Phosphorus and N are critical nutrients for agriculture but are also responsible for surface water enrichment that leads to toxic algal growth. Although P loading to surface waters has traditionally been thought to occur primarily in surface runoff, contributions from subsurface transport can also be significant. The primary objectives of this research were to evaluate several methods of representing macropore flow and transport in a finite element model using plot-scale infiltration and leaching data and to compare several models of various levels of complexity to simulate long-term P leaching. To determine flow and transport parameters, single-and dual-porosity models in HYDRUS-2D were calibrated with infiltration, Cl-, and P data from a 22-h plot-scale leaching experiment on a silt loam mantle with gravel subsoil. Both homogeneous and heterogeneous gravel profiles were simulated. The dual-porosity model with heterogeneous hydraulic conductivity best matched experimental data, with physical nonequilibrium (dual porosity) being more important than two-dimensional (2D) heterogeneity. Long-term (9 yr) P leaching to the water table (3 m below the soil surface) at the field site was simulated with both one-dimensional (1D) and 2D models using the calibrated parameters. There was little difference between analogous 1D and 2D models, suggesting that HYDRUS-1D may be sufficient to model long-term P leaching. Overall, the most important elements for accurately simulating P leaching in this silt loam and gravel soil profile were found to be (i) field-measured hydraulic conductivity of the limiting soil layer, (ii) calibrated dispersivity, and (iii) dual-porosity, in some circumstances.

In many agricultural regions, the human use of water for irrigation is often ignored or poorly represented in land surface models (LSMs) and operational forecasts. Because irrigation increases soil moisture, feedback on the surface energy balance, rainfall recycling, and atmospheric dynamics is not represented and may lead to reduced model skill. In this work, we describe four plausible and relatively simple irrigation routines that can be coupled to the next generation of hyper-resolution LSMs operating at scales of 1 km or less. The irrigation output from the four routines (crop model, precipitation delayed, evapotranspiration replacement, and vadose zone model) is compared against a historical field-scale irrigation database (2008–2014) from a 35 km2 study area under maize production and center pivot irrigation in western Nebraska (USA). We find that the most yield-conservative irrigation routine (crop model) produces seasonal totals of irrigation that compare well against the observed irrigation amounts across a range of wet and dry years but with a low bias of 80 mm yr−1. The most aggressive irrigation saving routine (vadose zone model) indicates a potential irrigation savings of 120 mm yr−1 and yield losses of less than 3 % against the crop model benchmark and historical averages. The results of the various irrigation routines and associated yield penalties will be valuable for future consideration by local water managers to be informed about the potential value of irrigation saving technologies and irrigation practices. Moreover, the routines offer the hyper-resolution LSM community a range of irrigation routines to better constrain irrigation decision-making at critical temporal (daily) and spatial scales (< 1 km).

Irrigation exhibits large variation across producer fields, even within same region and year. A knowledge gap exists relative to factors that explain this variation, in part due to lack of availability of high-quality irrigation data from multiple field-years. This study assessed sources of variation in irrigation using a large database collected during 9 years (2005-2013) from ca. 1400 maize and soybean producer fields in Nebraska, central USA (total of 12,750 field-year observations). The study area is representative of ca. 4.5 million ha of irrigated land sown with maize and soybean. Influence of biophysical (weather, soil, and crop type) and behavioral (producer skills, risk aversion) factors on irrigation was investigated. Field irrigation distributions showed a substantial number of fields received irrigation amounts that were well above average irrigation for same region-year. Variation in irrigation across fields, within the same region, was as large as year-to-year variation. Seasonal water deficit (defined as total reference evapotranspiration minus precipitation), soil available water holding capacity, and crop type explained about half of observed variation in field irrigation, indicating that producers adjusted irrigation depending upon site-year variation in these parameters. However, half of the variation in irrigation remained unexplained, indicating that producer behavior and skills play also an important role. There was evidence of a "neighbor" effect as fields that received large irrigation were surrounded by other fields with similarly large irrigation. Likewise, fields with above- or below-average irrigation in one year remained consistently above and below regional average irrigation, respectively, in other years despite similarity in weather and soil among fields. These findings indicate that irrigation decisions are influenced by both biophysical and behavioral factors, making predictions of field and regional irrigation extremely difficult. This study highlights the value of collecting on-farm irrigation data to understand producer decision-making and find opportunities to improve current water management in irrigated crop systems. (c) 2017 The Authors. Published by Elsevier B.V.

