Darrell G. Watts

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.

Simulation of water applied nitrogen distribution under surge irrigation

Fertigation during surge flow irrigation is a promising means of delaying nitrogen (N) fertilizer application on furrow irrigated corn and thereby reducing the potential for leaching of nitrate-nitrogen into groundwater. A computer model, SIFUM (Surge Irrigation Fertigation Uniformity Model), was developed to simulate the distribution, uniformity, and runoff losses of fertigation-applied N during surge irrigation of furrows. The model, SIFUM, was used to help develop fertigation management strategies for field application that can provide acceptable N distributions across a range of soil conditions and furrow flow rates for surge irrigation with free outflow of runoff with no runoff recovery. Simulation groups included injecting N during (a) all surges, (b) the advance surges only, (c) the post advance surges only, and (d) the middle surges (last of advance plus first of cutback). For each of these groups, fertigation was simulated both for the entire on-time of each surge cycle and for only a portion of the on-time. The application efficiency of the low quarter (AELQ) was used as an index for evaluating the efficiency of fertigation treatments. Simulation results show that to obtain the best N efficiency on high-intake soils, fertigation can occur during any portion or all of the on-time of a surge cycle, but should be applied during all cycles, i.e., for the entire irrigation. For medium and low intake soils, fertigation should be applied during all of the on-time of a surge cycle and may be applied for either all of the irrigation or just the advance surges.

Unsaturated Hydraulic Conductivity Determination by One-Step Outflow for Fine-Textured Soils

ABSTRACT UNSATURATED hydraulic conductivity and water diffusivity are difficult soil properties to determine on fme-textured soils. This study demonstrates the practicality of the one-step outflow method as a laboratory technique for undisturbed cores and evaluates improvements in procedures and data analysis that increase the reliability of estimated soil hydraulic properties. The one-step outflow method was used on Detroit silt loam cores collected from three layers at three sites. Data analysis was carried out by smoothing the mass outflow curves using either a quadratic or a power function. Calculation of soil hydraulic properties was greatly improved by the use of the power function because the resulting hydraulic properties are monotonic and do not require subsequent smoothing for use in simulation models.

Integrated Water and Nitrogen Management

Nitrate contamination of ground water is a concern throughout the world. Control of nitrate leaching is necessary to protect or improve water quality, but effective management is difficult because of the complex interactions between soil, water, and nitrogen (N). It is widely recognized that N and water management must be addressed simultaneously to develop production systems that reduce nitrate leaching. Difficulties with N and water management arise because of the various sources of N, uncertainties in N cycling and nitrate leaching imposed by climatic variability, and logistic considerations on the part of producers. Traditional approaches to N management usually involve applying ample fertilizer N early in the growing season to meet anticipated crop needs. An alternative is to apply small amounts of fertilizer early in the growing season and use tissue analysis to schedule additional fertilizer N as the season progresses. Fertilizer N can be applied in the irrigation water when the crop is too tall for typical machinery. Both leaf N concentration and use of chlorophyll meters are promising techniques for evaluating the N status of irrigated corn. Crop N and water needs can also be estimated using simulation models. Computer simulations and tissue testing procedures combined with improved irrigation techniques can help develop farming systems that conserve both water and N, while maintaining yields and reducing nitrate leaching.

Irrigation System Capacities for Corn Production

ABSTRACT IRRIGATION system capacity requirements for corn production in Nebraska were determined using a computer simulation model. A daily water balance was used to calculate moisture depletions and ultimately, to estimate the amount of transpiration by the crop. Yield reductions were estimated using the ratio of the seasonal crop transpiration to the potential seasonal transpiration for a crop without water stress. Using the daily soil moisture depletion and seasonal yield reduction data, histograms and cumulative density functions of soil moisture depletion and crop yield reduction were developed for various soils, locations and system flow rates. Graphical relationships between soil moisture depletion and system capacity and between crop yield reduction and system capacity were developed. The yield reduction-system capacity relationships developed were dependent upon the type of irrigation management strategy employed and the soil type. Four sites in Nebraska were studied, and system capacity recommendations for two sites were chosen as representative of the climatic variability across the state.

ENERGY REDUCTION THROUGH IMPROVED IRRIGATION PRACTICES

Several methods are presently available to reduce the energy required to pump water for irrigation, including: increased water use efficiency; increased pumping plant efficiency; irrigation scheduling; and reduced pumping pressures. The cost of electricity for pumping can be reduced further by scheduling pumping during times other than the period of peak demand on the utility system. An additional energy saving from improved water management is achieved through a reduction in losses of nitrogen fertilizer by leaching.

Modeling the effects of agricultural management practices on groundwater in Shelton, USA

An integrated methodology was developed to assess the impact of recharge rates, nitrate leaching from crop root zones, and irrigation pumping rates on groundwater quality and quantity. Monthly values for deep percolation of water, irrigation pumping and leaching of nitrate-nitrogen were calculated with a soilwater-plant model. An Intermediate Vadose Zone Model (IVZM) was developed and used together with models for groundwater flow (MODFLOW) and solute transport (MT3D) to simulate the movement of water and nitrate-nitrogen. The IVZM generates recharge files used by MODFLOW and MT3D. The methodology was applied to an area near the town of Shelton, Nebraska, where a shallow sand-gravel aquifer is highly contaminated by nitrate-nitrogen. MODFLOW was calibrated using historic groundwater level data from 1981 to 1996. Simulation results suggest that concentration reductions of 10% in the upper third of the saturated zone are possible after 10 years of improved irrigation and nitrogen fertilizer management. Groundwater quality changes slowly propagate down-gradient from the field and towards the lower part of the aquifer. A particle tracking technique showed that water from a well does not provide a good indication of the influence of improved management in the adjacent field. The quality of water pumped from a well represents the effect of management over 15 years and from more than 1.5 km up-gradient from the well. Regional groundwater quality depends on local management practices over a long period. Regulatory programs to improve water quality require therefore, widespread adoption and substantial time to provide significant water quality improvements.

A Portable Flow Metering Device for Furrow Irrigation Studies

ABSTRACT Aportable flow metering device was developed for use in furrow irrigation studies where gated pipe is used for water delivery. The clamp-on device utilizes commercially available orifices. Calibration of the metering device revealed that if the same discharge coefficient is used for all eight of the orifices tested, the uncertainty in measurement is less than 5% of actual flow. The uncertainty increases to slightly over 5% of actual flow if a partially open gate valve is located 12 diameters upstream from the orifice. An additional and useful characteristic of the device is that the desired flow can be established rapidly.