Derrel L. Martin

Crop Residue Cover Effects on Evaporation, Soil Water Content, and Yield of Deficit-Irrigated Corn in West-Central Nebraska

Competition for water is becoming more intense in many parts of the U.S., including west-central Nebraska. It is believed that reduced tillage, with more crop residue on the soil surface, conserves water, but the magnitude of water conservation is not clear. A study was initiated on the effect of residue on soil water content and corn yield at North Platte, Nebraska. The experiment was conducted in 2007 and 2008 on plots planted to field corn (Zea mays L.). In 2005 and 2006, soybean was grown on these plots. There were two treatments: residue-covered soil and bare soil. Bare-soil plots were created in April 2007. The residue plots were left untreated. In April 2008, bare-soil plots were recreated on the same plots as in 2007. The experiment consisted of eight plots (two treatments with four replications each). Each plot was 12.2 m × 12.2 m. During the growing season, soil water content was measured several times in each of the plots at six depths, down to a depth of 1.68 m, using a neutron probe. The corn crop was sprinkler-irrigated but purposely water-stressed, so that any water conservation in the residue-covered plots might translate into higher yields. In 2007, mean corn yield was 12.4 Mg ha-1 in the residue-covered plots, which was significantly (p = 0.0036) greater than the 10.8 Mg ha-1 in the bare-soil plots. Other research has shown that it takes 65 to 100 mm of irrigation water to grow this extra 1.6 Mg ha-1, which may be considered water conservation due to the residue. In 2008, the residue-covered soil held approximately 60 mm more water in the top 1.83 m compared to the bare soil toward the end of the growing season. In addition, mean corn yield was 11.7 Mg ha-1 in the residue-covered plots, which was significantly (p = 0.0165) greater than the 10.6 Mg ha-1 in the bare-soil plots. It would take 30 to 65 mm of irrigation water to produce this additional 1.1 Mg ha-1 of grain yield. Thus, the total amount of water conservation due to the residue was 90 to 125 mm in 2008. Water conservation of such a magnitude will help irrigators to reduce pumping cost. With deficit irrigation, water saved by evaporation is used for transpiration and greater yield, which may have even greater economic benefits. In addition, with these kinds of water conservation, more water would be available for competing needs.

Standardized ASCE Penman-Monteith: Impact of sum-of-hourly vs. 24-hour timestep computations at reference weather station sites

The standardized ASCE Penman-Monteith (ASCE-PM) model was used to estimate grass-reference evapotranspiration (ETo) over a range of climates at seven locations based on hourly and 24 h weather data. Hourly ETo computations were summed over 24 h periods and reported as sum-of-hourly (SOH). The SOH ASCE-PM ETo values (ETo,h,ASCE) were compared with the 24 h timestep ASCE-PM ETo values (ETo,d) and SOH ETo values using the FAO Paper 56 Penman-Monteith (FAO56-PM) method (ETo,h,FAO). The ETo,h,ASCE values were used as the basis for comparison. The ETo,d estimated higher than ETo,h,ASCE at all locations except one, and agreement between the computational timesteps was best in humid regions. The greatest differences between ETo,d and ETo,h,ASCE were in locations where strong, dry, hot winds cause advective increases in ETo. Three locations showed considerable signs of advection. Some of the differences between the timesteps was attributed to uncertainties in predicting soil heat flux and to the difficulty of ETo,d to effectively account for abrupt diurnal changes in wind speed, air temperature, and vapor pressure deficit. The ETo,h,FAO values correlated well with ETo,h,ASCE values (r2 > 0.997), but estimated lower than ETo,h,ASCE at all locations by 5% to 8%. This was due to the impact of higher surface resistance during daytime periods. Summing the ETo values over a weekly, monthly, or annual basis generally reduced the differences between ETo,d and ETo,h,ASCE. The differences suggest that using ETo,d rather than ETo,h,ASCE would result in underestimation or overestimation of ETo. Summing the ETo,d values over multiple days and longer periods for peak ETo months resulted in inconsistent differences between the two timesteps. The results suggest a potential improvement in accuracy when using the standardized ASCE-PM procedure applied hourly rather than daily. The hourly application helps to account for abrupt changes in atmospheric conditions on ETo estimation in advective and other environments when hourly climate data are available.

