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The impact of agrochemicals on groundwater quality has been the subject of considerable research and public debate. Mathematical models often are used to predict the fate of these chemicals and to develop regulations. In this research, we modified the pesticide degradation component of a management model to estimate soil temperature with depth and time and to incorporate the effect of temperature variation on the pesticide degradation rate. Estimated pesticide mass leaching beyond a depth of 1 m was two or more orders of magnitude greater when the temperature effect was incorporated into the model. Predicted soil temperatures at four different depths using measured surface soil temperatures followed the seasonal temperature variation of observed data with an average deviation

Calibration of neutron probes by transfer in laboratory media offers an accurate and fast method for probes of the same design. This paper develops a successful technique for such calibration, along with the materials needed, that can be duplicated readily in any laboratory. Three drums of stable hydrogenous media were prepared, and their apparent field water content was determined from two field calibrated probes. Media were alum, urea, and water. Ten additional probes were then calibrated from these drums using the apparent water contents. We developed a statistically based method of periodically checking calibration. It provides a calculation for a greatest acceptable difference between the latest count rate and the mean of the previous count rates. This greatest acceptable difference is a form of the statistical least significant difference and was highly satisfactory and simple to apply.

A chemical transport model, CMLS, and a weather simulation models, WGEN, were coupled to investigate the impact of possible and equally likely weather sequences on the travel time and relative amount of various chemicals penetrating given depths into the soil profile. Several combinations of geographical locations and sod chemical properties were investigated. Considerable variability in travel time from one weather sequence to another resulted. This variability is expressed in the form of probability distribution. Implications of this variability in terms of monitoring programs, the interpretation of model studies, and regulations on agricultural chemicals are discussed. 17 refs., 5 figs., 3 tabs.

The integration of geographic information systems (GIS) with distributed parameter hydrologic models is playing an increasing role in designing, calibrating, modifying, and comparing these models. Successful application of GIS technology in hydrologic modeling requires careful planning and extensive data manipulation. Three major tasks identified in most hydrologic applications with sophisticated computerized numerical models are: the spatial database construction, the integration of spatial model layers, and the GIS and model interface. The first task is often time consuming, but is becoming more justifiable with the ever increasing volume of data available from many organizations. The second task can be generalized in a series of spatial overlaying and/or projecting procedures that yield the final modeling layer. This task is becoming less tedious with rapid advancement of sophisticated GIS capabilities. The last task may require actual programming with difficulty varying from case to case. This article is both an introductory overview to GIS applications in hydrologic modeling, and a review of what has been achieved in this rapidly growing field.

Ammonia volatilization is an important issue in agricultural production and environmental protection. Experimental methods and numerical models exist to estimate the rate and amount of ammonia volatilization from commercial fertilizers and animal manures applied to a field. The existing models imposed assumptions on water movement in a soil profile that were judged to be inadequate for surface-applied swine (Sus Domesticus) effluent. In this research, a computer model was developed to estimate short-term ammonia volatilization from swine effluent applied to a field by flood or sprinkler irrigation. The model simulates simultaneous water flow, heat flow, and the transport and transformation of ammoniacal N in a soil profile using the finite difference method. Submodels were developed to evaluate concentrations of ammoniacal N in the infiltration pond of a flood irrigation event and in the droplets of sprinkler irrigation. The governing equations for the water flow, heat flow, and chemical transport modules and the irrigation submodules were derived from mass balance and energy balance employing constitutive laws established empirically. The model was tested against data from field experiments using parameters obtained from independent sources. The simulation results were in excellent agreement with experimental data in three out of six experiments. In the other three experiments, the predicted cumulative volatilization exceeded the measured amount by 5 to 30 kg ha -1 at the end of 1 wk. The differences were primarily in the first sampling period after the application. The simulated cumulative volatilization was most sensitive to temperature, pH of the soil system, and pH of the effluent applied.

