Wisconsin Central Sand Research Articles

Spatial and temporal forecasting of groundwater anomalies in complex aquifer undergoing climate and land use change

Monitoring groundwater (GW) level variations, or anomalies in multiple wells, over long periods of time is essential to understanding changes in regional groundwater resource availability. However, it is challenging to predict these GW anomalies over the long term in agricultural areas due to complicated boundary conditions, heterogeneous hydrogeological characteristics, and groundwater extraction, as well as nonlinear interactions among these factors. To overcome this challenge, we developed an advanced modeling framework based on a recurrent neural network of long short-term memory (LSTM) as an alternative to complex and computationally expensive physical models. GW anomalies were forecast two months in advance (t + 2) based on the evaluation of drivers that influence GW dynamics in densely irrigated agricultural regions. An application and evaluation of this new approach was conducted in the Wisconsin Central Sands (WCS) region in the U.S., one of the most productive agricultural regions. The modeling framework was developed for the period 1958–2020 by utilizing easily accessible dynamic and static variables to represent hydrometeorological and geological characteristics. GW anomaly observations were acquired from 26 piezometers (wells) installed in the sandy aquifer in WCS over 10–60 years. The subset of GW observations from ∼ 10–60 years, not used in model training, can forecast GW anomalies two months out with a coefficient of determination R2 ∼ 0.8. Additionally, MAE was less than 0.34 m/month across the study region for both temporal and spatial modeling frameworks. Groundwater anomalies showed high spatiotemporal variability, and their responses are influenced differently by boundary conditions, catchment geology, climate, and topography across locations. Sites with higher autocorrelation with previous two-months GW anomalies reduced bias by increasing R2. Land use change and irrigation pumping have interactive effects on GW anomalies forecasting. The novelty of this study is identifying the regional drivers of GW fluxes. This case-specific information and location-related simplification, modification, and assumption of LSTM is a unique contribution to the existing literature. Our framework can be used as an alternative method of simulating water availability and groundwater level changes in areas where subsurface properties are unknown or difficult to determine.

Quantifying the spatiotemporal variability of nitrate in irrigation water across the Wisconsin Central Sands.

The Wisconsin Central Sands is home to large scale vegetable production on sandy soils and managed with frequent irrigation, fertigation, and widespread nitrogen fertilizer application, all of which make the region highly susceptible to nitrate loss to groundwater. While the groundwater is used as the primary source of drinking water for many communities and rural residences across the region, it is also used for irrigation. Considering the high levels of nitrate found in the groundwater, it has been proposed that growers more accurately account for the nitrate in their irrigation water as part of nitrogen management plans. Our objectives were to 1) determine the magnitude of nitrate in irrigation water, 2) quantify the spatiotemporal variability of nitrate, and 3) determine key predictors of nitrate concentration in the region. We sampled irrigation water from 38 fields across six farms from 2018 to 2020. Across the 3 years of our study, nitrate concentration varied more across space than time. On average, our samples were tested at 19.0mg L-1 nitrate-nitrogen, or nearly two times the U.S. Environmental Protection Agency (EPA) threshold for safe drinking water, equivalent to 48.1kg ha-1 of applied nitrate-nitrogen with 25.4cm (or 10 in.) of irrigation. To better understand the spatiotemporal variability in nitrate levels, week of sampling, year, well depth, well casing, and nitrogen application rate were analyzed for their role as predictor variables. Based on our linear mixed effects model, nitrogen application rate was the greatest predictor of the nitrate concentration of irrigation water (p<0.05).

