed in two of the five sampled tributaries, suggesting steady inputs of high-nitrate groundwater. The results of this study reveal substantial spatial variation in surface-water nitrate concentrations in the headwaters despite the close proximity of sampling sites. This study demonstrates that sampling tributaries along with the main channel of a stream is beneficial in determining nitrate inputs, variability and overall contaminant loading to a watershed.

Joint Estimation of Gross Recharge, Groundwater Usage, and Hydraulic Properties within HydroSight.

Groundwater management decisions are often founded upon estimates of aquifer hydraulic properties, recharge and the rate of groundwater usage. Too often hydraulic properties are unavailable, recharge estimates are very uncertain, and usage is unmetered or infrequently metered over only recent years or estimated using numerical groundwater models decoupled from the drivers of drawdown. This paper extends the HydroSight groundwater time-series package ( http://peterson-tim-j.github.io/HydroSight/) to allow the joint estimation of gross recharge, transmissivity, storativity, and daily usage at multiple production bores. A genetic evolutionary scheme was extended from estimating time-series model parameters to also estimating time series of usage that honor metered volumes at each production bore and produces (1) the best fit with the observed hydrograph and (2) plausible estimates of actual evapotranspiration and hence recharge. The reliability of the approach was rigorously tested. Repeated calibration of models for four bores produced estimates of transmissivity, storativity, and mean recharge that varied by a factor of 0.22-0.32, 0.13-0.2, and 0.03-0.48, respectively, when recharge boundary effects were low and the error in monthly, quarterly, and biannual metered usage was generally <10%. Application to the 30 observation bores within the Warrion groundwater management area (Australia), produced a coefficient of efficiency of ≥0.80 at 22 bores and ≥0.90 at 12 bores. The aquifer transmissivity and storativity were reasonably estimated, and were consistent with independent estimates, while mean gross recharge may be slightly overestimated. Overall, the approach allows greater insights from the available data and provides opportunity for the exploration of usage and climatic scenarios.

Effects of management areas, drought, and commodity prices on groundwater decline patterns across the High Plains Aquifer

We use an 82-year record of water table data from the High Plains Aquifer to introduce a new application of segmented regression to hydrogeology, and evaluate the effects of droughts, crop prices, and local groundwater management on groundwater level trajectories. Across the High Plains, we find discernable regional cycles of faster and slower water table declines. A parsimonious Classification And Regression Tree (CART) analysis details correlations between select explanatory variables and changes in water table trajectories, quantified as changes in slope of well hydrographs. Drying relative to prior-year conditions is associated with negative changes in slope; in the absence of drying conditions, steep declines in commodity price are associated with positive changes in hydrograph slopes. Establishment of a groundwater management area is not a strong predictor for change in water table trajectories, but more wells tend to have negative changes in around the time of management areas are formation, suggesting that drought conditions are associated with both negative deflections in water table trajectory and enactment of management areas. Segmented regression is a promising tool for groundwater managers to evaluate change thresholds and the effectiveness of management strategies on groundwater storage and decline, using readily available water table data.

Groundwater Subsidy From Headwaters to Their Parent Water Watershed: A Combined Field‐Modeling Approach

AbstractHeadwater groundwater subsidy, defined here as out‐of‐catchment groundwater flow contribution from a headwater catchment to its larger parent watershed (i.e., higher‐order stream), can influence the water quality and quantity of regional water resources. But the integrated flow and transport modeling approaches currently being implemented to quantify this subsidy are limited by an absence of critical field observations, such as water table dynamics and groundwater age that are required to test such models. Here we couple tracer (and hydrometric) observations from the well‐studied 4.5‐ha M8 headwater catchment in the Maimai experimental watershed with a new semianalytical free‐surface integrated flow and transport model. Our main research goals are to quantify the magnitude, age, and flow paths of the headwaters groundwater subsidies at the Maimai experimental watershed. Additionally, we explore through virtual experiments the effects of watershed slope, watershed active thickness, and recharge rate on the age, flow path, and magnitude of out‐of‐catchment headwater groundwater subsidies versus within‐catchment (or local) groundwater flow contributions. Our results show that more than 50% of groundwater recharged in the Maimai headwaters subsidizes their parent watershed. The relative proportion of headwaters groundwater subsidies is inversely proportional to recharge rate and/or directly proportional to slope angle. Our results also show that the age of the headwater groundwater subsidies is more than 500 years, almost 9 times older than the age of within‐catchment groundwater flow contributions. These findings highlight the need to consider headwaters groundwater subsidies in groundwater management area considerations.

