South-central Nebraska Research Articles

Agronomic and economic evaluation of various furrow irrigation strategies for corn production under limited water supply

It is commonly known that furrow irrigation is a less efficient method of irrigation than a sprinkler center pivot system, but many fields9 irregular shapes prevent the use of center pivot irrigation. Restricted water allocations of surface and subsurface water are forcing farmers to implement irrigation strategies that will reduce water application on furrow-irrigated fields. The study site was located in south central Nebraska. The objective of this study was to evaluate two irrigation scheduling scenarios: (1) the every-furrow irrigation method with 50% (0.5), 70% (0.7), and 90% (0.9) field capacity (FC) treatments and (2) the every-furrow (EF) irrigation method compared to the every-other-furrow (EOF) method with both using the 75% field capacity (0.75 FC) treatment effects on corn (Zea mays L.) yield, net economic return, and residual soil nitrate-nitrogen. The experimental design was a randomized complete block with three replications. Grain yield showed no significant difference in both years for all irrigation treatments. Irrigation water application with the 0.5 FC strategy reduced the amount of applied water by approximately 70% and 200% compared to 0.7 FC and 0.9 FC, respectively, in both years. The water savings with the EOF method over the EF method was 23%. The economic return with 0.5 FC was 6% to 13% and 36% to 69% over 0.7 and 0.9 FC irrigation treatments, respectively. The 0.5 FC strategy showed no significant reduction in nitrate-nitrogen loss over 0.7 FC and 0.9 FC, while the EOF method reduced soil nitrate-nitrogen loss by 11% to 26% over the EF irrigation method in both years. The average economic return over two years with the 0.5 FC strategy was 9.5% and 52.5% over 0.7 FC and 0.9 FC irrigation treatments, respectively, while the average economic return with the EOF method over the EF method was 9.5%. Findings demonstrated that economic and environmental benefits of using 0.5 FC or the EOF method is much superior to other furrow irrigation strategies, especially in areas with limited water resources where less efficient irrigation methods may lead to significant water loss.

Grassland Bird Nesting on Restored and Remnant Prairies in South Central Nebraska

Grassland Bird Nesting on Restored and Remnant Prairies in South Central Nebraska

Characterization of Nighttime Evapotranspiration and Other Surface Energy Fluxes and Interactions with Microclimatic Variables in Subsurface Drip and Center-Pivot Irrigated Soybean Fields