How High to Fly? Mapping Evapotranspiration from Remotely Piloted Aircrafts at Different Elevations

Recent advancements in remotely piloted aircrafts (RPAs) have made frequent, low-flying imagery collection more economical and feasible than ever before. The goal of this work was to create, compare, and quantify uncertainty associated with evapotranspiration (ET) maps generated from different conditions and image capture elevations. We collected optical and thermal data from a commercially irrigated potato (Solanum tuberosum) field in the Wisconsin Central Sands using a quadcopter RPA system and combined multispectral/thermal camera. We conducted eight mission sets (24 total missions) during the 2019 growing season. Each mission set included flights at 90, 60, and 30 m above ground level. Ground reference measurements of surface temperature and soil moisture were collected throughout the domain within 15 min of each RPA mission set. Evapotranspiration values were modeled from the flight data using the High-Resolution Mapping of Evapotranspiration (HRMET) model. We compared HRMET-derived ET estimates to an Eddy Covariance system within the flight domain. Additionally, we assessed uncertainty for each flight using a Monte Carlo approach. Results indicate that the primary source of uncertainty in ET estimates was the optical and thermal data. Despite some additional detectable features at low elevation, we conclude that the tradeoff in resources and computation does not currently justify low elevation flights for annual vegetable crop management in the Midwest USA.

Characterizing Dominant Field-Scale Cropping Sequences for a Potato and Vegetable Growing Region in Central Wisconsin

Crop rotations are known to improve soil health by replenishing lost nutrients, increasing organic matter, improving microbial activity, and reducing disease risk and weed pressure. We characterized the spatial distribution of crops and dominant field-scale cropping sequences from 2008 to 2019 for the Wisconsin Central Sands (WCS) region, a major producer of potato and vegetables in the U.S. The dominant two- and three-year rotations were determined, with an additional focus on assessing regional potato rotation management. Our results suggest corn and soybean are the two most widely planted crops, occurring on 67% and 36% of all agricultural land at least once during the study period. The most frequent two- and three-year crop rotations include corn, soybean, alfalfa, sweet corn, potato, and beans, with continuous corn being the most dominant two- and three-year rotations (13.2% and 8.5% of agricultural land, respectively). While four- and five-year rotations for potato are recommended to combat pest and disease pressure, 23.2% and 65.9% of potato fields returned to that crop in rotation after two and three years, respectively. Furthermore, 5.6% of potato fields were planted continuously with that crop. Given potato’s high nitrogen (N) fertilizer requirements, the prevalence of sandy soils, and ongoing water quality issues, adopting more widespread use of four- or five-year rotations of potato with crops that require zero or less N fertilizer could reduce groundwater nitrate concentrations and improve water quality.

Predicting the color of sandy soils from Wisconsin, USA

Color is an important soil property and often used to infer soil properties and delineate soil horizons. We investigated the effect of soil texture, carbon, total elements, and particle-size fractions on the color of sandy soils using five color models. In total, 915 soil samples were collected to a maximum depth of 220 cm at 400 locations in the Wisconsin Central Sand Plains, and the soils were sandy throughout. The samples were scanned using a visible-near infrared spectrometer, with soil color models (HSV, RGB, CIE L*a*b*, CIE L*u*v*, and redness index (RI)) extracted from the reflectance spectra. Cubist models were used to predict each of the soil color coordinates from the soil properties. The models showed high prediction accuracy for V, R, SRGB, L*, a*, b*, u*, and v* color coordinates for both calibration and validation. The CIE L*a*b* color model was generally better than other color models. Silt and Fe were used in all of the Cubist models, while sand, clay, carbon, Al, Si, Mn, and Zn were used in most models. The 45–125 µm, 125–250 µm, and 1000–2000 µm fractions affected the soil color as opposed to the 250–500 µm and 500–1000 µm fractions. A clustering analysis on the CIE L*a*b* color model showed that soil lightness was higher with higher sand content, but lower with an increase in silt and clay content, carbon, Fe, Al, Zn, and Mn. The LUCAS dataset has approximately 20,000 soil samples and was used to explore the relationships between soil color and soil properties in sandy soils and to test the robustness of the model established with the Central Sands dataset. The Cubist models developed from either dataset (Central Sands, LUCAS) were not useful to predict CIE L*a*b* color using the other dataset.

