Long-term effects of nitrogen fertilizer use on ground water nitrate in two small watersheds

Changes in agricultural management can minimize NO3-N leaching, but then the time needed to improve ground water quality is uncertain. A study was conducted in two first-order watersheds (30 and 34 ha) in Iowa's Loess Hills. Both were managed in continuous corn (Zea mays L.) from 1964 through 1995 with similar N fertilizer applications (average 178 kg ha(-1) yr(-1)), except one received applications averaging 446 kg N ha(-1) yr(-1) between 1969 and 1974. This study determined if NO3-N from these large applications could persist in ground water and baseflow, and affect comparison between new crop rotations implemented in 1996. Piezometer nests were installed and deep cores collected in 1996, then ground water levels and NO3-N concentrations were monitored. Tritium and stable isotopes (2H, 18O) were determined on 33 water samples in 2001. Baseflow from the heavily N-fertilized watershed had larger average NO3-N concentrations, by 8 mg L(-1). Time-of-travel calculations and tritium data showed ground water resides in these watersheds for decades. "Bomb-peak" precipitation (1963-1980) most influenced tritium concentrations near lower slope positions, while deep ground water was dominantly pre-1953 precipitation. Near the stream, greater recharge and mixed-age ground water was suggested by stable isotope and tritium data, respectively. Using sediment-core data collected from the deep unsaturated zone between 1972 and 1996, the increasing depth of a NO3-N pulse was related to cumulative baseflow (r2 = 0.98), suggesting slow downward movement of NO3-N since the first experiment. Management changes implemented in 1996 will take years to fully influence ground water NO3-N. Determining ground water quality responses to new agricultural practices may take decades in some watersheds.     

In situ bioreactors and deep drain-pipe installation to reduce nitrate losses in artificially drained fields

Nitrate in water removed from fields by subsurface drain ('tile') systems is often at concentrations exceeding the 10 mg N L(-1) maximum contaminant level (MCL) set by the USEPA for drinking water and has been implicated in contributing to the hypoxia problem within the northern Gulf of Mexico. Because previous research shows that N fertilizer management alone is not sufficient for reducing NO(3) concentrations in subsurface drainage below the MCL, additional approaches are needed. In this field study, we compared the NO(3) losses in tile drainage from a conventional drainage system (CN) consisting of a free-flowing pipe installed 1.2 m below the soil surface to losses in tile drainage from two alternative drainage designs. The alternative treatments were a deep tile (DT), where the tile drain was installed 0.6 m deeper than the conventional tile depth, but with the outlet maintained at 1.2 m, and a denitrification wall (DW), where trenches excavated parallel to the tile and filled with woodchips serve as additional carbon sources to increase denitrification. Four replicate 30.5- by 42.7-m field plots were installed for each treatment in 1999 and a corn-soybean rotation initiated in 2000. Over 5 yr (2001-2005) the tile flow from the DW treatment had annual average NO(3) concentrations significantly lower than the CN treatment (8.8 vs. 22.1 mg N L(-1)). This represented an annual reduction in NO(3) mass loss of 29 kg N ha(-1) or a 55% reduction in nitrate mass lost in tile drainage for the DW treatment. The DT treatment did not consistently lower NO(3) concentrations, nor reduce the annual NO(3) mass loss in drainage. The DT treatment did exhibit lower NO(3) concentrations in tile drainage than the CN treatment during late summer when tile flow rates were minimal. There was no difference in crop yields for any of the treatments. Thus, denitrification walls are able to substantially reduce NO(3) concentrations in tile drainage for at least 5 yr.     

Hydrological variability and agricultural drainage ditch inorganic nitrogen reduction capacity