Conventional irrigation schedules are typically based on portions of the field where root zones hold the least available soil water. This leaves undepleted available water in areas with larger water holding capacities. The undepleted water could be used through variable-rate irrigation (VRI) management; however, the benefits of VRI without in-field mapping are unexamined. In this research, the field-averaged amount of undepleted available soil water in the root zone was calculated from the NRCS Soil Survey Geographic database for 49,224 center-pivot irrigated fields in Nebraska. Potential reductions in pumpage from mining undepleted available water were then estimated. Results of the analysis show that widespread adoption of zone control VRI technology based only on the pumping savings from mining undepleted available water may be unwarranted for current VRI costs and average pumping energy expenses in the Central Plains ($0.0026 m(-3) to $0.0947 m(-3)). Pumpage reductions exceeded 51 mm year(-1) for only 2% of the fields and exceeded 25 mm year(-1) for 13% of the fields; thus, reductions may be small compared to annual pumpage requirements. If VRI were implemented on all fields with a potential pumpage reduction greater than 51 or 25 mm year(-1), the volume of pumpage reduction would be approximately 0.35% or 1.3%, respectively, of the total irrigation pumpage in Nebraska. These data may be a conservative estimate of pumpage reduction in fields where the measured variability in soil properties exceeds that described by the NRCS Soil Survey, or if undepleted water is mined early in the season and the soil water profile is refilled by precipitation, allowing undepleted water to be mined again. Adoption of zone control VRI for mining undepleted available water is most feasible for fields where the pumpage reduction from VRI is large and pumping costs are above normal. Pivot fields with high undepleted water were sparsely distributed across Nebraska and were often located along streams and or in associated alluvial areas. The prevalence of fields with large quantities of undepleted water differed among and within soil associations. We were unable to assign feasibility of VRI based on the soil association, as the occurrence of undepleted water varied significantly within a soil association. These findings should assist producers and other entities interested in VRI technology; however, pumpage reduction through use of undepleted soil water is only one benefit of VRI technology and management. Producers are encouraged to consider all potential benefits when analyzing VRI investments.

Abstract. Much of the previous research quantifying the potential benefits of variable rate irrigation (VRI) consists of case studies with simulations using data from small numbers of intensely sampled fields. In this study, an indicator of the amount of root zone available water capacity that is unutilized by uniform rate irrigation was calculated for 49,224 center pivot irrigated fields in Nebraska using publicly available data exclusively. Based on the values of this indicator, potential seasonal irrigation reductions from increasing precipitation utilization with VRI were estimated to be high for a small fraction of analyzed center pivots but low on a regional scale. At current VRI and energy prices, pumping cost savings alone may fail to justify VRI adoption for most analyzed center pivots. Although the prevalence of center pivots with high indicator values differed among counties and among soil associations, ruling out with reasonable confidence the occurrence of either low or high indicator values in a county or soil association might be difficult. The study hopes to inform producers considering VRI and other entities interested in the potential impact of this particular application of VRI.