Geochemical Fingerprint and Soil Carbon of Sandy Alfisols

Soil carbon storage is affected by particle-size fractions and Fe oxides. We assessed soil carbon concentrations in different particle-size fractions, determined the soil chemical composition of the soil, and weathering and mineralogy of sandy soils of the Wisconsin Central Sands, USA. Three land uses were studied (agriculture, forest, and prairie). The soils contained a minimum of 830 g sand kg−1 up to 190 cm soil depth. Approximately 46% of the sand was in the 250–500 μm fraction, and 5% was &lt;125 μm. Soil carbon ranged from 5 to 13 g kg−1 in the topsoil, and decreased with depth. The &lt;45 μm fraction tended to have high concentrations of carbon, ranging from 19 to 43 g kg−1 in the topsoil. Silicon content was over 191 g Si kg−1, and was lowest in the Bt horizons (191–224 g Si kg−1). Up to 29 g Fe kg−1 and 39 g Al kg−1 were present in the soil, and were highest in the Bt horizons. These soils were mostly quartz, and diopside was found throughout the soil profiles. Weathering indices, such as the Ruxton Ratio, showed that the C horizons were the least weathered and the Bt horizons were more weathered. We conclude that most of the carbon in these soils is held in the &lt;45 μm fraction, and soil carbon and total Fe were lowest in the coarser size fractions.

Observation of irrigation-induced climate change in the Midwest United States.

Irrigated agriculture alters near-surface temperature and humidity, which may mask global climate change at the regional scale. However, observational studies of irrigation-induced climate change are lacking in temperate, humid regions throughout North America and Europe. Despite unknown climate impacts, irrigated agriculture is expanding in the Midwest United States, where unconfined aquifers provide groundwater to support crop production on coarse soils. This is the first study in the Midwest United States to observe and quantify differences in regional climate associated with irrigated agricultural conversion from forests and rainfed agriculture. To this end, we established a 60km transect consisting of 28 stations across varying land uses and monitored surface air temperature and relative humidity for 31months in the Wisconsin Central Sands region. We used a novel approach to quantify irrigated land use in both space and time with a database containing monthly groundwater withdrawal estimates by parcel for the state of Wisconsin. Irrigated agriculture decreased maximum temperatures and increased minimum temperatures, thus shrinking the diurnal temperature range (DTR) by an average of 3°C. Irrigated agriculture also decreased the vapor pressure deficit (VPD) by an average of 0.10kPa. Irrigated agriculture significantly decreased evaporative demand for 25% and 66% of study days compared to rainfed agriculture and forest, respectively. Differences in VPD across the land-use gradient were highest (0.21kPa) during the peak of the growing season, while differences in DTR were comparable year-round. Interannual variability in temperature had greater impacts on differences in DTR and VPD across the land-use gradient than interannual variability in precipitation. These regional climate changes must be considered together with increased greenhouse gas emissions, changes to groundwater quality, and surface water degradation when evaluating the costs and benefits of groundwater-sourced irrigation expansion in the Midwest United States and similar regions around the world.

Effects of carbon on moisture storage in soils of the Wisconsin Central Sands, USA

Quantifying soil water storage is important for plant growth and agricultural production. This is particularly important in sandy soils because they are widely cultivated and prone to drought. We investigated the effect of soil organic carbon (SOC), soil texture and gravel content on soil moisture characteristics in sandy soils. Soil moisture characteristics were analysed using standard soil physical characterization as well as visible–near infrared (vis–NIR) spectroscopy and pedotransfer functions (PTFs). Volumetric water content (−10, −33, −1500 kPa) and available water capacity (AWC) increased by 0.01 to 0.02 m3m−3 with a 2% increase in silt plus clay content. Available water capacity increased by about 0.05 m3m−3 with a 1% increase in SOC. A 5% increase in gravel in the subsoil decreased the volumetric water content of the soil by 0.01 m3m−3. Predictions of volumetric water contents and AWC by vis–NIR were the best when using a random forest model. Pedotransfer functions for AWC predictions performed best when sand, silt, clay, bulk density and SOC were included. Soil moisture storage ranged from 44 to 65 mm in the top 50 cm. At mean evapotranspiration (4.2 mm day−1), irrigation requirements were reduced by 6 days in the sandy soils that have larger SOC and silt contents in the topsoil. These soils are sandy throughout but have subtle differences in particle‐size distribution, SOC concentrations and gravel content. These differences affect soil moisture storage considerably. Soil management practices that increase the SOC contents in these sandy soils favour moisture storage and tend to reduce irrigation requirements.Highlights SOC, soil texture and gravel affect soil moisture characteristics in sandy soils. With vis–NIR and PTFs, soil water storage, water characteristics and AWC in soils with &gt;80% sand can be quantified and predicted. Available water capacity increased by about 0.05 m3m−3 with a 1% increase in SOC. Subtle differences in particle‐size distribution affect soil moisture storage and reduce irrigation requirements.