The sensitivities of chlorophyll fluorescence and fresh weight as bioassay techniques for the determination of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5 (4H)-one], diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] concentrations in soil were compared. The ratio of the initial inflection point (I) to the variable fluorescence maximum (P) of intact oat (Avena sativaL. ‘Chilocco’) leaves was determined for plants seeded directly into herbicide-treated soil and for those transplanted into treated soil after 14 days of growth in nutrient solution. Using the chlorophyll fluorescence of transplanted oats bioassay, 0-, 0.13-, 0.25-, and 0.50-ppm concentrations could be distinguished from one another within 8 h for metribuzin, within 24 h for diuron, and within 48 h for atrazine. These distinctions between rates could not be made 17 days after seeding into treated soil when using fresh weight as the bioassay indicator. Chlorophyll fluorescence of oats seeded directly into treated soil was also a reliable technique, but required much more time to attain sufficient plant size for convenient chlorophyll fluorescence determinations.

Comparaison du rapport I/P aux autres parametres de la fluorescence des chlorophylles afin d'etudier l'utilite de ces parametres pour l'indication de la concentration en herbicides inhibiteurs de la photosynthese dans le sol a l'aide d'un dosage biologique de la fluorescence chlorophyllienne sur plante intacte

The content of water and soil in physical intermixture was measured simultaneously and nondestructively by the attenuation of a dual‐energy beam of γ rays. The beam, 1 mm by about 3 cm in cross section, was devised by placing a 280‐mCi source of 137Cs behind a 389‐mCi source of 241Am, with lead collimators suitably aligned in front of each source and the scintillation probe. The probe was connected in parallel to two‐separate amplifier‐analyzer‐scaler systems, one being set in the integral mode to receive all pulses greater than 550 keV (for 137Cs, 662‐keV peak), with the other being set in the differential mode to receive all pulses in a band 35–85 keV (for 241Am, 60‐keV peak). When related to the count intensity in the high‐energy range, the count intensity e caused by 137Cs in the low‐energy band was empirically found to be independent of the material in the binary mixture (soil and/or water) placed in the beam for measurement. Also, e could be well expressed by a cubic polynomial that was then used al...

For possible use in resource management models we examined the relationship aspects of wetting front suction head τwf and near-saturated hydraulic conductivity Ks, the parameters of the Green and Ampt infiltration equation. Analysis of nonsimilar-media scaling and a capillary tube bundle implied the power function relationship of negative slope linearity of log τwf versus log Ks. Experimentally, τwf and Ks were obtained from field-measured hydraulic conductivity versus suction head for five soils. For each soil, of the total variation of log τwf regressed linearly on log Ks, less than half was due to regression, although the correspondingly small correlation coefficients were highly significant statistically. Hence log τwf versus log Ks displayed more independence than dependence. Also, based on regression line intercepts, only three soils could be reasonably grouped together. Whether such an indifferent relationship is useful, as in a large ensemble of τwf and Ks over spatially variable areas, must await further evaluation.

(...) A searching program and complete data base for the journals Weeds, Weed Science, and Weed Technology, were developed and combined into a total system named WeedLit. The software requirements include the data base «Citation» and the retrieval program «LifSearch». Hardware requirements include a computer capable of running MS-DOS or PC-DOS with a minimum of 512K bytes of RAM and a fixed disk with 4M bytes of free space for Citation (...)

An improved method of dual-energy gamma-ray attenuation was used to measure water content and bulk density rapidly and accurately, during one-dimensional, unsaturated water movement into a column of an equal-part mixture of initially air-dry, highly swelling bentonite and silt. The bulk density near the inlet end of the column decreased rapidly as water entered. Since the ends of the column were confined, the expansion of the wetted bentonite-silt produced a compensating compression in the remaining air-dry portion of the column. Several water-content variables were plotted against the so-called reduced material coordinate M/t to the 1/2 power (Boltmann variable expressed in terms of the material coordinate m and time t). Neither the volumetric water content nor the volumetric water ratio would coalesce the data when plotted against m/t to the 1/2 power. In contrast, a plot of water saturation vs. m/t to 1/2 power did coalesce the data into a single curve, except for very early times at positions nearer than 1 cm to the water inlet.