Abstract. Phosphorus (P) is a critical nutrient for plant growth in agriculture, but is also responsible for surface water enrichment that leads to toxic algal growth. While P loading to surface waters has traditionally been thought to occur from surface runoff, contributions from subsurface transport can also be significant. While P transport through many soil types is well-documented, the presence of highly conductive gravel outcrops and macropore networks can have a significant, yet poorly-documented effect on P movement to the aquifer. Floodplain soils in the Ozark ecoregion generally contain coarse chert gravel layers that exhibit macropore behavior. Previous research has evaluated short-term P transport in plot trials ranging from 1 m 2 to 100 m 2 across many Ozark ecoregion floodplain sites. Traditional methods of estimating P loading and soil saturation do not account for macropore flow and likely underestimate P transport to the water table. To address this concern, long-term P modeling was performed in HYDRUS-2D/3D using data collected from short-term plot experiments. Calibration was performed using single- and dual-porosity models with both homogeneous and heterogeneous gravel profiles. The dual-porosity model with heterogeneous hydraulic conductivity best matched experimental data, although the dual-porosity model with homogenous soil layers also performed well. Long-term P transport to a 3 m-deep water table was simulated using 9 years of both daily and 5 minute rainfall data with a P flux consistent with yearly poultry litter applications. Long-term simulations with 5 minute rainfall data found that 113 kg ha -1 reached the water table over 9 years, or 21% of P applied.

The benefits of grass barriers or hedges for reducing offsite transport of non‐point‐source water pollutants from croplands are well recognized, but their ancillary benefits on soil properties have received less attention. We studied the 15‐yr cumulative effects of narrow and perennial switchgrass (Panicum virgatum L.) barriers on soil organic C (SOC), total N, particulate organic matter (POM), and associated soil structural properties as compared with the cropped area on an Aksarben silty clay loam (fine, smectitic, mesic Typic Argiudoll) with 5.4% slope in eastern Nebraska. Five switchgrass barriers were established in 1998 at ∼38‐m intervals parallel to the crop rows in a field under a conventional tillage and no‐till grain sorghum [Sorghum bicolor (L.) Moench]–soybean [Glycine max (L.) Merr.]–corn (Zea mays L.) rotation. Compared with the cropped area, switchgrass barriers accumulated about 0.85 Mg ha−1 yr−1 of SOC and 80 kg ha−1 yr−1 of total soil N at the 0 to 15 cm soil depth. Switchgrass barriers also increased coarse POM by 60%. Mean weight diameter of water‐stable aggregates increased by 70% at 0 to 15 cm and by 40% at 15 to 60 cm, indicating that switchgrass barriers improved soil aggregation at deeper depths. Large (4.75–8 mm) macroaggregates under switchgrass barriers contained 30% more SOC than those under the cropped area. Switchgrass‐induced changes in SOC concentration were positively associated with aggregate stability (r = 0.89***) and porosity (r = 0.47*). Overall, switchgrass barriers integrated with intensively managed agroecosystems can increase the SOC pool and improve soil structural properties.

A surface energy balance model (SEB) was extended by Lagos et al. Irrig Sci 28:51–64 ( 2009 ) to estimate evapotranspiration (ET) from variable canopy cover and evaporation from residue-covered or bare soil systems. The model estimates latent, sensible, and soil heat fluxes and provides a method to partition evapotranspiration into soil/residue evaporation and plant transpiration. The objective of this work was to perform a sensitivity analysis of model parameters and evaluate the performance of the proposed model to estimate ET during the growing and non-growing season of maize ( Zea Mays L. ) and soybeans ( Glycine max ) in eastern Nebraska. Results were compared with measured data from three eddy covariance systems under irrigated and rain-fed conditions. Sensitivity analysis of model parameters showed that simulated ET was most sensitive to changes in surface canopy resistance, soil surface resistance, and residue surface resistance. Comparison between hourly estimated ET and measurements made in soybean and maize fields provided support for the validity of the surface energy balance model. For growing season’s estimates, Nash–Sutcliffe coefficients ranged from 0.81 to 0.92 and the root mean square error (RMSE) varied from 33.0 to 48.3 W m −2 . After canopy closure (i.e., after leaf area index (LAI = 4) until harvest), Nash–Sutcliffe coefficients ranged from 0.86 to 0.95 and RMSE varied from 22.6 to 40.5 W m −2 . Performance prior to canopy closure was less accurate. Overall, the evaluation of the SEB model during this study was satisfactory.