The application of inorganic nitrogen fertilizers on agricultural landscapes has the potential to generate concerns of environmental degradation at fine to coarse scales across the catchment and landscape. Inorganic nitrogen species (NO3*, NO2*, and NH3) are typically associated with subsurface flow processes; however, surface runoff from rainfall events in no-till agriculture with inorganic surface fertilizers might contribute to downstream eutrophication. Inorganic nitrogen reduction capacity of agricultural drainage ditches under no-till cotton was determined under natural, variable rainfall conditions in northern Mississippi. Monthly grab baseflow samples and storm-generated flow samples were variably sampled temporally within two experimental farm ditches over 2 yr. Inorganic nitrogen concentrations, in conjunction with Manning's equation and Natural Resources Conservation Service dimensionless hydrographs, provided individual water volumes per storm event and thus maximum effluent and outflow nitrogen loads. Base and stormflow regression results indicate drainage ditches reducing NO3* and NH3 over the length of the ditch for growing and dormant seasons. Overall, maximum storm loads of dissolved inorganic nitrogen (DIN) from the farm over the 2-yr sampling period accounted for 2.2% of the initial fertilizer application, of which 1.1% left the ditch (0.84 kg ha(-1) yr(-1)) (a 57% ditch reduction of DIN load over 2 yr). Long-term sampling incorporating data on application and loss of fertilizers and farm management will provide critical information for farmers and scientists on the potential of economic gains and downstream ecosystem eutrophication, respectively.     

Pesticide fate and transport throughout unsaturated zones in five agricultural settings, USA

Pesticide transport through the unsaturated zone is a function of chemical and soil characteristics, application, and water recharge rate. The fate and transport of 82 pesticides and degradates were investigated at five different agricultural sites. Atrazine and metolachlor, as well as several of the degradates of atrazine, metolachlor, acetochlor, and alachlor, were frequently detected in soil water during the 2004 growing season, and degradates were generally more abundant than parent compounds. Metolachlor and atrazine were applied at a Nebraska site the same year as sampling, and focused recharge coupled with the short time since application resulted in their movement in the unsaturated zone 9 m below the surface. At other sites where the herbicides were applied 1 to 2 yr before sampling, only degradates were found in soil water. Transformations of herbicides were evident with depth and during the 4-mo sampling time and reflected the faster degradation of metolachlor oxanilic acid and persistence of metolachor ethanesulfonic acid. The fraction of metolachlor ethanesulfonic acid relative to metolachlor and metolachlor oxanilic acid increased from 0.3 to >0.9 at a site in Maryland where the unsaturated zone was 5 m deep and from 0.3 to 0.5 at the shallowest depth. The flux of pesticide degradates from the deepest sites to the shallow ground water was greatest (3.0-4.9 micromol m(-2) yr(-1)) where upland recharge or focused flow moved the most water through the unsaturated zone. Flux estimates based on estimated recharge rates and measured concentrations were in agreement with fluxes estimated using an unsaturated-zone computer model (LEACHM).     

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.     

Surface water and groundwater nitrogen dynamics in a well drained riparian forest within a poorly drained agricultural landscape

The effectiveness of riparian zones in mitigating nutrient in ground and surface water depends on the climate, management, and hydrogeomorphology of a site. The purpose of this study was to determine the efficacy of a well drained, mixed-deciduous riparian forest to buffer a river from N originating from a poorly drained grass seed cropping system. The study site was adjacent to the Calapooia River in the Willamette Valley, Oregon. Water was found to move from the rapid drainage of swale surface water. During winter hydrological events, the riparian forest also received river water. Low nitrate (NO3-) concentrations (0.2-0.4 mg NO3- -NL(-1)) in the shallow groundwater of the cropping system were associated with low rates of mineralization and nitrification (33 kg N ha(-1) yr(-1)) and high grass seed crop uptake of N (155 kg N ha(-1) yr(-1)). The riparian forest soil had higher rates of mineralization (117 kg N ha(-1) yr(-1)) that produced quantities of soil N that were within the range of literature values for plant uptake, leading to relatively low concentrations of shallow groundwater NO3 (0.6-1.8 mg NO3- -NL(-1)). The swale that dissected the cropping system and riparian area was found to have the highest rates of denitrification and to contribute dissolved organic C to the river. Given the dynamic nature of the hydrology of the Calapooia River study site, data suggest that the riparian forest plays a role not only in reducing export of NO3- from the cropping system to the river but also in processing nutrients from river water.     