The lack of knowledge and data on the driving forces of nighttime (nocturnal) evapotranspiration (ET) for various vegetation surfaces under different climatic and management conditions led this study to investigate the magnitude of nighttime ET (ETnight) and its interactions with other nighttime surface energy fluxes, i.e., soil heat flux (Gnight), sensible heat flux (Hnight), and net radiation (Rn _ night), and microclimatic variables, i.e., wind speed at 3 m (u3 _ night), vapor pressure deficit (VPDnight), and air temperature (Tnight). Soybean [(G lycine max (L.) Merr.] canopies under two different irrigation methods in subsurface drip- and center-pivot irrigated (SDI and CP) fields in south central Nebraska were studied. Hourly energy flux and meteorological data from the SDI field for the 2007 and 2008 seasons and from the CP field for 2008 were analyzed. The study period was divided into five sub-periods based on plant and canopy development to evaluate nighttime energy balances and driving forces at various plant growth and development stages. The five sub-periods are: pre-planting (from mid-March to plant emergence, EM), early season (from emergence to full canopy cover, leaf area index, LAI < 3, ES), mid-season (LAI > 3, MS), late season (full canopy cover to harvest, LAI < 3, LS), and bare soil (from harvest to mid-November, PH). In 2007, seven nights had greater than 0.50 mm per night of ET, and there were 13 such nights in 2008. Daily ETnight was a maximum of 35% (0.42 mm night-1) and 55% (0.48 mm night-1) of daily total ET (sum of hourly (ETSOH) in 2007 and 2008, respectively. The maximum ETnight usually occurred on windy nights. During LS in 2007, ETnight averaged 5% of ETSOH. In 2008, ETnight was greatest (0.17 mm night-1) during ES; ETnight rates were also high (0.11 mm night-1) during EM. During these two periods, ETnight accounted for 4% and 6% of ETSOH). The variables that had the most impact in driving the ETnight varied with the stage of growth. For example, in 2008, during EM when ETnight averaged over 6% of daily ETSOH at 0.11 mm night-1, ETnight was significantly (p < 0.05) correlated to u3 _ night. ETnight was significantly correlated to Hnight in ES when ETnight was about 4% of ETSOH at 0.17 mm night-1 and during MS when ETnight was at a minimum. However, ETnight was not significantly correlated to any of the microclimate variables during LS when ETnight was -3% of ETSOH at -0.04 mm night-1. ETnight was significantly correlated to u3 _ night and Rn _ night during PH when ETnight was 2% of ETSOH at 0.03 mm night-1. ETnight values at the SDI and CP fields were typically within 0.20 mm night-1 of each other, with ETnight at the CP field averaging 0.03 mm night-1 greater. Maximum ETnight was about the same at the two fields (1.72 and 1.92 mm for SDI and CP) and occurred on the same day. However, negative ET fluxes occurred more frequently and with greater magnitude at the SDI field. The contributions of H and G to ETnight at the CP field were similar to those at the SDI field. While nighttime fluxes away from the canopy were generally greater at CP, the similarity of ETnight at the SDI and CP fields (particularly the maximum ETnight occurring on the same day) suggests that ETnight is highly a function of regional weather conditions, while microclimate conditions contribute to variations in magnitude.

Spectral Data-Based Estimation of Soil Heat Flux

Numerous existing spectral-based soil heat flux (G) models have shown wide variation in performance for maize and soybean cropping systems in Nebraska, indicating the need for localized calibration and model development. The objectives of this article are to develop a semi-empirical model to estimate G from a normalized difference vegetation index (NDVI) and net radiation (Rn) for maize (Zea mays L.) and soybean (Glycine max L.) fields in the Great Plains, and present the suitability of the developed model to estimate G under similar and different soil and management conditions. Soil heat fluxes measured in both irrigated and rainfed fields in eastern and south-central Nebraska were used for model development and validation. An exponential model that uses NDVI and Rn was found to be the best to estimate G based on r2 values. The effect of geographic location, crop, and water management practices were used to develop semi-empirical models under four case studies. Each case study has the same exponential model structure but a different set of coefficients and exponents to represent the crop, soil, and management practices. Results showed that the semi-empirical models can be used effectively for G estimation for nearby fields with similar soil properties for independent years, regardless of differences in crop type, crop rotation, and irrigation practices, provided that the crop residue from the previous year is more than 4000 kg ha-1. The coefficients calibrated from particular fields can be used at nearby fields in order to capture temporal variation in G. However, there is a need for further investigation of the models to account for the interaction effects of crop rotation and irrigation. Validation at an independent site having different soil and crop management practices showed the limitation of the semi-empirical model in estimating G under different soil and environment conditions.

Ponderosa Pine Family Growth Comparisons in the Central Great Plains of Kansas

Ponderosa pine (Pinus ponderosa Laws.) has been planted widely in the Great Plains. Recommendations based on a 1968 study were to use material from south central South Dakota and north central Nebraska. A second test to further delineate seed sources (provenance/families) in this region was established in 1986. This paper reports results for survival, height, diameter, and D2H measurements in Kansas at 15 years. Results identify a wide range of suitable families within the Great Plains region.A majority of the tested sources performed well especially those from central Nebraska. Those sources from eastern Montana and western Nebraska performed poorly where environmental or geographic conditions were the poorest, thus verifying the original recommendations.

