NU Water-Related Research in Nuckolls County

This study examines the water use of winter annual weeds. More winter annual weeds grow now than 20 years ago because of the adoption of reduced tillage systems, where the soil is not disturbed between harvest and planting. Winter annuals typically germinate in the fall, overwinter as small plants, and grow rapidly as temperatures warm in the spring; these weeds are especially well adapted to limited summer rainfall. Common winter annuals in Nebraska are downy brome, henbit, field pennycress, wild mustard, marestail (horseweed), foxtail barley, shepherdspurse, speedwell, and prickly lettuce. This project is investigating whether allowing winter annual weeds to grow too long in the spring depletes the soil of moisture that would benefit the crop later in the summer.

Estimated potential nitrogen immobilization by winter annual weeds may be calculated as:

• 500 lbs/ac of winter annual biomass growth at planting time (this would be a relatively dense, uniform stand of weeds).
• As a general statement, nitrogen composes approximately 3% of plant biomass.
• $0.58/lb of nitrogen fertilizer (based on $950/ton of anhydrous ammonia)

Based on these assumptions, a dense, uniform stand of winter annuals could tie up approximately 15 lb of nitrogen per acre (500 x 0.03), or $8.70 per acre (15 x 0.58) of nitrogen intended for a corn crop.

Estimating the irrigation cost to replace water used by the same 500 lbs/A of winter annual biomass by assuming:

• 500 lbs/A of winter annual weed biomass at planting time,
• 800 lbs of water is required to produce 1 lb of winter annual weed biomass.
• At $2.50 diesel fuel, applying 1 inch of irrigation water per acre would cost $9.66.

The 500 lbs of winter annual biomass would use 400,000 lbs of water per acre (500 x 800), or 47,920 gallons of water (400,000 lb x 0.1198 gal/lb). This equals 1.75 acre inches of soil water (47,920 gal /{27,158 gal/acre in}) used by these weeds. Based on a cost of $9.66 to apply 1 inch of irrigation water, it would cost approximately $17.00 per acre to replenish the water used by winter annual weeds in this scenario.

The goals of a recently funded project to measure the potential benefits of developing a groundwater trading market in Nebraska is discussed in this Cornhusker Economics article. Groundwater is a major component of agricultural water use. In extensive regions of the Western United States, rural agricultural economies rely entirely on groundwater. At the same time as providing water for human needs, groundwater is also an input to streams, wetlands and riparian areas that provide important ecosystem services. Ongoing groundwater pumping will deplete flows in adjacent streams, leading to potential conflict between human and environmental uses of water. In the last decades, many conflicts over transboundary allocations of water, endangered species and instream and riparian habitat have been driven by surface water-groundwater interaction. For example, claims have been filed with the United States Supreme Court over the impacts of groundwater use on flows of transboundary rivers for the Pecos River (Texas vs. New Mexico), the Arkansas River (Kansas vs. Colorado) and the Republican River (Kansas vs. Nebraska and Colorado). Groundwater has typically been viewed as private property, and its use in agriculture is generally neither regulated nor quantified precisely. However, there is growing interest in moving to systems that regulate groundwater use. The ability to trade groundwater allocations is often a part of such conversations.

The South Central Agricultural Laboratory is a University of Nebraska research farm located about 15 miles east of Hastings immediately south of Highway 6. The primary focus of this farm is the development and refinement of irrigated crop production practices for Nebraska agriculture and beyond. A number of research projects are currently underway on site and in conjunction with producers in the region.

1. Crop water use efficiency, nitrogen use efficiency, and best irrigation and fertigation management practices for subsurface-irrigated corn and soybeans.
2. Measurement of soil evaporation under no-till, conventional (disk) till, and ridge till practices for corn using frequency-domain reflectometers under three irrigation frequencies and five irrigation levels for corn.
3. Development of best deficit irrigation management strategies for soybeans.
4. Center pivot irrigation engineering and evapotranspiration research: measurement of crop coefficients, evapotranspiration, and yield of corn under deficit irrigation settings.
5. Measurement of crop water use and crop water use efficiency of eight corn hybrids under full and deficit irrigation and dryland settings.
6. Measurement of maximum allowable crop water stress that can be imposed on corn, stress versus crop growth-yield- and available soil water relationships.
7. Crop canopy temperature measurements to quantify crop water stress index for corn and soybeans.
8. Measurements of hydraulics and uniformity coefficients, crop water use efficiency of a new low pressure irrigation system for soybeans.
9. Measurement of energy fluxes and crop coefficients using high frequency techniques such as Bowen ratio energy balance system and Eddy covariance system to provide improved evapotranspiration data for corn, soybeans, and natural grassland.
10. Measurement of non-growing (dormant season) evaporative losses to quantify annual evaporation and other water balance components.
11. Operational characteristics of atmometers (ETgage) to measure reference evapotranspiration and Watermark granular matrix sensors to monitor soil water status and their practical applications and demonstrations for effective irrigation management.
12. Rootworm pressure effect on crop water uptake under center pivot irrigation.
13. On-farm demonstration of limited irrigation strategies for making maximum use of water resources. The project is being conducted in partnership with the NRCS and Nebraska Corn Board in Hordville, Geneva, York, Edgar, Ord, West Point, Schuyler, and Mead in grower fields.