CROP ROTATIONS WITH FULL AND LIMITED IRRIGATION AND DRYLAND MANAGEMENT

ABSTRACT Irrigated cropping systems need to maximize the economic value of both rainfall and irrigation water, especially in areas of declining groundwater. This study compared water management systems in a winter wheat (Triticum aestivum, L.)-corn {Zea mays, L.)-soybean (Glycine max, L.) (W-C-S) and continuous corn (CC) rotation in west central Nebraska for dryland, limited irrigation (150 mm/yr), and full irrigation. Crop yield, evapotranspiration, and soil water storage were determined from field studies conducted at North Platte, Nebraska, on a Cozad silt loam (Fluventic HaplustoU) soil. Dryland com used 21.5% more evapotranspiration (ET) in the W-C-S rotation compare to CC. ET for the limited and full irrigation com was 4.6% and 4.9% more for the W-C-S rotation compared to the CC and was statistically significant at the P > 0.08 level. Water use efficiency, defined by the slope of the linear relationship between grain yield and ET (3Y 3ET-^), was the same for com in the W-C-S and CC rotations. Com grain yield response to irrigation and ET was more than the yield response of winter wheat and soybean. The W-C-S rotation increased com grain yields in two out of three years at this location for dryland management and increased the seasonal ET of corn compared to continuous corn. Full irrigation management did not consistently increase winter wheat and soybean grain yields above the limited irrigation treatments. Soil water storage for the full irrigation management was greatly reduced compared to dryland and limited irrigation management for both rotations.

Testing of a Water Loss Distribution Model for Moving Sprinkler Systems

Field water balance measurements using monolithic lysimeters were used in validating the Cupid-DPE model for predicting water loss partitioning during sprinkler irrigation from a moving lateral system fitted with impact sprinklers and spray nozzles. The model combines equations governing water droplet evaporation and droplet ballistics with a comprehensive plant-environment energy balance model. Comparisons indicate good agreement between measured and modeled transpiration, and the measured and modeled soil evaporation during the day of irrigation. Total predicted evapotranspiration during the day of irrigation was greater than measured totals using the monolithic lysimeters. However, part of this difference was because the lysimeters could not measure water use during irrigation. Total measured and predicted evapotranspiration agreed well for the day following irrigation. Predicted soil evaporation rates matched well for the period immediately following irrigation, and cumulative soil evaporation was nearly identical to the measured total through the end of the next day. During irrigation, the main water loss was shifted from transpiration to evaporation of the wetted-canopy. For equal application volumes, the duration of this effect was greater using impact sprinklers due to the greater wetted diameter and lower average application rate compared to spray nozzles. Predicted water flux rates during irrigation were up to 50% greater for canopy evaporation than for transpiration rates predicted immediately prior to the start of irrigation. Canopy evaporation amounted to 69% and 63% of the total predicted water use during impact and spray irrigation, respectively. It also was 0.69 and 0.28 mm greater, respectively, than the predicted transpiration total during this same time span assuming no irrigation had been applied. About 13 and 5% of the water applied by overhead sprinkling was evaporated or transpired during impact and spray irrigation, respectively. However, the net increase in predicted water loss during irrigation was only 5.8% and 2.4% of the irrigated water depth applied for the impact and spray cases, respectively, because transpiration and soil evaporation would have occurred even without irrigation. Although droplet evaporation represented less than 1% of the total water loss for the day using either type of sprinkler, irrigation water did influence the energy transfer between the plant-environment and water droplets during flight, on the canopy, and the soil.

Effects of Water and Nitrogen Management on Nitrate Leaching Loss from Sands

ABSTRACT Afield calibrated computer model was used to estimate the probable impact of different management prac-tices on nitrogen uptake and the loss of water and nitrate from the root zone of irrigated corn on sandy soil. The overall objective was to compare the contributions of nitrate to the ground water system, that may result from the wide range of existing management practices on ir-rigated sands in the central Great Plains. Three nitrogen sources, two nitrogen amounts, and a wide range of ir-rigation applications were simulated for wet, normal and dry growing seasons at North Platte, Nebraska. Control of irrigation amounts to minimize percolation and pro-per selection of nitrogen amount and source all significantly reduced nitrate leaching loss. However, model results indicate that it is impossible to reduce nitrate losses to near zero amounts and maintain present production levels. This would appear to have serious im-plications for ground-water quality in the future as ir-rigation continues to expand on sandy soils.

operating Rules for Deficit Irrigation Management

ABSTRACT Methods to develop operating rules for deficit irrigation management of a limit water supply were developed. Techniques to predict the optimal irrigated area for a given amount of water available at the start of a season were described. Dynamic programming was used to develop an operating rule to annually distribute a limited water allocation over a multi-seasonal period. Operating rules developed from the optimization models were simple which allows for producer acceptance and use. Utilizing the techniques developed in the paper should help irrigators develop management plans to adapt to changing water availability conditions.