Spatial-temporal analysis of soil water storage and deep drainage under irrigated potatoes in the Central Sands of Wisconsin, USA

Knowledge of soil water storage and deep drainage is important for improving irrigation efficiency, maximizing crop water use, and understanding groundwater table fluctuation. This is particularly important in sandy soils that depend on irrigation to produce high crop yields. In sandy soils under potato in the Wisconsin Central Sand Plains, soil water storage using tension probes was measured in three irrigation zones over the 2014 and 2015 growing seasons. Soil water storage was estimated across a 78 ha field using apparent electrical conductivity maps. Deep drainage was estimated using the Richards’ equation and Hydrus-1D software. It was found that the average soil water storage ranged from 74 to 110 mm in the top 0.45 m in three irrigation zones in 2014, and from 70 to 95 mm in 2015. Rainfall and irrigation was 387 and 269 mm in 2014, and 328 and 281 mm in 2015. Estimated deep drainage was uniform in three irrigation zones, and ranged from 222 to 244 mm in 2014, and from 167 to 180 mm in 2015. A negative correlation was found between soil water storage and potato yields possibly due to over-irrigation. The methods used in this study can be applied to improve irrigation and water use efficiency.

Apparent electrical conductivity predicts physical properties of coarse soils

Precision agriculture informed by electromagnetic induction surveys could reduce groundwater withdrawals and nitrogen leaching from coarse soils. However, coarse, nonsaline soils often have extremely narrow ranges of mapped apparent electrical conductivity (ECa) and the efficacy of ECa for predicting soil physical properties is uncertain in this context. For this reason, it is also uncertain as to whether electromagnetic induction surveys are valuable for guiding precision agriculture on coarse, nonsaline soils. Additionally, the need to ground-truth electromagnetic induction surveys for individual agricultural fields with soil sampling and statistical model development hampers adoption of precision agriculture at the regional scale. Our research objectives were to quantify the variation in mapped ECa and develop statistical relationships between ECa and soil physical properties both within and across several agricultural fields in the Wisconsin Central Sands, a distinct hydropedological region with coarse, glaciolacustrine soils. We used nonparametric correlation analyses to identify associations and quantile regression, a statistical approach with no assumptions of normality or homoscedasticity, to identify predictive relationships between ECa and soil physical properties. We found strong, significant (p < 0.05) correlative and predictive relationships between ECa and topsoil (0–0.3 m) particle size fraction, organic matter content, and field capacity within and across several fields. Yet, we did not observe many significant relationships between ECa and subsoil (0.5–0.6 m) physical properties, which we attribute to heterogeneous soil layering and the low depth resolution of our soil sampling approach. Our findings demonstrate that proximal sensing of ECa can identify intrafield variability in soil properties under extremely narrow observed ECa ranges (0–11 mS m−1). Moreover, we found that interfield quantile regression models predicted soil physical properties across several agroecosystems. Heteroscedasticity was present in interfield ECa relationships with physical properties, which resulted in the need for different quantile regression models across the conditional distribution. The flexibility for accommodating heteroscedasticity in soils and simplicity of modeled functions make quantile regression a promising approach for developing interfield or regional models of ECa to predict soil physical properties in distinct, hydropedological regions with coarse soils.