Terraces in the Republican River Basin total about 2 million acres; about 15% of the basin above Hardy, Nebraska is terraced. The goal of this project is to better understand how on-farm conservation practices, specifically terraces and small ponds, affect the basin's water supplies. Data has been collected at five dryland fields near Culbertson, Curtis, and Stamford, Nebraska and Colby and Norton, Kansas. The Kansas sites are in areas where three main tributaries of the Republican River - Beaver, Sappa, and Prairie Dog creeks - flow toward Harlan County Reservoir. The field data collected will be used to determine if computer models created for the Republican River Basin accurately measure the impact of conservation terraces and small reservoirs on the basin.

Initial research results show:

• About 16% of land in the Republican Basin is protected by terraces, and an equal number by small reservoirs
• About 45% of runoff into a terrace channel goes to evapotranspiration (ET), 45% goes to groundwater recharge, and 10% overtops the terraces
• Small reservoirs retain about 90% of inflow, most of which goes to groundwater recharge - little evaporation
• Much overland flow is loss in transmission as recharge or ET from plants in the creek

Integrated values for the basin be completed by the end of 2010. For more information, see the following slides presented at the 2010 Greater Platte Basins Symposium:

• Derrel Martin, Field-Scale Hydrologic Impacts of Conservation Terraces and Small Reservoirs in the Republican River Basin
• Dean Eisenhauer, Water Storage Capacity and Overland Runoff Impacts of Conservation Terraces in the Republican River Basin

As part of this project, Harvey and his students are conducting research to unravel the hydrology of central Nebraska's Rainwater Basin wetlands. These wetlands are of international importance as habitat for millions of migratory water birds. In addition, these playa wetlands may contribute to groundwater recharge and water quality improvement. However, many of the wetlands have been drained, and those remaining suffer functional impairment due to sedimentation and pesticide and fertilizer runoff.

Most of the remaining Rainwater Basin wetlands are geographically isolated. Currently research is underway to investigate the role of these wetlands in providing groundwater recharge and water quality improvement. This is a significant environmental issue of concern because groundwater in this region is of vital importance, providing drinking and irrigation water. Moreover, groundwater levels are declining throughout much of the region. Levels of nitrate and atrazine exceed drinking water standards in some parts of the basin. Most of the remaining playa wetlands are impaired by sediment and there is an active program to restore the wetlands by removing this sediment. Study sites are located in Phelps, Kearney, Clay, Fillmore, and York counties (see map below).

Three components need to be measured when establishing a hydrologic budget for these closed basin wetlands which are surface storage, evapotranspiration (ET), and groundwater recharge. ET is calculated by the Bowen-Ratio Energy Budget (BREB) Method with the aid of a Bower Tower. Surface storage and recharge data will be aided by stilling wells and drive-point wells, respectively. Hydroperiods and plant community diversity are being determined before and after sediment removal to evaluate the impact of in-washed sediments on recharge and underlying groundwater quality. Chloride concentrations obtained from upland runoff collectors will be incorporated into the Chloride Mass-Balance Method along with precipitation and sediment chloride concentrations to obtain recharge fluxes for a wetland site.

Two Master's theses and one Bachelor's thesis have been completed as part of this project:

• Wilson, Richard D. (2010), Evaluating Hydroperiod Response in the Rainwater Basin Wetlands of South-Central Nebraska, MS Thesis, UNL School of Natural Resources, 163 p.
• Foster, Sarah E., (2010), Temporal and Spatial Variations of Ions, Isotopes and Agricultural Contaminants in Surface Waters and Groundwater of Nebraska’s Rainwater Basin Wetland Region, MS Thesis, UNL Department of Earth and Atmospheric Sciences, 185 p.