South-central Nebraska is one of the most extensively irrigated areas in the U. S., with over 65,000 active irrigation wells, and maize is the major agronomical crop produced. Maize production in this region requires supplementary irrigation for maximum productivity. Effective on-farm implementation of full and limited irrigation practices for potential improvements of crop productivity requires knowledge of locally developed crop yield response to water functions. In this study, the effects of full and limited irrigation practices on maize (Zea mays L.) plant height, leaf area index (LAI), grain yield and biomass production, actual crop evapotranspiration (ETa), yield production functions, yield response factors (K-y), and harvest index (HI) were investigated. Field experiments were conducted in 2009 and 2010 under center-pivot irrigation at the University of Nebraska-Lincoln, South Central Agricultural Laboratory near Clay Center, Nebraska. Four irrigation regimes [fully irrigated treatment (FIT), 75% FIT, 60% FIT, and 50% FIT] and a rainfed treatment were evaluated each year. Maize ETa, LAI, biomass production, grain yield, and HI were significantly affected by the irrigation regimes. Maize yields varied from 9.05 Mg ha(-1) for the rainfed treatment to 15.5 Mg ha(-1) for FIT in 2009 and from 11.7 to 15.5 Mg ha(-1) for the respective treatments in 2010. HI ranged between 0.49 for rainfed and 0.57 for FIT with an all-treatment average of 0.54. ETa ranged from 481 mm for rainfed treatment to 620 mm for FIT in 2009 and from 579 to 634 mm for the same treatments in 2010. Strong yield vs. irrigation relationships (R-2 >= 0.98 in both years) and yield vs. ETa relationships (R-2 = 0.94 in 2009 and R-2 = 0.97 in 2010) were measured. There was a strong linear increase in ETa with increasing irrigation amounts (R-2 = 0.97). The yield-irrigation and yield-ETa relationships showed variation between the two years due to the impact of weather variability on these relationships, indicating the importance of accounting for weather variability impact on the slopes of crop yield production functions. Based on the slopes of the ETa vs. grain yield relationships, 1.2 Mg ha(-1) (in 2009) and 1.7 Mg ha(-1) (in 2010) of grain yield was produced per 25.4 mm of ETa beyond 280 mm (in 2009) and 403 mm (in 2010) of ETa that was used by maize to start producing grain yield, which is also called the amount of ETa required for establishing grain yield. Yield response factors varied between treatments and with year for the same treatment and averaged 1.65 in 2009 and 2.85 in 2010, with a two-year average of 1.82. No statistically significant difference (p > 0.05) in grain yield was found between 75% FIT and 100% FIT. In terms of crop response to water performance, the 75% FIT and 60% FIT treatments were very comparable to the fully irrigated treatment and are viable practices in increasing crop water productivity of maize with supplementary irrigation under these experimental, soil and crop management, and climatic conditions.