Drivers of Potential Recharge from Irrigated Agroecosystems in the Wisconsin Central Sands

Core Ideas Groundwater in humid regions should be managed for services, not depletion. Climate variability can be a greater driver of potential recharge than crop type. Soil texture can be a significant driver of point‐based potential recharge estimates. Crop phenology may not be predictive of point‐based potential recharge estimates. The expansion of irrigated agriculture on landscapes underlain by coarse‐grained, glacial aquifers in Wisconsin, Minnesota, and Michigan changes the timing and magnitude of groundwater recharge. Water managers require improved estimates of groundwater recharge to manage pumping impacts on groundwater‐fed streams, lakes, and wetlands. We implemented a network of 25 passive capillary lysimeters to infer potential groundwater recharge and evapotranspiration (ET) from irrigated potato (Solanum tuberosum L.), sweet corn and field corn (Zea mays L.), and pea (Pisum sativum L.)–pearl millet [Pennisetum glaucum (L.) R. Br.] rotations in the Wisconsin Central Sands (WCS) from June through November of 2013 to 2016. We found that interannual climate variability, subtle differences in soil texture, and cropping system type drove potential recharge to varying degrees during the summer and fall seasons. Relatively finer soil texture was positively correlated to point estimates of potential recharge. This correlation was the strongest following large precipitation events. June to November cumulative potential recharge for 2013 to 2016 averaged 71 ± 235 mm across all lysimeters. Our findings suggest that aquifer depletion will be an episodic process that leaves surface waters most vulnerable to pumping and recharge impacts during and following drier years in the WCS. Differences among cropping systems were most pronounced under average precipitation conditions, which facilitated potential groundwater losses under field corn and pea–pearl millet rotations and potential groundwater gains under potato rotations. We conclude that regional water management strategies could be effective in buffering against the interannual climate variability of recharge, while localized management strategies could increase irrigation efficiency by targeting crop and soil textural drivers.

Groundwater recharge under compacted agriculturalsoils, pineand prairie in Central Wisconsin, USA

The water table in Wisconsin Central Sand Plain (CSP) dropped 1.8 m between 2002 and 2010, and several lakes in the area suffer from low water levels. The lower groundwater level has been attributed to agricultural cropping practices, specifically irrigation, and a reduction in ground-water recharge. Dominant soils in this area are sands (Udipsamments). The objective of our research was to quantify groundwater recharge under (i) irrigated agricultural crops, (ii) prairie, and (iii) a 50-year old pine tree plantation in the CSP. Equipment was installed at five sites to monitor water table elevation, soil water content, and precipitation at 15-minute intervals. It was found that, when the soil was at field moisture capacity, precipitation during the growing season resulted in 1.4 cm more water table rise under a prairie than under irrigated agricultural fields. Agricultural crops used groundwater through irrigation, but natural vegetation relied on soil available water, and capillary rise of water from the shallow groundwater table (1 to 2 m to water table), for daily transpiration. After snowmelt, prairie vegetation yielded greater rise, up to 16 cm, in the water table than agricultural fields. Lack of crop residue on the soil surface of agricultural fields resulted in a continuous layer of frost in the soil profile that extended to about a meter depth. This thick, frozen layer was enhanced by greater soil compaction under irrigated crops compared to limited or no compaction in prairie areas. The key finding was that this deep frost in the soil profile inhibited snowmelt water from infiltrating and recharging the groundwater. Thus, compacted-irrigated agricultural soils in the CSP alter groundwater recharge characteristics during frozen and non-frozen ground periods. Increased crop residues on the surface of agricultural fields might enhance groundwater recharge during winter snow melt periods.