Hispid Cotton Rat (Sigmodon hispidus) in Nebraska: Distribution, Reproduction, and Seasonal Activity

ABSTRACT. The hispid cotton rat (Sigmodon hispidus) has occurred in Nebraska for about 50 years and entered the state from the south via northward movements through Kansas. However, little is known about its status or distribution in the state in recent decades. We conducted surveys for S. hispidus in southern Nebraska at historical sites and sites without previous records to determine its current status. From 2004 to 2008, we documented cotton rats at 9 of 12 historical sites in south-central and extreme southeastern parts of the state, and we captured this species at 13 new sites in southwestern and south-central Nebraska. In the past 3 decades, the distribution of S. hispidus has expanded westward but not northward in the state. An explanation for such a shift is unclear. At this northern edge of its geographical range, pregnant females were captured from early April to early October, and nonreproductive females were captured from November to January. Males with enlarged testes occurred from late March...

Finding the Smoothest Path to Success: Model Complexity and the Consideration of Nonlinear Patterns in Nest-Survival Data

Abstract. Quantifying patterns of nest survival is a first step toward understanding why birds decide when and where to breed. Most studies of nest survival have relied on generalized linear models (GLM) to explore these patterns. However, GLMs require assumptions about the models' structure that might preclude finding nonlinear patterns in survival data. Generalized additive models (GAM) provide a flexible alternative to GLMs for estimating linear and nonlinear patterns in data. Here we present a comparison of GLMs and GAMs for explaining variation in nest-survival data. We used two different model-selection criteria, the Bayes (BIC) and Akaike (AIC) information criteria, to select among simple and complex models. Our study was focused on the analysis of Redwinged Blackbird (Agelaius phoeniceus) nests in the Rainwater Basin wetlands of south-central Nebraska. Under BIC, our quadratic model of nest age had the most support, and the model predicted a concave pattern of daily nest survival. We found more mo...

Soil water recharge in a semi-arid temperate climate of the Central U.S. Great Plains

Abstract The amount of soil water at the beginning of the growing season has a large impact on crop yields in rainfed agriculture, especially in semi-arid regions and in years with below-average rainfall in more humid climates. Robust algorithms are needed to estimate soil water storage before planting to aid crop management decisions. The main objectives of this paper are to investigate soil water recharge during the non-growing season (October 20 to May 1) in a semi-arid, temperate ecosystem in south-central Nebraska (USA) and to evaluate empirical models to estimate soil water content at the beginning of the summer-crop growing season. A database of soil water content measurements collected over 5 years at nine locations in south-central Nebraska was used to estimate available water-holding limits in the soil profile and to determine the change in available soil water during the non-growing season. Regression analysis was performed to analyze the relationship among soil water recharge, residual soil water (i.e., soil water content at the end of the previous growing season), total precipitation, and available water-holding capacity (AWHC) in the root zone to 1.5 m. Precipitation storage efficiency (PSE) was calculated as the quotient of soil water recharge and total non-growing season precipitation. Predictive models to estimate soil water content at the beginning of summer-crop growing season were derived from these analyses. A large portion of the variation in soil water recharge was explained by residual soil water and precipitation. PSE averaged 28% across site-years; low PSE values were associated with high residual soil water and/or low AWHC. Two predictive models (linear and linear-plateau) that used residual soil water, total precipitation, and AWHC as independent variables explained 75–80% of the variation in the measured soil water content at the beginning of the summer-crop growing season. These empirical models represent a new tool to estimate soil water content by planting date of summer crops. Site-management conditions such as residue amount and its architecture, tillage system, soil texture, and terrain slope are not currently accounted for in these models and would likely improve predictive capacity.