Multiple-year water balance of soil covers in a semiarid setting

Surface covers are used to isolate contaminants in hazardous and low-level radioactive sites for time frames ranging from hundreds of years to millennia or more. In the absence of data for such durations, the long-term performance of surface barriers can only be represented with short-term tests or inferred from analogs and modeling. This paper provides evidence of field performance of soil covers for periods up to 17 yr. The results of lysimeter studies from a semiarid site in Washington State show that a cover design known as the Hanford Barrier, which consists of 1.5 m of silt loam above a sand-gravel capillary break, can nearly eliminate drainage. The results were similar if plants were present or not, demonstrating the robustness of the design. Furthermore, reducing the silt loam thickness to 1.0 m (as might occur via erosion), with or without plants, did not lead to drainage. When irrigated to mimic 3x average precipitation conditions, the vegetated Hanford Barrier continued to prevent drainage. Overall, the results showed no loss in performance during the 17 yr of testing. Only when plants were eliminated completely from the 3x precipitation test did drainage occur (rates ranged from 6 to 16 mm yr(-1)). In a separate test, replacing the top 0.2 m of silt loam with dune sand and reducing the plant cover did not lead immediately to the onset of drainage, but soil matric heads within the silt loam noticeably increased. This observation suggests that dune sand migration onto a surface cover has the potential to reduce a cover's ability to minimize deep drainage.     

Movement of lagoon-liquor constituents below four animal-waste lagoons

Movement of liquor constituents from animal-waste lagoons has the potential to degrade ground water quality. The depth of movement and concentrations of lagoon-liquor constituents in the soil underlying three cattle (Bos taurus)-waste retention lagoons and one swine (Sus scrofa)-waste lagoon were determined. Samples were taken by using a direct-push coring machine, dissected by depth, and analyzed for total N, organic C, CaCO3, pH, cation exchange capacity (CEC), texture, and extractable NO3, NH(4), P, Cl, Ca, Mg, K, and Na. Ammonium N concentrations were greatest in the upper 0.5 m of soil under all four lagoons with concentrations ranging from 94 to 1139 mg kg(-1). Organic N was determined to make up between 39 and 74% of the total N beneath all lagoons. The swine lagoon had 2.4 kg N m(-2) in the underlying soil whereas the cattle lagoon with highest quantity of N had 1.2 kg N m(-2) in the underlying soil. Although N concentrations decreased with depth, N was greater than expected background levels at the bottom of some cores, indicating that the sampling efforts did not reach the bottom of the N plume. Nitrate N concentrations were generally less than 5 mg kg(-1) immediately below the lagoon floor. In the uppermost 0.5 m of soil underlying the swine and three cattle lagoons, NH4+ occupied 44% and between 1 and 22% of the soil cation exchange sites, respectively. The depth of movement of N under these lagoons, as much as 4 m, may pose remediation difficulties at lagoon closure.     

Nitrate exported in drainage waters of two sprinkler-irrigated watersheds

Nitrate contamination of surface waters has been linked to irrigated agriculture across the world. We determined the NO3-N loads in the drainage waters of two sprinkler-irrigated watersheds located in the Ebro River basin (Spain) and their relationship to irrigation and N management. Crop water requirements, irrigation, N fertilization, and the volume and NO3-N concentration of drainage waters were measured or estimated during two-year (Watershed A; 494 irrigated ha) and one-year (Watershed B; 470 irrigated ha) study periods. Maize (Zea mays L.) and alfalfa (Medicago sativa L.) were grown in 40 to 60% and 15 to 33% of the irrigated areas, respectively. The seasonal irrigation performance index (IPI) ranged from 92 to 100%, indicating high-quality management of irrigation. However, the IPI varied among fields and overirrigation occurred in 17 to 44% of the area. Soil and maize stalk nitrate contents measured at harvest indicated that N fertilizer rates could be decreased. Drainage flows were 68 mm yr(-1) in Watershed A and 194 mm yr(-1) in Watershed B. Drainage NO3-N concentrations were independent of drainage flows and similar in the irrigated and nonirrigated periods (average: 23-29 mg L(-1)). Drainage flows determined the exported mass of NO3-N, which varied from 18 (Watershed A) to 49 (Watershed B) kg ha(-1) yr(-1), representing 8 (Watershed A) and 22% (Watershed B) of the applied fertilizer plus manure N. High-quality irrigation management coupled to the split application of N through the sprinkler systems allowed a reasonable compromise between profitability and reduced N pollution in irrigation return flows.     