Irrigated maize is produced on about 3.5 Mho in the U.S. Great Plains and western Corn Belt. Most irrigation water comes from groundwater Persistent drought and increased competition for water resources threaten long-term viability of groundwater resources, which motivated our research to develop strategies to increase water productivity without noticeable reduction in maize yield. Results from previous research at the University of Nebraska-Lincoln (UNL) experiment stations in 2005 and 2006 found that it was possible to substantially reduce irrigation amounts and increase irrigation water use efficiency (IWUE) and crop water use efficiency (CWUE) (or crop water productivity) with little or no reduction in yield using an irrigation regime that applies less water during growth stages that are less sensitive to water stress. Our hypothesis was that a soil moisture-based irrigation management approach in research fields would give similar results in large production-scale, center-pivot irrigated fields in Nebraska. To test this hypothesis, IWUE, CWUE, and grain yields were compared in extensive on-farm research located at eight locations over two years (16 site-years), representing more than 600 ha of irrigated maize area. In each site-year two contiguous center-pivot irrigated maize fields with similar topography, soil properties, and crop management practices received different irrigation regimes: one was managed by UNL researchers, and the other was managed by the farmer at each site. Irrigation management in fanner-managed fields relied on the farmers 'traditional visual observations and personal expertise, whereas irrigation timing in the UNL-managed fields was based on pre-determined soil water depletion thresholds measured using soil moisture sensors, as well as crop phenology predicted by a crop simulation model using a combination of real-time (in-season) and historical weather data. The soil moisture-based irrigation regime resulted in greater soil water depletion, which decreased irrigation requirements and enabled more timely irrigation management in the UNL-managed fields in both years (34% and 32% less irrigation application compared with farmer-managed fields. in 2007 and 2008, respectively). The average actual crop evapotranspiration (ETC) for the UNL- and farmer-managed fields for all sites in 2007 was 487 and 504 mm, respectively. In 2008, the average UNL and average farmer-managed field had seasonal ETC of 511 and 548 mm, respectively. Thus, when the average of all sites is considered, the UNL-managed fields had 3% and 7% less ETC than the farmer-managed fields in 2007 and 2008, respectively, although the percentage was much higher for some of the farmer-managed fields. In both years, differences in grain yield between the UNL and farmer-managed fields were not statistically significant (p = 0.75). On-farm implementation of irrigation management strategies resulted in a 38% and 30% increase in IWUE in the UNL-managed fields in 2007 and 2008, respectively. On average, the CWUE value for the UNL-managed fields was 4% higher than those in the farmer-managed fields in both years. Reduction in irrigation water withdrawal in UNL-managed fields resulted in $32.00 to $74.10 ha(-1) in 2007 and $44.46 to $66.50 ha(-1) in 2008 in energy saving and additional net return to the farm income. The results from this study can have significant positive implications in future irrigation management of irrigated maize systems in regions with similar soil and crop management practices.

Depression storage plays a key role in retaining excess precipitation. When modeling irrigation systems and surface hydrology, depression storage is often treated as static storage, meaning that the maximum storage volume must be filled before overland runoff occurs. However, several researchers have documented that runoff can begin before all depressions fill. The objectives of this study were to (1) analyze the process of dynamic filling of surface depressions and (2) determine the effect of slope and microrelief on maximum depression storage. Plaster of Paris molds of 12 soil surfaces were collected from plots that are part of a tillage study which includes plow, disk, and no-till treatments. The molded surfaces were subjected to a 100mm/hr simulated rain on a bed with adjustable slopes. Dynamic filling of surface depressions was analyzed by measuring depressional storage at points before maximum depressional storage was achieved. An empirical relationship relating depressional storage to precipitation excess is proposed, and suggests that the dynamic nature of depressional storage may be predicted if the maximum depressional storage and excess precipitation hyetograph are known.

Surface water contamination can often be reduced by passing runoff water through perennial grass filters. Research was conducted in 2006 to 2008 to evaluate the size of cool season grass filters consisting primarily of tall fescue (Festuca arundinacea Schreb) with some orchard grass (Dactylis glomerata L.) relative to drainage area size in reducing runoff sediment and phosphorus (P). The soil was Pohocco silt loam Typic Eutrochrepts with a median slope of 5.5 %. The grass filters occupying 1.1 and 4.3 % of the plot area were compared with no filter with four replications. The filters were planted in the V-shaped plot outlets which were 3.7 × 11.0 m in size. The filter effect on sediment and P concentration was determined from four natural runoff events when nearly all plots had runoff. Filter effect on runoff volume and contaminant load was determined using total runoff and composites of samples collected from 12 runoff events. Sediment concentration was reduced by 25 % with filters compared with no filter (from 1.10 to 1.47 g L−1), but P concentration was not affected. The 1.1 and 4.3 % filters, respectively, compared with having no grass filter, reduced: runoff volume by 54 and 79 %; sediment load by 67 and 84 % (357 to 58 kg ha−1); total P load by 68 and 76 % (0.58 to 0.14 kg ha−1); particulate P (PP) load by 66 and 82 % (0.39 to 0.07 kg ha−1); and dissolved reactive P (DRP) load by 73 and 66 % (0.2 to 0.07 kg ha−1), respectfully. A snowmelt runoff event had 56 % greater DRP concentration compared with rainfall-induced runoff events. Grass filters reduced sediment and P load largely by reducing runoff volume rather than reducing concentration. Well-designed and well-placed grass filters that occupy 1.0 to 1.5 % of the drainage area and intercept a uniform flow of runoff from a drainage area can reduce sediment and nutrient loss in runoff by greater than 50 %.

