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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 (Ky), 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 ram 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 (R² ≥ 0.98 in both years) and yield vs. ETa relationships (R² = 0.94 in 2009 and R² = 0.97 in 2010) were measured. There was a strong linear increase in ETa with increasing irrigation amounts (R² ≥ 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. Held 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. [ABSTRACT FROM AUTHOR]

Irrigated maize is produced on about 3.5 Mha 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 farmer-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 ETQ of 511 and 548 mm, respectively. Thus, when the average of all sites is considered, the UNL-managed fields had 3% and 7% less ETQ 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. [ABSTRACT FROM AUTHOR]

Conservation buffers, such as filter strips and riparian forest buffers, are widely prescribed to improve and protect water quality in agricultural landscapes. These buffers intercept field runoff and retain some of its pollutant load before it reaches a waterway. A buffer typically is designed to have uniform width along a field margin and to intercept runoff that flows uniformly to it. However, spatial analysis of field conditions and runoff patterns indicate that more runoff is likely to flow to some locations along a field margin than to others which can substantially limit a buffer's effectiveness. We propose that precision conservation, the use of precision spatial information, technologies, and procedures to implement conservation practices, can be used to improve the design of buffers and ensure their effectiveness. Precision conservation can integrate detailed landscape data with mathematical models in a geographic information system. We can then analyze spatial patterns of runoff and design variable-width buffers that precisely match the needs of every location along a waterway. Greater cost of precision conservation is offset partly by greater water-quality benefit from each acre of buffer. Many of the required data sources and modeling components already exist, substantial improvements are possible that can produce even greater conservation efficiency. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Vegetative buffers may function better for filtering agricultural runoff in some locations than in others because of intrinsic characteristics of the land on which they are placed. The objective of this study was to develop a method based on soil survey attributes that can be used to compare soil map units for how effectively a buffer installed in them could remove pollutants from crop field runoff. Three separate models were developed. The surface runoff models for sediment and for dissolved pollutants were quantitative, based mainly on slope, soil, and rainfall factors of the Revised Universal Soil Loss Equation (RUSLE), and were calibrated using the Vegetative Filter Strip Model (VFSMOD) for a standard buffer design and field management. The groundwater model categorized map units by the presence or absence of suitably-shallow groundwater and hydric conditions for interaction with the root zone of a buffer. The models were applied to a ~65 km2 (~25 mi2) agricultural watershed in northwestern Missouri. Data acquisition, calculations, and map production utilized the Soil Survey Geographic Database (SSURGO). For surface runoff, soil survey-based values correlated strongly with corresponding VFSMOD estimates for sediment (R2 = 0.94) and dissolved pollutant trapping efficiency (R2 = 0.83) for a wide range of soil, slope, and rainfall conditions. A strong negative correlation between trapping efficiency and field runoff load was indicated. Mapped results revealed large differences in buffer capability for surface runoff across the test watershed (21 to 99 percent for sediment and seven to 47 percent for dissolved pollutants). Trapping efficiency for dissolved pollutants was much smaller than for sediment in every map unit. Lower values of trapping efficiency were associated with map units where runoff loads are higher and where a buffer will trap greater loads of sediment, but smaller loads of dissolved pollutants, than in units with higher values. Comparative rankings can be adjusted somewhat for site conditions that depart from the reference conditions, and recalibration may be desired to better account for them. For groundwater, the confluence of hydric conditions and shallow water table occurred only in the highest reaches of the test watershed, but a buffer can also interact with groundwater in most upland and riparian locations due to the prevalence of a seasonally shallow water table. By this approach, soil surveys may be used as a screening tool to guide planners to locations where buffers are likely to have a greater impact on water quality and away from those where impact is likely to be small. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Concentrated flow of surface runoff from agricultural fields may limit the capability of riparian buffers to remove pollutants. This study was conducted on four farms in southeastern Nebraska to develop a method for assessing the extent of concentrated flow in riparian buffers and for evaluating the impact that it has on sediment-trapping efficiency. Field methods consisted of mapping field runoff areas and their pathways to and through riparian buffers to streams. Mathematical relationships were developed from a model (VFSMOD) that estimates sediment-trapping efficiency from the ratio of buffer area to field runoff area. Among the farms surveyed, riparian buffers averaged 9 to 35 m wide, and gross buffer area ranged from 1.5 to 7.2 ha, but the effective buffer area that actually contacts runoff water was only 0.2 to 1.3 ha. Patterns of topography and microrelief in fields and riparian zones prevented uniform distribution of field runoff across entire buffer areas. Using the mathematical relationships, it is estimated that riparian buffers at each of the four farms could potentially remove 99%, 67%, 59%, and 41% of sediment from field runoff if the runoff is uniformly distributed over the entire gross buffer area. However, because of non-uniform distribution, it is estimated that only 43%, 15%, 23%, and 34%, respectively, would actually be removed. The results indicate that concentrated flow through riparian buffers can be substantial and may greatly limit filtering effectiveness in this region. [ABSTRACT FROM AUTHOR]