Aquifer Drawdown and Recovery in the Northeast Groundwater Management Area, Wisconsin, USA: A Century of Groundwater Use

The Northeast Groundwater Management Area of Wisconsin, USA contains two major cones of depression in a confined sandstone aquifer. Each cone is centered near cities that have used groundwater for over 100 years. Near one of these cities (Green Bay), episodic changes in the development of groundwater and surface water resources during this period have resulted in major changes to the potentiometric surface. On two occasions, roughly 50 years apart, reductions in groundwater withdrawals have resulted from the construction of pipelines drawing surface water from Lake Michigan. In each case, rapid recovery of the potentiometric surface by as much as 70 m has occurred in the northern of the two pumping cones. The most recent switch occurred during 2006 and 2007 when eight communities stopped pumping groundwater, reducing daily withdrawals by approximately 46.37 million liters. The rate of water level recovery has diminished in some areas, with a return to a flowing artesian state for some municipal and residential wells. Although the northern portion of the groundwater management area has returned to a sustainable condition in the confined aquifer, the portion with the southern cone of depression remains in a state of prolonged drawdown.

Groundwater Quality and Nitrogen Use Efficiency in Nebraska's Central Platte River Valley

Groundwater nitrate contamination has been an issue in the Platte River Valley of Nebraska since the 1960s, with groundwater nitrate-N concentrations frequently in excess of 10 mg L. This article summarizes education and regulatory efforts to reduce the environmental impact of irrigated crop production in the Platte River Valley. In 1988, a Groundwater Management Area (GWMA) was implemented in the Central Platte Natural Resources District to encourage adoption of improved management practices. Since 1988, there have been steady declines in average groundwater nitrate-N concentrations of about 0.15 mg NO-N L yr in much of the GWMA (from 19 to 15 mg NO-N L). However, N use efficiency (NUE) (partial factor productivity for N [PFP]) has increased very little from 1988 to 2012 (60-65 kg grain kg N), whereas statewide PFP increased from 49 to 67 kg grain kg N in the same period. Although growers are encouraged to credit N from sources besides fertilizer (e.g., soil residual, legumes, irrigation water, and manure), confidence in and use of credits tended to decrease as credits became larger; there was a tendency toward an average N rate regardless of credit-based recommendations. This information, coupled with data from other studies, suggests that much of the decline in groundwater nitrate can be attributed to improved irrigation management-especially conversion from furrow to sprinkler irrigation-and to a lesser extent to improved timing of N application. The development and adoption of improved N management practices, such as fertigation, controlled-release N formulation, and use of crop canopy sensors for in-season N application may be required for further significant NUE gains in these irrigated systems.

A fuzzy simulation–optimization approach for optimal estimation of groundwater availability under decision maker uncertainty

As groundwater is a slowly replenishing resource that can be depleted relatively easily, there is a growing interest in actively managing aquifer resources. Participatory, multi-stakeholder driven approaches are increasingly being adopted to plan groundwater use such that the resource is available for present as well as future needs. The state of Texas requires neighboring GCDs (local regulatory bodies) within a groundwater management area (GMA) to engage in joint planning activities and define desired future conditions (DFCs) for the aquifers they regulate. The DFCs are then used to estimate modeled available groundwater which defines how much water is available within an aquifer in a given region. The groundwater joint planning process was modeled using a combined simulation–optimization modeling scheme in this study. The response surface methodology was used to establish regional-scale aquifer stress-response relationships. In addition to average county-wide drawdown, other aquifer responses including stream-aquifer exchanges, coastal-aquifer exchanges and GMA-wide drawdown were considered to define the DFCs. A constrained linear regression was used in conjunction with a regional groundwater flow model to obtain the necessary response functions which formed the basis for a crisp optimization model whose objective was maximizing groundwater production while ensuring that the prescribed DFCs are not violated (constraints). This model was transformed into a fuzzy linear programming model to account for the fact that groundwater planners find it difficult to specify DFCs with a high degree of precision. Using linear membership functions, the decision makers’ preferences were captured using two values––a minimum preferred cut-off and the maximum allowable value for the metric. For estimating groundwater availability, the fuzzy optimization model reconciles production and maximizes the goal and the constraints representing the DFCs. The developed framework was illustrated by applying it to joint planning in Groundwater Management Area 15 in South Texas. The optimization models were highly sensitive to acceptable average drawdowns, while the coastal-aquifer interactions had secondary impacts. The fuzzy optimization model yielded lower estimates of groundwater availability in comparison to the crisp optimization scheme. The fuzzy optimization model is therefore consistent with the precautionary principle and recommended for use in the early stages of groundwater planning where incomplete understanding of the aquifer dynamics precludes specification of precise limits for the DFCs.