Groundwater recharge under compacted agriculturalsoils, pineand prairie in Central Wisconsin, USA

The water table in Wisconsin Central Sand Plain (CSP) dropped 1.8 m between 2002 and 2010, and several lakes in the area suffer from low water levels. The lower groundwater level has been attributed to agricultural cropping practices, specifically irrigation, and a reduction in ground-water recharge. Dominant soils in this area are sands (Udipsamments). The objective of our research was to quantify groundwater recharge under (i) irrigated agricultural crops, (ii) prairie, and (iii) a 50-year old pine tree plantation in the CSP. Equipment was installed at five sites to monitor water table elevation, soil water content, and precipitation at 15-minute intervals. It was found that, when the soil was at field moisture capacity, precipitation during the growing season resulted in 1.4 cm more water table rise under a prairie than under irrigated agricultural fields. Agricultural crops used groundwater through irrigation, but natural vegetation relied on soil available water, and capillary rise of water from the shallow groundwater table (1 to 2 m to water table), for daily transpiration. After snowmelt, prairie vegetation yielded greater rise, up to 16 cm, in the water table than agricultural fields. Lack of crop residue on the soil surface of agricultural fields resulted in a continuous layer of frost in the soil profile that extended to about a meter depth. This thick, frozen layer was enhanced by greater soil compaction under irrigated crops compared to limited or no compaction in prairie areas. The key finding was that this deep frost in the soil profile inhibited snowmelt water from infiltrating and recharging the groundwater. Thus, compacted-irrigated agricultural soils in the CSP alter groundwater recharge characteristics during frozen and non-frozen ground periods. Increased crop residues on the surface of agricultural fields might enhance groundwater recharge during winter snow melt periods.

Irrigation Effects in the Northern Lake States: Wisconsin Central Sands Revisited

Irrigated agriculture has expanded greatly in the water-rich U.S. northern lake states during the past half century. Source water there is usually obtained from glacial aquifers strongly connected to surface waters, so irrigation has a potential to locally decrease base flows in streams and water levels in aquifers, lakes, and wetlands. During the nascent phase of the irrigation expansion, water availability was explored in works of some fame in the Wisconsin central sands by Weeks et al. (1965) on the Little Plover River and Weeks and Stangland (1971) on "headwater area" streams and lakes. Four decades later, and after irrigation has grown to a dominant landscape presence, we revisited irrigation effects on central sands hydrology. Irrigation effects have been substantial, on average decreasing base flows by a third or more in many stream headwaters and diminishing water levels by more than a meter in places. This explains why some surface waters have become flow and stage impaired, sometimes to the point of drying, with attendant losses of aquatic ecosystems. Irrigation exerts its effects by increasing evapotranspiration by an estimated 45 to 142 mm/year compared with pre-irrigated land cover. We conclude that irrigation water availability in the northern lake states and other regions with strong groundwater-surface water connections is tied to concerns for surface water health, requiring a focus on managing the upper few meters of aquifers on which surface waters depend rather than the depletability of an aquifer.

Late Pleistocene dune construction in the Central Sand Plain of Wisconsin, USA

Wisconsin's Central Sand Plain east of the Wisconsin River is composed of eolian sand forming high-relief dunes surrounded by sand sheets and scattered low-relief dunes. To establish a maximum age for dune formation, three samples for optical dating were taken from glacial Lake Wisconsin lacustrine sediment that underlies eolian sand. These age estimates range from 19.3 to 13.6ka. Age estimates taken from within or at the base of the dunes range from 14.0 to 10.6ka. Samples taken from < 2m of the ground surface were slightly younger, indicating dunes were stabilized between 11.8 and 5.5ka. The younger ages near the surface of some dunes were most likely the result of pedoturbation or localized problems with applying the optical dating method. The majority of the optical age estimates from dunes (18 of 21) indicated that most of the dunes were active between 14 and 10ka and that most dune activity ended by 10ka. These ages suggest that localized activity on dune crests may have occurred in the Holocene but would have been limited to < 1m of sand accumulation. The timing of dune activity and the lack of any significant Holocene reactivation suggest that dune activation in this setting cannot be attributed solely to changes in aridity. Instead, we attribute dune formation to changes in sediment availability from either sand inputs from the Wisconsin River or the melting of permafrost.