Transport and fate of nitrate and pesticides: hydrogeology and riparian zone processes

There is continuing concern over potential impacts of widespread application of nutrients and pesticides on ground- and surface-water quality. Transport and fate of nitrate and pesticides were investigated in a shallow aquifer and adjacent stream, Cow Castle Creek, in Orangeburg County, South Carolina. Pesticide and pesticide degradate concentrations were detected in ground water with greatest frequency and largest concentrations directly beneath and downgradient from the corn (Zea mays L.) field where they were applied. In almost all samples in which they were detected, concentrations of pesticide degradates greatly exceeded those of parent compounds, and were still present in ground waters that were recharged during the previous 18 yr. The absence of both parent and degradate compounds in samples collected from deeper in the aquifer suggests that this persistence is limited or that the ground water had recharged before use of the pesticide. Concentrations of NO(-)(3) in ground water decreased with increasing depth and age, but denitrification was not a dominant controlling factor. Hydrologic and chemical data indicated that ground water discharges to the creek and chemical exchange takes place within the upper 0.7 m of the streambed. Ground water had its greatest influence on surface-water chemistry during low-flow periods, causing a decrease in concentrations of Cl(-), NO(-)(3), pesticides, and pesticide degradates. Conversely, shallow subsurface drainage dominates stream chemistry during high-flow periods, increasing stream concentrations of Cl(-), NO(-)(3), pesticides, and pesticide degradates. These results point out the importance of understanding the hydrogeologic setting when investigating transport and fate of contaminants in ground water and surface water.     

Mitigation of shallow groundwater nitrate in a poorly drained riparian area and adjacent cropland

Riparian ecosystems, through their unique position in the agricultural landscape and ability to influence nutrient cycles, can potentially reduce NO3 loading to surface and ground waters. The purpose of this study was to determine the fate of NO3 in shallow groundwater moving along a lateral flowpath from a grass seed cropping system through an undisturbed mixed-species herbaceous riparian area. Soil A (30-45 cm) and C horizon (135-150 cm) NO3, dissolved oxygen, and nitrous oxide concentrations were significantly higher in the cropping system than the adjacent riparian area. Nitrate concentrations in both horizons of the riparian soil were consistently at or below 0.05 mg N L(-1) while cropping system concentrations ranged from 1 to 12 mg N L(-1). Chloride data suggested that NO3 dilution occurred from recharge by precipitation. However, a sharp decrease in NO3/Cl ratios as water moved into the riparian area indicated that additional dilution of NO3 concentrations was unlikely. Riparian area A horizon soil water had higher dissolved organic carbon than the cropping system and when the riparian soil became saturated, available electron acceptors (O2, NO3) were rapidly reduced. Dissolved inorganic carbon was significantly higher in the riparian area than the cropping system for both horizons indicating high biological activity. Carbon limitation in the cropping system may have led to microbial respiration using primarily O2 and to a lesser degree NO3. Within 6 m of the riparian/cropping system transition, NO3 was virtually undetectable.     

Nitrate retention in riparian ground water at natural and elevated nitrate levels in north central Minnesota

The relationship between local ground water flows and NO(3)(-) transport to the channel was examined in three well transects from a natural, wooded riparian zone adjacent to the Shingobee River, MN. The hillslope ground water originated as recharge from intermittently grazed pasture up slope of the site. In the hillslope transect perpendicular to the stream, ground water NO(3)(-) concentrations decreased from approximately 3 mg N L(-1) beneath the ridge (80 m from the channel) to 0.01 to 1.0 mg N L(-1) at wells 1 to 3 m from the channel. The Cl(-) concentrations and NO(3)/Cl ratios decreased toward the channel indicating NO(3)(-) dilution and biotic retention. In the bankside well transect parallel to the stream, two distinct ground water environments were observed: an alluvial environment upstream of a relict beaver dam influenced by stream water and a hillslope environment downstream of the relict beaver dam. Nitrate was elevated to levels representative of agricultural runoff in a third well transect located approximately 5 m from the stream to assess the effectiveness of the riparian zone as a NO(3)(-) sink. Subsurface NO(3)(-) injections revealed transport of up to 15 mg N L(-1) was nearly conservative in the alluvial riparian environment. Addition of glucose stimulated dissolved oxygen uptake and promoted NO(3)(-) retention under both background and elevated NO(3)(-) levels in summer and winter. Disappearance of added NO(3)(-) was followed by transient NO(2)(-) formation and, in the presence of C(2)H(2), by N(2)O formation, demonstrating potential denitrification. Under current land use, most NO(3)(-) associated with local ground water is biotically retained or diluted before reaching the channel. However, elevating NO(3)(-) levels through agricultural cultivation would likely result in increased NO(3)(-) transport to the channel.     