A statistical power analysis approach to estimate groundwater-monitoring network size in Victoria County Groundwater Conservation District, Texas

A properly designed groundwater-monitoring network is critical to evaluate groundwater management policies and regulations and establish a scientifically credible, risk-informed approach towards management of aquifer resources. To achieve proper design, the network must comprise a sufficient number of wells that comprehensively covers the area of interest. In this study, statistical power analysis is utilized to define the size of a proposed monitoring network within the Victoria County Groundwater Conservation District (VCGCD). In particular, the network seeks to provide empirical information to evaluate whether the VCGCD is meeting the criteria of the desired future conditions (DFC) established by the Groundwater Management Area 15 (GMA 15) through the joint planning process, as required by the statutes of Texas. The evaluation of DFCs is expressed as a hypothesis-testing problem with the null hypothesis stating that the VCGCD complies with the DFC, and the alternative hypothesis stating that the VCGCD does not comply with the DFC. The power analysis quantifies the ability of the statistical test to correctly reject the null hypothesis. The power of the statistical test is a function of the sample size and this relationship can be exploited to determine the size of the required groundwater-monitoring network if the effect size can be specified. However, prior to establishing a monitoring network, the groundwater level monitoring tends to be ad hoc; therefore, the statistical moments required for characterizing the effect size are not likely to be known with a high degree of certainty. As such, an innovative framework that integrates power analysis with bootstrap resampling protocols was developed to estimate monitoring network size under uncertainty. The results of the study indicated that a minimum set of approximately 70 wells is required within the VCGCD to statistically evaluate the compliance with DFCs with 90 % reliability and at a significance level of 10 % and 90 % power. The number of wells increased to about 85 when the significance level was reduced to 5 %. Geostatistical analysis indicated that these monitoring wells must be at least 3 miles apart to ensure statistically independent information. The developed methodology provides a practical framework to size a groundwater-monitoring network under data sparse situations.

NITRATE CONTAMINATION IN OREGON WELL WATER: GEOLOGIC VARIABILITY AND THE PUBLIC'S PERCEPTION

ABSTRACT: Approximately 4.5 million people in the United States who rely on well water are exposed to nitrate-N concentrations exceeding the 10 mg/l standard. In this study in the Southern Willamette Valley in Oregon we reassessed nitrate-N in rural wells sampled in 2000–2001, compared nitrate-N concentrations among geological units, and surveyed residents about their perceptions of well water quality. Nitrate-N concentrations were again sampled in 2002 and found to have increased significantly from the previous period. With rapid population growth in the area, the potential health risk in drinking well water that exceeds 10 mg/l nitrate-N warrants continued public education. Nitrate-N concentrations were found to be higher in the Holocene alluvium of the Willamette River and the Pleistocene sand and gravel post-Missoula Flood deposits. Researchers conducting future studies may choose to stratify and monitor wells by geologic unit and by other parameters that estimate input of nutrients to the environment. Opinions differed between agricultural landowners and nonagricultural landowners with regard to the impact that agricultural fertilizers may have on water quality. Participants were supportive of a range of regulatory actions that might be used by homeowners or landowners to address ground water contamination. Given that the area is now designated a Groundwater Management Area, understanding local stakeholders’perceptions is critical and strategic and has the potential to help public agencies manage potential conflicts of opinion among stakeholders, build consensus, and help guide the approach to restoring ground water quality.

Arsenic-rich groundwater in an urban area experiencing drought and increasing population density, Perth, Australia

Groundwater in the Gwelup groundwater management area in Perth, Western Australia has been enriched in As due to the exposure of pyritic sediments caused by reduced rainfall, increased groundwater abstraction for irrigation and water supply, and prolonged dewatering carried out during urban construction activities. Groundwater near the watertable in a 25–60 m thick unconfined sandy aquifer has become acidic and has affected shallow wells used for garden irrigation. Arsenic concentrations up to 7000 μg/L were measured in shallow groundwater, triggering concerns about possible health effects if residents were to use water from household wells as a drinking water source. Deep production wells used for public water supply are not affected by acidity, but trends of progressively increasing concentrations of Fe, SO 4 and Ca over a 30-a period indicate that pyrite oxidation products extend to the base of the unconfined aquifer. Falling Eh values are triggering the release of As from the reduction of Fe(III) oxyhydroxide minerals near the base of the unconfined aquifer, increasing the risk that groundwater used as a drinking water source will also become contaminated with high concentrations of As.