Discussion of “Application of SEBAL Model for Mapping Evapotranspiration and Estimating Surface Energy Fluxes in South-Central Nebraska” by Ramesh K. Singh, Ayse Irmak, Suat Irmak, and Derrel L. Martin

Discussion of “Application of SEBAL Model for Mapping Evapotranspiration and Estimating Surface Energy Fluxes in South-Central Nebraska” by Ramesh K. Singh, Ayse Irmak, Suat Irmak, and Derrel L. Martin

No Other River

It's an annual rite. Sometime between late February and early April, I drive west on Interstate 80. In south-central Nebraska, both the Platte River and the interstate swing low toward the Nebraska-Kansas state line. Somewhere along the bottom of this catenary curve, I pull off the interstate and drive gravel roads south of the river. What I'm looking for is the world's largest gathering of migrating sandhill cranes. They're easy to find. The half-million cranes are concentrated, more or less, within the forty-mile stretch of the river between Grand Island and Kearney, the narrow waist on the hourglass-shaped Central Flyway and the narrowest part of the crane's migratory route. Because this part of Nebraska is relatively flat and open, one doesn't have to look hard or far to see the elegant, leggy, four-foot tall, red-capped, gray birds. I come to watch them feast and dance and drop out of and rise into the sky. Most of the cranes congregating on the Platte are lesser sandhills, the crane with the longest migration. The remaining cranes are greater sandhills and the endangered whooping cranes. After the roughly 600-mile trip from their winter homes in eastern New Mexico, northwestern Texas, and northern Mexico, the sandhill cranes are lean and hungry. Once in Nebraska, they glean corn left from the harvest and slurp up snails, worms, grubs, insects, snakes, and crayfish in the wet meadows near the Platte. Each crane stays in Nebraska about four weeks and adds a pound or so of fat during this time. At night, they return to the shallow river where they sleep standing in water, usually on one leg in roosts of as many as 10,000 to 15,000 cranes per half mile of river. From the air, these sleeping masses, some of which are a mile long, look sandbar-shaped, and the individual cranes are evenly spaced, as if, writes Steve Grooms in The Cry of the Sandhill Crane, positioned on a grid of four-foot squares, each bird staying just beyond the reach of its neighbors' sharp bills. At dawn, the cranes head for the fields near the river. And there, they dance, bowing, strutting, pumping their heads, raising their bills, flapping their wings, tossing corncobs into the air, and gracefully hopping as high as twenty feet. Great bouncing

Estimation of Crop Coefficients Using Satellite Remote Sensing

Crop coefficient ( Kc ) based estimation of crop evapotranspiration ( E Tc ) is one of the most commonly used methods for irrigation water management. The standardized FAO56 Penman-Monteith approach for estimating E Tc from reference evapotranspiration and tabulated generalized Kc values has been widely adopted worldwide to estimate E Tc . In this study, we presented a modified approach toward estimating Kc values from remotely sensed data. The surface energy balance algorithm for land model was used for estimating the spatial distribution of E Tc for major agronomic crops during the 2005 growing season in southcentral Nebraska. The alfalfa-based reference evapotranspiration ( E Tr ) was calculated using data from multiple automatic weather stations with geostatistical analysis. The Kc values were estimated based on E Tc and E Tr (i.e., Kc =E Tc /E Tr ). A land use map was used for sampling and profiling the Kc values from the satellite overpass for the major crops grown in southcentral Nebraska. Finally,...

A Field System for Isoflurane Anesthesia of Multiple Species of Mesopredators

Abstract Inhaled isoflurane is one of many chemical immobilizing agents available for field anesthesia of wildlife. We examined the use of inhaled isoflurane for field immobilization of mesopredators. Five species of mammals were anesthetized in a wetland complex in south-central Nebraska with isoflurane; we developed an induction chamber to use with a portable anesthesia machine to provide anesthesia at the capture location. Sixteen raccoons (Procyon lotor), 20 striped skunks (Mephitis mephitis), 4 Virginia opossums (Didelphis virginiana), 2 American badgers (Taxidea taxus) and 1 mink (Mustela vison) were captured and immobilized; we radio-marked 11 raccoons and 9 skunks. Induction was smooth and uneventful for all species immobilized. Mean induction times for raccoons and skunks were 10.7 min (SD = 1.1) and 11.0 min (SD = 2.4) respectively. Mean handling times (post-induction) were 7.1 min (SD = 3.9) for raccoons and 7.3 min (SD = 4.8) for skunks. All animals, with the exception of opossums, wer...

