D. C. Slack

Silver as a residual disinfectant to prevent biofilm formation in water distribution systems.

ABSTRACT Biofilms can have deleterious effects on drinking water quality and may harbor pathogens. Experiments were conducted using 100 μg/liter silver to prevent biofilm formation in modified Robbins devices with polyvinyl chloride and stainless steel surfaces. No significant difference was observed on either surface between the silver treatment and the control.

Tilled soil subsidence during repeated wetting.

ABSTRACT A total of 15.2 cm of water was applied without raindrop impact energy to six replicates of four freshly tilled soils to determine its effect on subsidence, random roughness, bulk density, and hydraulic conductivity. Texture of the soils ranged from sandy loam to clay loam. Random roughness degraded to a relatively constant value about 0.5 cm below initial values for all soils except one which was degraded to 1.3 cm below the initial value. Bulk density increased about 0.1 g/cm3 for three soils while the fourth increased 0.2 g/cm3. Regression equations were developed to predict these changes. Coefficients of determination ranged from 0.84 to 0.97 for both random roughness and bulk density. Hydraulic conductivity also decreased with added water. Maximum decreases ranged from 3.0 cm/h to 8.9 cm/h...

Irrigation Scheduling Using Crop Canopy-Air Temperature Difference

ABSTRACT AN irrigation scheduling model was developed using crop canopy-air temperature difference as the dependent variable and net radiation, relative humidity and available soil water as independent variables. This model was used to schedule irrigations on a corn crop during the 1980 growing season. Crop yield and water use was compared with that of corn grown under irriga-tions scheduled by use of electrical resistance blocks and a water balance (checkbook) method. The yield of the treatment irrigated with the temperature difference scheduling approach was not significantly different from that of other treatments. However, the water balance method and resistance method called for additional water applications of 39 and 18 percent, respectively, when compared to the temperature difference method.

Electro-migration of nitrate in sandy soil

Migration of nitrate to groundwater has become a serious threat in many agricultural areas. This paper presents the results of experimental laboratory tests studying the nitrate gradient developed in response to an electrical potential. Two systems were tested; the first had no flow (closed system) and the second had flow opposite to the direction of the electrical current. A solution of sodium nitrate in sandy soil was used in both systems. The tests showed that the electro-kinetic process effectively concentrated and retained nitrate close to the anode. The movement of NO(3)(-) through the soil column was significantly influenced by the development of a pH gradient. Statistical analysis was performed to determine best-fit equations relating the nitrate gradient to the electrical input and pH gradient. A simple one-dimensional finite difference model was used to predict the pH gradient developed during the electro-kinetic process. The experimental measurements closely agreed with the predicted spatial and temporal distribution of the nitrate gradient for both closed and open system configurations.

Inactivation of Pseudomonas aeruginosa and Aeromonas hydrophila by silver in tap water

This study was conducted to assess the efficacy of silver as a secondary disinfectant to replace or reduce the level of chlorine utilized in water distribution systems. Pseudomonas aeruginosa and Aeromonas hydrophila are opportunistic pathogens present in drinking water and have been associated with waterborne disease. After 8 hours of exposure to 100 μ g/L of silver, there was a > 6-log10 reduction in P. aeruginosa in tap water at room temperature at pH7 and a 5.55-log10 reduction in the presence of 3 mg/L humic acid. Similar reductions were observed at pH9. At 4°C, reductions greater than 4-log10 were observed after 24 hours. For A. hydrophila, a > 6-log10 reduction occurred at both pH7 and pH9 within nine hours. The World Health Organization has determined that this amount of silver could be used for water disinfection without health risks. Furthermore, silver shows promise as a secondary disinfectant, even in the presence of organic matter in concentrations that would reduce the effectiveness of free chlorine.

Modeling infiltration under moving sprinkler irrigation systems.