Engineering systems to enhance irrigation performance

The desirable irrigation system applies water at a rate that allows all water to infiltrate and distributes the water in space and time to match crop requirements in each parcel of the field. Various types of irrigation systems and management strategies have been developed in attempts to achieve the “desired” system. Our objective is to review various methods of enhancing irrigation performance. Although the “desired” system has not been attained, considerable improvements have been made based upon selection and management technologies which generate profits within the constraints of environmental prudence. Each irrigation system has inherent opportunities for enhancing irrigation performance. Like-wise, each has limitations in achieving maximum crop productivity per unit of applied water. Methods to improve the performance or surface irrigation can be grouped into those that increase the uniformity of water intake, reduce runoff losses, or decrease spatial variability. Two surface irrigation systems that enhance performance are surge-flow and level-basin. The uniformity and efficiency of sprinkler systems can be enhanced by computer-based design procedures and, in some cases, by applying low-energy, precision application concepts. Advantages of microirrigation are less surface area wetted, which minimizes evaporation and weed growth, and improved application uniformity which is specifically designed into the distribution network. An appropriate management strategy is necessary to attain the potential of an irrigation system engineered to match crop water requirements, and soil and environmental conditions. The best irrigation method applies the amount of water desired at the appropriate time while providing for leaching requirements, agronomic operations, and environmental considerations. With enhanced engineering and computer capabilities and improved knowledge of the soil-plant-water continuum, irrigators will adopt “prescription” irrigation. Prescription systems apply precisely the prescribed amounts of water, nutrients, and pesticides to match the production capacity of each parcel of land.

MODELING SEEPAGE FROM AN UNLINED BEEF CATTLE FEEDLOT RUNOFF STORAGE POND

A site-specific water balance model was developed to evaluate the effects of sludge accumulation, starting stage, and annual precipitation on seepage from an unlined beef cattle feedlot runoff storage pond. The computer model predicted daily inflows due to precipitation and runoff, and outflows due to evaporation and seepage. The seepage component was estimated using the SWMS_2D finite element saturated/unsaturated flow model, while feedlot runoff was estimated using the Natural Resource Conservation Service runoff method. Evaporation, precipitation, and temperature data from a nearby weather station were used in the model. Based on results of 9,100 annual simulations, the mean seepage volume ranged from 31 900 m3/y with no sludge accumulation to 19 300 m3/y with 22 years of sludge accumulation (1.5 m of sludge). The mean seepage rate ranged from 1.11 cm/day with no sludge accumulation to 0.50 cm/day with 22 years of sludge accumulation. Sidewall seepage volumes ranged from 49 to 73% of the total pond seepage volume. Increasing the pond stage from 0 to 250 cm at the beginning of the simulations caused a 200% increase in annual seepage volumes, yet only a 20% increase in annual seepage rates. Annual seepage volumes increased as much as 62% when annual precipitation increased from 44 to 96 cm/y. Average annual seepage rates varied little with varying annual precipitation. Seepage losses were 1.5 to 3.2 times as great as evaporation losses. This research provides information on variability in seepage rates that will be valuable to regulatory personnel when writing new environmental regulations, and to engineers when designing new storage ponds and lagoons.

Spatial Interpolation of Climate Variables in Nebraska

Temperature and rainfall are important climatological parameters, and knowledge of their temporal and spatial patterns is useful for researchers working in many disciplines. In this study, spatial interpolation techniques were implemented in a Geographic Information System (GIS) to study the spatial variability of climate variables (maximum air temperature, minimum air temperature, and seasonal and annual rainfall) in Nebraska. Thirty years (1971-2000) of climate data (average monthly maximum and minimum temperatures and rainfall) from 215 National Weather Service Cooperative Observer Network (COOP) weather stations distributed throughout Nebraska and surrounding states were used in the analyses. Literature suggests that there is no single preferred method of interpolation, and the selection of interpolation method is usually based on the available data, desired level of accuracy, and available resources. We analyzed three different commonly used interpolation methods (inverse distance weighted, spline, and kriging) and evaluated their performance. Overall, the summary of all statistical parameters showed no significant difference between interpolation techniques in predicting the spatial variability in 30-year climate normals. Investigation of interpolation errors at individual weather stations agreed with summary statistics. Spatial variability, in this instance, is likely smoothed due to long-term averaging of the data (30 years), resulting in similar errors for all the interpolation techniques. Subjective assessment of maps for all climate variables showed that the kriging method produced smoother maps compared to spline and inverse distance weighted. Considering the degree to which accurate spatial interpolation could be accomplished with relative ease and less bias, the spline method proves the better option.

Decision Support System for Design of Center Pivots

Sprinkler irrigation package selection for center pivot systems depends on soil type and affects water and energy use. A decision support system (DSS) was developed to provide designers and irrigators with a rapid method to analyze many design alternatives. A digitized soils database is accessed by the DSS to determine the predominate soil for each 0.64-ha parcel of land in a center pivot field or site. Flow rates needed to meet crop water requirements and expected application losses are calculated and spatially averaged based on these water requirements for each cell. Uniformity of the irrigation applications are estimated for level land only. Energy usage for various sprinkler packages and power sources can be analyzed. Potential runoff from each cell is displayed by the DSS. The user selects the parameters for multiple system comparisons. The DSS enhances the analyses of management information seldom utilized in current design methods and presents the results of database lookups and numerical model calculations in easy-to-interpret graphical formats. The design can then integrate the users’ attitudes and values regarding energy and water savings.