Reference and Crop Evapotranspiration in South Central Nebraska. I: Comparison and Analysis of Grass and Alfalfa-Reference Evapotranspiration

In Nebraska, historically, there have been differences among the water regulatory agencies in terms of the methods used to compute reference evapotranspiration ( ETref ) to determine actual crop water requirements and hydrologic balances of watersheds. Because simplified and/or empirical temperature or radiation-based methods lack some of the major weather parameters that can significantly affect grass and alfalfa-reference ET ( ETo and ETr ) the performance of these methods needs to be investigated to help decision makers to determine the potential differences associated with using various ETref equations relative to the standardized ASCE Penman–Monteith (ASCE-PM) equations. The performance of 12 ETo and five ETr equations were analyzed on a daily basis for south central Nebraska from 1983 to 2004. The standardized ASCE-PM ETo and ETr values were used as the basis for comparisons. The maximum ASCE-PM ETo value was estimated as 12.6 mm d−1 , and the highest ETr value was estimated as 19 mm d−1 on June 21,...

Application of SEBAL Model for Mapping Evapotranspiration and Estimating Surface Energy Fluxes in South-Central Nebraska

Knowledge of spatiotemporal distribution of evapotranspiration ET on large scales, as quantified by satellite remote sensing techniques, can provide important information on a variety of water resources issues such as evaluating water distributions, water use by different land surfaces, water allocations, water rights, consumptive water use and planning, and better management of ground and surface water resources. The objective of this study was to assess the operational characteristics and performance of the surface energy balance algorithm for land SEBAL model for estimating crop ET ETc and other energy balance components, and mapping spatial distribution and seasonal variation of ETc on a large scale in south-central Nebraska climatic conditions. A total of seven cloud free Landsat Thematic Mapper TM/Enhanced Thematic Mapper ETM satellite images May 19, June 20, July 22, August 7, September 8, September 16, and October 18, 2005 were processed to generate ETc maps and estimate surface energy fluxes. Predictions from the SEBAL model were compared with the Bowen ratio energy balance system BREBS-measured fluxes on an instantaneous and daily basis. The ETc maps generated by the model for seven Landsat overpass days showed a very good progression of ETc with time during the growing season in 2005 as the surface conditions continuously changed. The predictions for some surface energy fluxes were very good. Overall, a very good correlation was found between the BREBS-measured and SEBAL-estimated ETc with a good r 2 of 0.73 and a root-mean-square difference RMSD of 1.04 mm day 1 . The estimated ETc was within 5% of the measured ETc. The model was able to predict growing season from emergence to physiological maturity cumulative daily corn ET reasonable well within 5% of the BREBS-measured values. The model overestimated the surface albedo by about 26% with a RMSD of 0.05. The difference between the measured and predicted albedo was the greatest on May 19, early in the growing season before the full canopy cover. The second largest difference between the two albedo values was on October 18, a day after harvest. The model significantly under predicted soil heat flux with a large RMSD of 80 W m 2 and most of the underestimation occurred in the late growing season. Local calibration of soil heat flux significantly improved the agreement between the measured and predicted values. Furthermore, the sensible heat flux was underestimated between September 20 after physiological maturity and October 18 a day after harvest. While our results showed that SEBAL can be a viable tool for generating ETc maps to assess and quantify spatiotemporal distribution of ET on large scales as well as estimating surface energy fluxes, its operational assessment for estimating sensible heat flux and ETc, especially during the drier periods for different surfaces, needs further development.

Late-seasonal Activity and Diet of the Evening Bat (Nycticeius humeralis) in Nebraska

In North America, Nebraska represents part of the northwestern edge of the distribution for the evening bat (Nycticeius humeralis). To date, little information on this bat's natural history has been published from the state or from other parts of the Great Plains. Here we report on aspects of its natural history in Nebraska from 2 localities. In late summer and early autumn of 2006, we documented individuals farther west in Nebraska (Harlan County) than previously reported and determined that individuals fed mainly on Coleoptera and Hymenoptera. In 2006, evening bats appeared to migrate from Nebraska during late September–early October, and individuals were extremely fat, about 15 g, prior to migration. Evening bats likely are more widespread and common in south central Nebraska than previously documented. On 6 October 2005, we reported on an individual from eastern Nebraska (Douglas County), which represents the latest seasonal record of N. humeralis from the state.