This study examines how much water trees use in different forests. In riparian forests, invasive plants affect the quantity of water infiltrating and running off land by intercepting water and transpiring water. Sap flow sensors are being used to monitor how removal of invasive tree species affects tree-level and stand-level evapotranspiration. In addition, vegetation transects will reveal how understory plants respond to invasive tree removal. Researchers are gathering data using eddy covariance towers and satellite images. Study results will be used to test whether removing eastern Red Cedar and Russian Olive Trees can significantly improve water yields.

In regards to the understory response following the removal of invasive woody species from a cottonwood riparian forest, this research shows that:

• Removing invasive species increased the frequency of invasive form and warm season grasses.
• Species diversity increased when invasive tree species were removed.
• Changes in understory species following thinning were most pronounced where eastern redcedars were removed.
• Light level differences observed under different canopy types likely explain differences in understory species response to thinning. This will be examined in future studies.

The State of Nebraska is attempting to aggressively manage invasive species along the riparian corridors of the Platte River and the Republican River. Although the impetus for the removal is different, in both basins state agencies and weed management districts are using herbicides and mechanical removal to control a combination of invasive species led by Phragmites australis (common reed), Tamarix ramosissima Ledeb. (saltcedar), and Elaeagnus angustifolia L. (Russian olive).

Along the central stretch of the Platte River, invasive species have overtaken sandbars and side channels which are invaluable wildlife habitat. In an attempt to reclaim this habitat for bird species such as the Piping Plover and Whooping Crane, the state is removing large stretches of common reed. Along the Republican River, the state is removing all invasive species to reduce riparian evapotranspiration. By reducing evapotranspiration the hope is to increase stream flow along the Republican River. Since 2007 invasive species along the mainstem of the river and along the banks of the Harlan Reservoir have been sprayed with herbicide or mechanically removed.

In order to understand the impacts of removing invasive species along riparian corridors researchers at the University of Nebraska-Lincoln and collaborators at other universities have developed a multi-faceted research project.

On the Republican River basin researchers are directly measuring evapotranspiration rates from native and invasive plants. Researchers are also using a regional water balance model to estimate the water savings that could be achieved by removing all invasive species within the basin.

On the Platte River researchers are monitoring water quality changes associated with a controlled herbicide treatment of common reed. Using a combination of in situ instruments and grab samplers researchers are determining the impacts of species removal. Other experiments are being conducted to measure how invasive species alter biogeochemical processes and sediment characterization.

Visit the project website for more information, including real-time meteorological data from a riparian wetland near the Republican River, real-time water quality data at a stream site on the Platte River, and quarterly project reports.

Final Report on Riparian Vegetation Impacts on Water Quantity, Quality, and Stream Ecology

This study is using a combination of techniques including remote sensing, to develop reliable estimates of evapotranspiration from riparian zones and determine varying water use rates for typical and invasive species in the Republican River Basin. The project will provide datasets of evapotranspiration and the annual water balance for a range of conditions in the riparian areas along the Republican River. Specific deliverables of the project include:

• Map of surface energy fluxes, including evapotranspiration, across three watersheds in the Lower, Middle and Upper Republican Natural Resources Districts for different spatial and temporal (i.e. daily, seasonal and annual) scales.
• Map of riparian vegetation classification across three watersheds using high resolution remote-sensing and ground truth observations.
• Comparison of water use and water availability on riparian vegetation and adjacent treated research area by measuring evapotranspiration rates, using various methods.
• Data for planners and decision-makers to develop water management policies.
• Extension and education materials to inform and communicate results to stakeholders.

The Rainwater Basin is one of the most endangered wetland ecosystems in North America. This ecosystem is critical to many species including migratory waterfowl. Land use and runnoff from agriculture and cattle confinement operations are likely to be reducing the basin's health and diversity, however little information exists concerning macroinvertebrates. Twenty-two locations were assessed for water quality parameters, vegetation composition, and macroinvertebrates identified to genus. Samples were collected biweekly starting in April through July for three years. Macroinvertebrate diversity was impacted in areas with little buffer although the effects were not pronounced. Institution of a more effective vegetative buffers strip may reverse this trend to improve ecosystem quality and provide for invertebrate resources for migratory birds.

Click here to see a poster about this research