Nitrate removal in riparian wetlands: interactions between surface flow and soils

Riparian wetlands containing springs are thought to be ineffective at removing nitrate because contact times between the upwelled ground water and the underlying microbially active soils are short. Tracer experiments using lithium bromide (LiBr) and nitrate (NO3-N) injected at the surface were used to quantify residence times and NO3-N removal in a riparian swale characteristic of New Zealand hill-country pasture. An experimental enclosure was used with collecting trays at the downstream end to measure flow and concentration, shallow wells to measure subsurface concentrations, and an array of logging conductivity probes to monitor tracer continuously. The majority of added tracer reached the outlet more slowly than could be explained by surface flow, but more quickly than could be explained by Darcy seepage flow. There was evidence from the wells of tracer diffusing vertically to a depth of at least 5 cm into the surface soil layer, which was permanently saturated and highly porous. During dry weather 24 +/- 9% of added NO3-N was removed over a distance of 1.5 m largely by denitrification. The net uptake length coefficient for this wetland (K = 0.08 +/- 0.03 m(-1)) is slightly higher than the range (K = 0.01-0.07 m(-1)) measured in a small stream channel infested with macrophytes. Nitrate removal is expected to decrease with increasing flow. Seepage flow is estimated to have removed only 7 +/- 4% of the added NO3-N and we hypothesize that vertical diffusion substantially increases NO3-N removal in this type of wetland. Riparian wetlands with springs and surface flows should not be dismissed as having low NO3-N removal potential without checking whether there is significant vertical mixing.     

Dairy slurry application method impacts ammonia emission and nitrate in no-till corn silage

Reducing ammonia (NH3) emissions through slurry incorporation or other soil management techniques may increase nitrate (NO3) leaching, so quantifying potential losses from these alternative pathways is essential to improving slurry N management. Slurry N losses, as NH3 or NO3 were evaluated over 4 yr in south-central Wisconsin. Slurry (i.e., dairy cow [Bos taurus] manure from a storage pit) was applied each spring at a single rate (-75 m3 ha(-1)) in one of three ways: surface broadcast (SURF), surface broadcast followed by partial incorporation using an aerator implement (AER-INC), and injection (INJ). Ammonia emissions were measured during the 120 h following slurry application using chambers, and NO3 leaching was monitored in drainage lysimeters. Yield and N3 uptake of oat (Avena sativa L.), corn (Zea mays L.), and winter rye (Secale cereale L.) were measured each year, and at trial's end soils were sampled in 15- to 30-cm increments to 90-cm depth. There were significant tradeoffs in slurry N loss among pathways: annual mean NH3-N emission across all treatments was 5.3, 38.3, 12.4, and 21.8 kg ha(-1) and annual mean NO3-N leaching across all treatments was 24.1, 0.9, 16.9, and 7.3 kg ha' during Years 1, 2, 3, and 4, respectively. Slurry N loss amounted to 27.1% of applied N from the SURF treatment (20.5% as NH3-N and 6.6% as NO,-N), 23.3% from AER-INC (12.0% as NH3-N and 11.3% as NO3-N), and 9.19% from INJ (4.4% as NH3-N and 4.7% as NO3-N). Although slurry incorporation decreased slurry N loss, the conserved slurry N did not significantly impact crop yield, crop N uptake or soil properties at trial's end.     