An Evaluation of Evapotranspiration Model Complexity Against Performance in Comparison with Bowen Ratio Energy Balance Measurements

We investigated the relevance of model complexity in 11 evapotranspiration (ET) models, both for two-step and direct predictions for sum-of-hourly, daily, and seasonal time scales and compared their performances against the Bowen ratio energy balance system-measured ET for corn (Zea mays L.) in south central Nebraska. The simplest model we used was the water equivalent of net radiation, the intermediate was the Priestley-Taylor equation, and the complex models were the ASCE-Penman-Monteith (ASCE-PM) based two-step applications (i.e., grass and alfalfa reference surfaces). We also estimated ET with the soil water balance equation from the soil water content measured using a neutron probe soil moisture meter. By constructing the models from the simple towards more complicated, the effects of different components of the energy balance on ET during the growing season were investigated. We reported coefficients of determination (R2) and Nash-Sutcliffe efficiency (R2NS), and root mean square error (RMSE) to assess the models' performance against measured data. The ET from the soil water balance approach agreed well with those measured with the BREBS throughout the growing season. Over a seasonal time scale, the simplistic radiation-based parameterizations gave similar results as those more complex models. In the early season before canopy closure is attained (stage I) and in the late season after crop physiological maturity (stage III), the model predictions significantly deviated from measured data for both daily and seasonal estimates. The two-step ET calculation procedure was found to work very well during the mid-season (stage II) when the crop canopy was fully developed. In stages I and III, all the methods we used revealed negative R2 and R2NS values between the daily predicted and measured ET. We found significant sensible heat losses, indicating low rates of transpiration in both stages I and III. Compared to stage II, the time-dependent crop coefficients were found less capable in scaling the reference ET to represent daily actual crop ET fluctuations in stages I and III. Both the ASCE-PM-based two-step estimates as well as the direct estimates from the Priestley-Taylor and other methods gave more accurate results under full canopy cover. In this period, a sensible heat parameterization implicit in the models improved the daily predictions approximately 10% to 15%, despite model deficiencies. In the mid-season, our results suggested that the daily ASCE-PM reference grass-based approach provided arguably better estimates with respect to both daily and seasonal total fluxes compared to the sum-of-hourly calculations. Compared to the water equivalent of net radiation method, the complex models achieved up to 17% improvement in explaining the variability in daily ET during the mid-season. However, these methods did not improve seasonal ET estimates. Our results suggested that radiation is the dominant driver of evaporative losses over seasonal time scales. Other meteorological variables gained importance at daily and sum-of-hourly calculations.

Identifying Fecal Sources in a Selected Catchment Reach Using Multiple Source‐Tracking Tools

Given known limitations of current microbial source-tracking (MST) tools, emphasis on small, simple study areas may enhance interpretations of fecal contamination sources in streams. In this study, three MST tools-Escherichia coli repetitive element polymerase chain reaction (rep-PCR), coliphage typing, and Bacteroidales 16S rDNA host-associated markers-were evaluated in a selected reach of Plum Creek in south-central Nebraska. Water-quality samples were collected from six sites. One reach was selected for MST evaluation based on observed patterns of E. coli contamination. Despite high E. coli concentrations, coliphages were detected only once among water samples, precluding their use as a MST tool in this setting. Rep-PCR classification of E. coli isolates from both water and sediment samples supported the hypothesis that cattle and wildlife were dominant sources of fecal contamination, with minor contributions by horses and humans. Conversely, neither ruminant nor human sources were detected by Bacteroidales markers in most water samples. In bed sediment, ruminant- and human-associated Bacteroidales markers were detected throughout the interval from 0 to 0.3 m, with detections independent of E. coli concentrations in the sediment. Although results by E. coli-based and Bacteroidales-based MST methods led to similar interpretations, detection of Bacteroidales markers in sediment more commonly than in water indicates that different tools to track fecal contamination (in this case, tools based on Bacteroidales DNA and E. coli isolates) may have varying relevance to the more specific goal of tracking the sources of E. coli in watersheds. This is the first report of simultaneous, toolbox approach application of a library-based and marker-based MST analyses to flowing surface water.