ABSTRACT A modified Mein and Larson model was employed to predict time to surface ponding and corresponding volume of water infiltrated under two types of center pivot irrigation systems. Predictions were made for two soil types with differing surface cover. Results were com-pared with those observed in the field. The model did a good job of predicting time to surface ponding for the soil with a well protected surface. For a soil with in-complete cover, the prediction of both time to ponding and the corresponding volume was poor.

HYDRAULIC PROPERTIES OF SOILS AS AFFECTED BY SURFACE APPLICATION OF WASTEWATER.

ABSTRACT WASTEWATER was applied to three soils to deter-mine the sealing affect or change in hydraulic properties. Saturated conductivities of disturbed soils were measured weekly under laboratory conditions over a 40-wk period. For the Lester clay loam and Waukegan silt loam, saturated conductivities of the soils were less than the saturated conductivities of the wastewater seal. In Hubbard loamy sand, an immediate seal was formed due to the application of wastewater.

DEVELOPMENT AND TESTING OF A SMALL WEIGHABLE LYSIMETER SYSTEM TO ASSESS WATER USE BY SHALLOW-ROOTED CROPS

A mobile weighing system was designed in 1994 to weigh small lysimeters to measure water use by shallow-rooted crops. Using a reconditioned pesticide sprayer, a hoist, and a weighmeter, small lysimeters were weighed in the 1995, 1996, and 1997 growing seasons. The lysimeters were constructed of 4.8-mm hot-rolled steel and were 0.91 x 1.02 x 0.61 m in size with an internal area of 0.929 m 2 . The weight of a lysimeter containing only moist soil was 1.110.27 kg. At the beginning of each season of use, several tests were conducted on the weighing system to determine if it was sensitive enough to give adequate results of crop water use. In 1995, tests were conducted on linearity, repeatability, thermal shift, and creep errors. The values were ±0.062 kg, ±0.12 kg, -0.007 kg/ ° C, and -0.242 kg, respectively. The terminal linearity was 0.082 kg (1996) and 0.043 kg (1997). The measurement uncertainty decreased as the number of lifts increased. The results showed that the system was capable of producing adequate results for determining crop water use.

Comparison of three resistance methods for estimating heat flux under stable conditions

Aerodynamic resistance to sensible heat transfer (rah) is an important parameter in bulk resistance models of energy exchange between the Earths surface and the atmosphere. To estimate sensible heat flux density (H) over irrigated fields, it is often necessary to evaluate rah under stable atmospheric conditions. This is because irrigated fields are often cooler than air temperature as a result of net radiative cooling at night and advective influences. Sensible heat fluxes are generally small at night, but may become a relatively large component of the surface energy budget during advective conditions. The purpose of this paper was to compare H modeled for stable conditions over an extensive area of irrigated bermudagrass (Cynodon dactylon) cv. Midiron) using three rah methods from the literature, and to determine the sensitivities of the methods to the various parameter inputs. In addition to comparing modeled H values with each other, comparisons were made with H values measured by eddy correlation over the bermudagrass. The three methods studied were expressed as functions of the bulk Richardson number (RiB). The methods differ mainly in the stability correction function, and the added resistance term (kB−1, where k is von Karmans constant and B−1 is a non-dimensional bulk parameter), which arises as a result of dissimilarities between heat and momentum transfer mechanisms near surface obstacles. Application of the methods to the bermudagrass data resulted in marked differences between the various modeled H values, and between modeled and measured H values. The modeled H values were more sensitive to surface emissivity and temperature inputs.

Modeling Soil Moisture and Effects of Basin Tillage

ABSTRACT A soil-water model was developed and tested. The model provides satisfactory predictions of soil-water content at various depths for corn at a location in south-west Minnesota. The model was used to study the poten-tial effects of basin tillage on soil moisture levels at two locations, one in Minnesota and another in south central Texas. Results of these projections indicated that basin tillage increases soil moisture and decreases crop stress significantly as compared to conventional tillage by in-creasing infiltration of excess precipitation during the growing season, the off-season or both.