Nitrate impacts on groundwater from irrigated-vegetable systems in a humid north-central US sand plain

Groundwater is frequently susceptible to nitrate pollution in irrigated regions possessing a humid climate, coarse soil, and shallow water table. Such pollution degrades local drinking water resources and increases watershed nitrate export. The irrigated-vegetable production agroecosystem of the Wisconsin Central Sand Plain (WCSP), north-central USA, exemplifies the problem. This study’s research goal was to assess groundwater nitrate loading in this agroecosystem, with a view to manage groundwater quality and nitrate export in the WCSP and similar regions. Nitrate loading was measured beneath a 44 ha field for 4 years using a novel “water-year” method, during three crops of sweet corn ( Zea mays L.) and one of potato ( Solanum tuberosum L.). Nitrate-N concentrations averaged 20 mg l −1 in shallow (upper 3 m) groundwater beneath the study field. Nitrate-N loading from the sweet corn ranged from 126 to 169 kg ha −1 per year; loading from potato was 228 kg ha −1 per year. Groundwater loading amounted to 61% of total available N and 77% of fertilizer N. Measured loadings compared well with those calculated using a budget approach, supporting the validity of both methods. N budgets were calculated from average regional harvests and two fertilizer rates: a typical grower rate, and the more conservative University Extension rate. Budget-derived nitrate-N loadings from sweet corn are 151 kg ha −1 per year (typical) and 119 kg ha −1 per year (University Extension). For potato, typical and recommended fertilizer rates are equal, and the budget-derived nitrate-N loading is 203 kg ha −1 per year. Budget-derived loadings imply that limiting the basin-averaged nitrate-N concentration in groundwater to 10 mg l −1 (the US drinking water standard) would require each irrigated-vegetable hectare to be offset by 4.5–6.5 ha of land supplying nitrate-free groundwater recharge. Further, a WCSP watershed with half irrigated-vegetable cover, even with no other nitrate sources, would export 50–74 kg ha −1 per year of nitrate-N. This export is greater on a per-hectare basis than the large maize- and soybean-producing basins of Iowa, USA. Most orthodox nitrate control strategies (e.g., decreasing and splitting fertilizer, irrigation scheduling) have already been implemented in the WCSP, and have little further potential to substantially reduce loading if profitability must be maximized. Better control will require new and unconventional approaches for controlling nitrate leaching from mineralized crop residue as well as additional curbing of direct fertilizer nitrogen leaching.

Paper mill residuals and compost effects on soil carbon and physical properties.

Use of organic by-products as soil amendments in agricultural production exemplifies a strategy for converting wastes to resources. The overall objective of this research was to evaluate the short- and intermediate-term effects of repeatedly amending sandy soil with paper mill residuals (PMR) and composted PMR in a vegetable rotation in Wisconsin's Central Sands. Specifically, we investigated the effects of PMR and composted PMR on total soil C and related these to changes in water-holding capacity and plant-available water (PAW). Amendment effects on irrigation requirements were estimated with a simple soil water balance model. The experimental design was replicated five times as a randomized complete block with four organic amendments: raw PMR, PMR composted alone (PMRC), PMR composted with bark (PMRB), and peat applied at two rates and a non-amended control. All amended treatments significantly increased total soil C relative to the nonamended control following applications in 1998 and 1999. One year following the second serial amendment, all PMR treatments increased PAW by 5 to 45% relative to the control. There was a significant positive linear relationship between total soil C and PAW. All amended treatments reduced the average amount of irrigation water required for potato production by 4 to 30% and the number of irrigation events by 10 to 90%. There was a clear trend of greater reduction in irrigation requirements with more carbon added. The cumulative effects of repeated additions of PMRB suggest that certain composts might sustain elevated PAW and reduce irrigation requirements beyond one year.

Nitrate and chloride loading to groundwater from an irrigated north-central U.S. sand-plain vegatable field.