Field water balance of landfill final covers

Landfill covers are critical to waste containment, yet field performance of specific cover designs has not been well documented and seldom been compared in side-by-side testing. A study was conducted to assess the ability of landfill final covers to control percolation into underlying waste. Conventional covers employing resistive barriers as well as alternative covers relying on water-storage principles were monitored in large (10 x 20 m), instrumented drainage lysimeters over a range of climates at 11 field sites in the United States. Surface runoff was a small fraction of the water balance (0-10%, 4% on average) and was nearly insensitive to the cover slope, cover design, or climate. Lateral drainage from internal drainage layers was also a small fraction of the water balance (0-5.0%, 2.0% on average). Average percolation rates for the conventional covers with composite barriers (geomembrane over fine soil) typically were less than 12 mm/yr (1.4% of precipitation) at humid locations and 1.5 mm/yr (0.4% of precipitation) at arid, semiarid, and subhumid locations. Average percolation rates for conventional covers with soil barriers in humid climates were between 52 and 195 mm/yr (6-17% of precipitation), probably due to preferential flow through defects in the soil barrier. Average percolation rates for alternative covers ranged between 33 and 160 mm/yr (6 and 18% of precipitation) in humid climates and generally less than 2.2 mm/yr (0.4% of precipitation) in arid, semiarid, and subhumid climates. One-half (five) of the alternative covers in arid, semiarid, and subhumid climates transmitted less than 0.1 mm of percolation, but two transmitted much more percolation (26.8 and 52 mm) than anticipated during design. The data collected support conclusions from other studies that detailed, site-specific design procedures are very important for successful performance of alternative landfill covers.     

Aluminum output fluxes from forest ecosystems in Europe: a regional assessment

Data from 89 forested catchments and plots across Europe were used to define empirical relationships between aluminum leaching and input fluxes of major ions, output fluxes of major ions, ecosystem parameters such as soil pH, and combinations of these. Forests that release dissolved Al to seepage or surface waters are located primarily in areas receiving the highest loading of acid rain, and the output flux of Al shows the highest correlations to the throughfall flux of inorganic nitrogen, the output fluxes of NO3-, H+, and SO4(2-), and the mineral soil pH. If the speciation of Al is taken to be Al3+ (an overestimate), Al is released in a nearly 1:1 molar charge ratio with the sum of NO3- and SO4(2-) in runoff or seepage water over a wide range of basepoor bedrock types and acid deposition across Europe. The empirical data point to a threshold range of N deposition of 80 to 150 mmolc N m(-2) yr(-1) and a (less clearly defined) range of S deposition of 100 to 200 mmolc SO4(-2) m(-2) yr(-1) above which Al released from forests exceeds 100 mmolc Al m(-2) yr(-1). Within this threshold range, the sites that release little or no dissolved Al are those that continue to assimilate input N and/or have high soil pH (>4.5).     

Micro-trench experiments on interflow and lateral pesticide transport in a sloped soil in northern Thailand

During recent decades, a change in land use in the mountainous regions of Northern Thailand has been accompanied by an increased input of agrochemicals. We identified lateral water flow and pesticide transport pathways and mechanisms in a Hapludult on a sloped litchi orchard in Northern Thailand. During two rainy seasons, two micro-trench experiments were performed at the plot scale (2 by 3 m). The first experiment was performed at the footslope of the orchard; the second was performed at a midslope position. Two salt tracers (bromide and chloride) and two pesticides {methomyl [S-methyl-N-(methylcarbamoyloxy)thioacetimidate] and chlorothalonil (2,4,5,6-Tetrachlor-1,3-benzdicarbonitril)} were applied in stripes parallel to the slope 150 and 300 cm away from the trench. At the trench, soil water was collected by wick samplers. Tensiometers and time-domain reflectometry probes were installed. At the end of the experiment, soil samples were taken and analyzed for residual concentrations of tracers and pesticides. Lateral subsurface flow of water occurred exclusively along preferential flow paths and was mainly observed at 0- to 30- and 60- to 90-cm depth. Lateral transport of pesticides was negligible, but both pesticides were found beneath the application area at 90 cm depth. Therefore, they may pose a groundwater contamination risk. The amount of wick flow and the location of interflow were mainly a function of rain amount and antecedent soil water suction. During dry periods, water flow was restricted to the topsoil. After heavy rain events and wet periods, interflow was mainly observed in the subsoil. The cumulative rain amount between samplings necessary to induce interflow was 20 mm. At the footslope, the interflow was seven times higher, and the network of water-bearing pores increased compared with the midslope position.     