Reemergence of Goss's Wilt and Blight of Corn to the Central High Plains

Goss's bacterial wilt and blight, also known as leaf freckles and wilt was first identified in Dawson Co. in south central Nebraska in 1969. The disease was soon identified in 54 Nebraska counties and six bordering states. Widespread development of symptoms in NE, WY, and CO early in the 2006 growing season led to submission of more than 50 samples to the UNL Panhandle Research and Extension Center's Plant Disease Diagnostic Clinic in Scottsbluff, NE for analysis. Pathogen identification was made based on a combination of test results. Accepted for publication 5 April 2007. Published 19 September 2007.

Hydrologic connections of a stream–aquifer-vegetation zone in south-central Platte River valley, Nebraska

Field investigation and numerical modeling approaches were used to examine the hydrologic relations between the Platte River and the adjacent alluvial aquifer and riparian zone in south-central Nebraska. Field methods include direct-push techniques for coring and electric logging in the river channel, permeameter tests for estimating the hydraulic conductivity of the streambed, and monitoring of groundwater levels responding to changes in stream stages and to groundwater evapotranspiration. The channel sediments consist mostly of coarse sand and gravels at the study site with large values of horizontal and vertical hydraulic conductivity. Groundwater in the riparian zone responded nearly simultaneously to the changes in stream stages, and diurnal fluctuations of the water table are correlated with fluctuations of stream stage in summer. All these indicate a well-connected river-aquifer-vegetation hydrologic system. Numerical models, based on the Galerkin finite element method, were developed to construct detailed flow nets for examining the changes in the patterns of groundwater flow dynamics resulting from the use of groundwater and stream water by riparian vegetation. Simulation results suggest that a number of hydrologic factors, such as the thickness of the aquifer and vertical anisotropy of aquifer hydraulic conductivity, also affect the flow patterns. Vertical flow is a major component, more significant than the horizontal flow below the river and the vegetation zone in the growing season. Groundwater evapotranspiration can bring deeper groundwater to the water table by hydraulic lift. This function of riparian vegetation could cause a complicated situation in an investigation of groundwater quality in riparian zones.

Statistical and geostatistical features of streambed hydraulic conductivities in the Platte River, Nebraska

This paper presents streambed hydraulic conductivities of the Platte River from south-central to eastern Nebraska. The hydraulic conductivities were determined from river channels using permeameter tests. The vertical hydraulic conductivities (K v ) from seven test sites along this river in south-central Nebraska belong to one statistical population. Its mean value is 40.2 m/d. However, the vertical hydraulic conductivities along four transects of the Ashland test site in eastern Nebraska have lower mean values, are statistically different from the K v values in south-central Nebraska, and belong to two different populations with mean values of 20.7 and 9.1 m/d, respectively. Finer sediments carried from the Loup River and Elkhorn River watersheds to the eastern reach of the Platte River lowers the vertical hydraulic conductivity of the streambed. Correlation coefficients between water depth and K v values along a test transect indicates a positive correlation – a larger K v usually occurs in the part of channel with deeper water. Experimental variograms derived from the vertical hydraulic conductivities for several transects across the channels of the Platte River show periodicity of spatial correlation, which likely result from periodic variation of water depth across the channels. The sandy to gravelly streambed contains very local silt and clay layers; spatially continuous low-permeability streambed was not observed in the river channels. The horizontal hydraulic conductivities were larger than the vertical hydraulic conductivities for the same test locations.