Nonuniform field runoff can reduce the effectiveness of filter strips that are a uniform size along a field margin. Effectiveness can be improved by placing more filter strip where the runoff load is greater and less where the load is smaller. A modeling analysis was conducted of the relationship between pollutant trapping efficiency and the ratio of filter strip area to upslope contributing area, i.e., buffer area ratio. The results were used to produce an aid for designing filter strips having consistent effectiveness along field margins where runoff load is nonuniform. Simulations using the process-based Vegetative Filter Strip Model show that sediment and water trapping efficiencies of a filter strip increase nonlinearly as the buffer area ratio gets larger. Site characteristics, including slope, soil texture, and upslope soil cover management practices, help to define this relationship more accurately. Using the Vegetative Filter Strip Model simulation results, a graphical design aid was developed for estimating the buffer area ratio required to achieve specific trapping efficiencies for different pollutants under a broad range of agricultural site conditions. A single graph was produced showing simulation results for seven scenarios as a family of lines that divide the full range of possible relationships between trapping efficiency and buffer area ratio and into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering slope, soil texture, and field cover management practices. Relationships for sediment-bound and dissolved pollutants are interpreted from the Vegetative Filter Strip Model results for sediment and water. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. The use of this design aid will enable a more precise fit between filter size and runoff load where runoff from agricultural fields is nonuniform.

Targeting specific locations within agricultural watersheds for installing vegetative buffers has been advocated as a way to enhance the impact of buffers and buffer programs on stream water quality. Existing models for targeting buffers of Hortonian, or infiltration-excess, runoff are not well developed. The objective was to improve on an existing soil survey–based approach that would provide finer scale resolution, account for variable size of runoff source area to different locations, and compare locations directly on the basis of pollutant load that could be retained by a buffer. The method couples the Soil Survey Geographic database with topographic information provided by a grid digital elevation model in a geographic information system. Simple empirical equations were developed from soil and topographic variables to generate two indexes, one for deposition of sediment and one for infiltration of dissolved pollutants, and the equations were calibrated to the load of sediment and water, respectively, retained by a buffer under reference conditions using the process-based Vegetative Filter Strip Model. The resulting index equations and analytical procedures were demonstrated on a 67 km2 (25.9 mi2) agricultural watershed in northwestern Missouri, where overland runoff contributes to degraded stream water quality. For both indexes, mapped results clearly mimic spatial patterns of water flow convergence into subdrainages, substantiating the importance of size of source area to a given location on capability to intercept pollutants from surface runoff. A method is described for estimating a range of index values that is appropriate for targeting vegetative buffers. The index for sediment retention is robust. However, the index for water (and dissolved pollutant) retention is much less robust because infiltration is very small, compared to inflow volumes, and is relatively insensitive to the magnitude of inflow from source areas. Consequently, an index of inflow volume may be more useful for planning alternative practices for reducing dissolved pollutant loads to streams. The improved indexes provide a better method than previous indexes for targeting vegetative buffers in watersheds where Hortonian runoff causes significant nonpoint pollution.