MODELING EFFECTS OF BRUSH MANAGEMENT ON THE RANGELAND WATER BUDGET: EDWARDS PLATEAU,TEXAS1

ABSTRACT: The Soil and Water Assessment Tool (SWAT) was used to evaluate the influence of woody plants on water budgets of semi‐arid rangelands in karst terrain. The model was configured for the hydrologic evaluation of the North Fork of the Upper Guadalupe River watershed and was calibrated and verified using measured flow data. Nash and Sutcliffe fit efficiencies for daily and monthly verification periods were 0.09 and 0.50, respectively. Streamflow, baseflow, and evapotranspiration (ET) rates were comparable to published field data. Simulated deep recharge was considerably higher than the published values for the Edwards Plateau. The model was then used to simulate conditions with existing brush cover and four different brush removal scenarios. Scenarios were created to represent existing brush and the removal of brush from only locations that were either covered by heavy brush, were on a moderate slope, or were in shallow soils. Resulting data was compared to previous studies of both field experiments and model simulations. Maximum brush removal resulted in a reduction in ET equal to 31.94 mm/yr depth over the entire basin, or 46.62 mm/yr depth over the treated area. Removal of heavy brush cover resulted in the greatest changes in evapotranspiration, surface runoff, base‐flow, and deep recharge. Slope was found to have the greatest effect on lateral subsurface flow.     

Tillage erosion and its effect on soil properties and crop yield in Denmark

Tillage erosion had been identified as a major process of soil redistribution on sloping arable land. The objectives of our study were to investigate the extent of tillage erosion and its effect on soil quality and productivity under Danish conditions. Soil samples were collected to a 0.45-m depth on a regular grid from a 1.9-ha site and analyzed for 137Cs inventories, as a measure of soil redistribution, soil texture, soil organic carbon (SOC) contents, and phosphorus (P) contents. Grain yield was determined at the same sampling points. Substantial soil redistribution had occurred during the past decades, mainly due to tillage. Average tillage erosion rates of 2.7 kg m(-2) yr(-1) occurred on the shoulderslopes, while deposition amounted to 1.2 kg m(-2) yr(-1) on foot- and toeslopes. The pattern of soil redistribution could not be explained by water erosion. Soil organic carbon and P contents in soil profiles increased from the shoulder- toward the toeslopes. Tillage translocation rates were strongly correlated with SOC contents, A-horizon depth, and P contents. Thus, tillage erosion had led to truncated soils on shoulderslopes and deep, colluvial soils on the foot- and toeslopes, substantially affecting within-field variability of soil properties. We concluded that tillage erosion has important implications for SOC dynamics on hummocky land and increases the risk for nutrient losses by overland flow and leaching. Despite the occurrence of deep soils across the study area, evidence suggested that crop productivity was affected by tillage-induced soil redistribution. However, tillage erosion effects on crop yield were confounded by topography-yield relationships.     

Artificial Groundwater Recharge Zones Mapping Using Remote Sensing and GIS: A Case Study in Indian Punjab

Artificial groundwater recharge plays a vital role in sustainable management of groundwater resources. The present study was carried out to identify the artificial groundwater recharge zones in Bist Doab basin of Indian Punjab using remote sensing and geographical information system (GIS) for augmenting groundwater resources. The study area has been facing severe water scarcity due to intensive agriculture for the past few years. The thematic layers considered in the present study are: geomorphology (2004), geology (2004), land use/land cover (2008), drainage density, slope, soil texture (2000), aquifer transmissivity, and specific yield. Different themes and related features were assigned proper weights based on their relative contribution to groundwater recharge. Normalized weights were computed using the Saaty’s analytic hierarchy process. Thematic layers were integrated in ArcGIS for delineation of artificial groundwater recharge zones. The recharge map thus obtained was divided into four zones (poor, moderate, good, and very good) based on their influence to groundwater recharge. Results indicate that 15, 18, 37, and 30 % of the study area falls under “poor,” “moderate,” “good,” and “very good” groundwater recharge zones, respectively. The highest recharge potential area is located towards western and parts of middle region because of high infiltration rates caused due to the distribution of flood plains, alluvial plain, and agricultural land. The least effective recharge potential is in the eastern and middle parts of the study area due to low infiltration rate. The results of the study can be used to formulate an efficient groundwater management plan for sustainable utilization of limited groundwater resources.