Groundwater pollution and associated effects on drinking water have increased with the expansion of irrigated agriculture in north-central U.S. sand plains. Controlling this pollution requires an ability to measure and predict pollutant loading by specific agricultural systems. We measured NO3 and Cl loading to groundwater beneath a Wisconsin central sand plain irrigated vegetable field using both a budget method and a new monitoring-based method. By relying on frequent monitoring of shallow groundwater, the new method overcomes some limitations of other methods. Monitoring-based and budget methods agreed well, and indicated that loading to groundwater was 165 kg ha(-1) NO3-N and 111 kg ha(-1) Cl for sweet corn (Zea mays L.) in 1992, and 228 kg ha(-1) NO3-N and 366 kg ha(-1) Cl for potato (Solanum tuberosum L.) in 1993. Nitrate N loading was 56 to 60% of available N, or 66 to 70% of fertilizer N. Sweet corn NO3 loading was about typical for this region, but potato NO3 loading was probably 50% greater than typical because heavy rains provoked extra fertilizer application. Our results imply that typical NO3-N loading would be 119 kg ha(-1) for sweet corn and 203 kg ha(-1) for potato, even with strict adherence to University Extension fertilizer recommendations. To keep average groundwater NO3-N within the 10 mg L(-1) U.S. drinking water standard, each irrigated vegetable field would need to be offset by five to eight times as much land supplying NO3-free groundwater recharge.

Groundwater Quality Beneath Irrigated Vegetable Fields in a North‐Central U.S. Sand Plain

AbstractThe dramatic expansion of irrigated agriculture since about 1970 in the north‐central USA has been accompanied by NO3 and pesticide pollution of groundwater. The expansion has been concentrated in areas with sandy soils and shallow water tables, such as the Wisconsin central sand plain. In some parts of this sand plain, most wells contain detectable pesticide residues and NO3‐N concentrations that exceed the 10 mg L−1 U.S. drinking water maximum contaminant level (MCL). To evaluate the effects on groundwater quality of this agricultural system, we monitored solutes 23 times during a 2‐yr period in the upper 3 m of the aquifer beneath and immediately upgradient of four irrigated vegetable fields. Groundwater beneath fields had significantly greater concentrations of most solutes and lower pH than upgradient groundwater. Especially pronounced were Ca, Cl, K, Mg, and NO3 differences, with concentrations 5 to 26 times greater under fields. Nitrate N concentrations averaged 21 mg L−1 under fields, compared with 1 mg L−1 upgradient. Pesticide residues were ubiquitous beneath fields, and generally persisted for many months after application. Pesticide concentrations often exceeded Wisconsin preventive‐action limits (PALs), but seldom exceeded federal MCLs. Even when agricultural management approximated best management practice (BMP) recommendations, the NO3 concentration beneath these fields approached double the MCL, indicating a need for new approaches to control agricultural groundwater pollution.

Impacts of Irrigated Vegetable Agriculture on a Humid North‐Central U.S. Sand Plain Aquifer

AbstractWe determined the impacts of irrigated vegetable agriculture on ground water quality in a 29 km2 portion of the Wisconsin central sand plain (WCSP). Vegetable fields cover 22% of the study area and are used for growing potatoes, sweet corn, snap beans, peas, field corn, and soybeans. We found that contaminant plumes from fields underlay about 54% of the area. Plumes were 3 to &gt;16 m thick and in places occupied the entire aquifer thickness. Impacted ground water retained the chemical signature observed under fields, and compared with unimpacted ground water, had elevated NO3‐N (median 13.7 mg L−1 impacted vs. 0.5 for unimpacted) and Cl (median 23 vs. 2.0 mg L−1) and 1.4 to 2.5 times the Ca, Mg, K, Na, and SO4. NO3‐N detections exceeded 10 mg L−1 in 58% of impacted monitoring wells compared with none of the unimpacted wells. Evidence of denitrification was observed, but rarely. Residues of the pesticides alachlor, atrazine, carbofuran, metolachlor, and metribuzin were detected, but usually at summed concentrations of &lt;1 μg L−1 outside of fields. The maximum observed sum of pesticide residues was 8 μg L−1. Not all potential pesticide residues were detectable by the analytical method.Agricultural impacts limit the aquifer's value as a source of potable water, because wells are unable to tap unimpacted ground water through much of the area. Results of this study are generally applicable to other humid sandy areas in the north‐central United States and elsewhere. Impacts might be more severe elsewhere because the agricultural land use is frequently denser and ground water flowpaths are longer.