Climate and vegetation strongly influence the water cycle on local to regional scales. A change in the surface energy and water balance, especially in dry climatic regions, can have a significant impact on local water availability and, therefore, water resource management. The purpose of this study is to quantify the energy and water balance of a riparian wetland in a subhumid region of the central US, as well as the role of seasonal climate variability and vegetation phenology. The site is located in the Republican River basin in south-central Nebraska, where decreases in streamflow have been observed in recent decades. In an effort to reduce consumptive water use from evapotranspiration (ET), and thereby reclaim surface water, invasive species such as Phragmites australis have been removed throughout the riparian corridor of the river basin. In this study, we used energy/water balance monitoring stations, a Large Aperture Scintillometer (LAS), and numerous water and soil temperature probes to determine the energy and water balance during the 2009 growing season (April 11-October 3). Sensible heat flux was measured using the LAS, while ET was calculated as a residual of the energy balance (i.e., net radiation minus sensible heat flux and heat storage rates in the canopy, water, and soil). Rigorous quality control and uncertainty analyses were performed, and comparisons were also made with ET rates calculated via the simpler Priestley-Taylor method.Results of the energy budget analysis indicate that the average ET rate for the wetland during the growing season was 4.4 mm day(-1), with a maximum daily rate of 8.2 mm day(-1) (occurring on June 29). Precipitation during the same 176-day period averaged 2.7 mm day(-1). Net radiation and vegetation phenology were found to be the two largest drivers of seasonal variability in ET. Sensible heat flux was significantly larger than latent heat flux early in the season, when standing vegetation in the wetland was still dry and brown. By late May and early June, however, Bowen ratios had declined well below 0.5 in response to greener and more abundant vegetation, higher transpiration rates, and reduced sensible heat flux. Heat storage rates in the wetland were dominated by changes in water temperature (as compared to soil or canopy heat storage) and comprised a significant portion of the hourly energy balance. On daily mean timescales, changes in the rate of heat storage corresponded to similar to 13% of the variability in net radiation, while for the season-long average, the heat storage term was found'to be essentially negligible. The Priestley-Taylor equation provided a reasonable estimate of ET during the height of the growing season but significantly overestimated ET during the beginning of the season (since it could not account for large sensible heat fluxes from the dry vegetation). Analysis of the wetland water balance showed seasonal variations in water level that were similar to changes in cumulative water inputs (i.e., precipitation minus ET). Portions of the season when the two curves had differing rates of change indicated periods of net water influx or outflux from other sources (primarily groundwater). (C) 2011 Elsevier B.V. All rights reserved.

Infiltration is an important process that affects the efficiency of center-pivot irrigation systems. No-till planting systems potentially influence the hydraulic properties of soils and, consequently infiltration. The result of long-term no-till use could be higher infiltration, lower irrigation runoff, and may allow for lower-pressure nozzle packages, thus reducing energy requirements for irrigation. This potential was investigated at two locations in eastern and south central Nebraska. Paired treatments were used at each location, one that has been no-till planted for over seven years and one that uses two to three operations per year for seed-bed preparation and cultivation. The Green and Ampt equation was used to develop infiltration functions that are applicable to the analysis of infiltration under center pivots. Saturated hydraulic conductivities were higher in no-till fields (6.2-8.21 cm/h) compared to tilled (2.8-3.9 cm/h). Green and Ampt curves illustrated zero percent runoff with a depth applied in three of the fields. In one tilled field, 11% runoff was calculated. Greater runoff is expected under center pivots on tilled fields.

Surface application of animal manure increases nutrients at the soil surface with increased potential for nutrient loss in runoff. Non-application setbacks are often required with the intent to reduce nutrient loss to surface water. The objective of this research was to determine the effect of setback distance on phosphorus and sediment in runoff. The research was conducted in eastern Nebraska on 24 ha of a terraced field with a 4 to 7% slope and predominantly Yutan silty clay loam Mollic Hapludalf soil. Experimental units were drainage areas served by risers for the tile-outlet terrace system. The seven treatments included no manure applied and setbacks with radii of 0, 5, 10, 20, 30, and 40 m; the setback distances were limited on the terrace side by the top of the terrace bund. Composted feedlot manure was applied at approximately 74 Mg ha-1 with 222 kg Pha-1applied. Over a two-year period, the mean precipitation was 770 mm per year and six major runoff events occurred. Setback distance did not affect dissolved P, particulate P, total P, and sediment concentrations or losses except that sediment loss was greater by 51% with increased setback distance, confirming the value of manure in reducing soil erodibility. Sediment and P concentrations were related to time since the on-set of a runoff event and peak intensity of rainfall events. The results show that manure application setbacks around risers on fields protected with tile-outlet terraces are ineffective in reducing P and sediment runoff. Key words: CAFO, compost, concentrated animal feeding operations, ISCO, terraces, water quality.