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Changes in soil water retention of the surface soil brought about by tillage can significantly alter the amount of rain water that infiltrates into the root zone and is available for plant growth. Soil tillage generally increases porosity and changes the pore‐size distribution, leading to changes in the soil water retention curve and hydraulic conductivities. The objective of this study was to investigate some simple ways of estimating the soil water retention curve of a tilled soil from that of an untilled soil, knowing the change in soil porosity or bulk density due to tillage. The study of literature and empirical analysis of the available data indicated: (i) under field conditions the tillage did not significantly change the air‐entry value of the soil; (ii) tillage increased the absolute value of the slope of the log‐log relationship below the air‐entry value; and (iii) the changes due to tillage in the retention curve occurred only in the larger pore‐size range, approximately between the air‐entry pressure head value and 10 times the air‐entry value. Assuming these observations hold in general, two simple methods of estimating the water retention curve of a tilled soil from that of its untilled condition are proposed. The first method is a simple imposition of the Brooks and Corey function between the air‐entry value and 10 times this value. The second method assumes that the change in soil water content at a given pressure head in the above range of pressure heads was inversely proportional to the value of the pressure head. The tests on four pairs of measured water retention curves on three different soils showed that these methods provided good approximations.

On agricultural lands, animal waste disposal as fertilizer has been practiced since the beginning of agriculture. However, the practice has been an environmental concern in recent years due to over disposal of animal waste in some instances. This study evaluated soil NO3 response to beef‐manure application on a corn (Zea mays L.) field and tested the Root Zone Water Quality Model (RZWQM) for manure management. The experiment site was located in Northeastern Colorado on a silage‐corn field with a history of fertilization with beef manure every fall after corn harvest. To study the residual effect of long‐term manure application, 582 kg ha‐1 of manure‐N was applied to the east side of the field in the Fall of 1993, 1994, and 1995, while the west side received manure in 1993 only. Average silage‐corn yields from the west site were 25.4, 31.9, and 22.5 Mg ha‐1 for 1994, 1995, and 1996, respectively, which were not significantly different from that harvested from the east site (25.1, 30.9, and 24.3 Mg ha‐1, respectively). Average soil NO3 concentrations decreased significantly from 14.9 to 8.5 mg N kg‐1 in the top 30 cm of soil, and from 5.4 to 3.7 mg N kg‐1 in the 30‐ to 60‐cm soil profile after stopping manure application. No significant difference in soil NO3 concentrations between the manured and not‐manured sites was found below 60 cm. Average plant N uptake ranged from 140 to 362 kg N ha‐1 and was not significantly different between the two sites. The RZWQM was calibrated on the basis of the measured silage‐corn yield and plant N uptake, and was then used to predict soil NO3 concentration and total water storage in the soil profile. Generally, the calibrated model provided adequate predictions for both NO3 and soil water content with r2 > 0.83. The model was further used to evaluate alternative scenarios of manure and water management.

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may decrease infiltration (Savabi and Stott, 1994) and is an issue we do not consider in this study. Surface sealing of bare soils often reduces rain infiltration, and The relationship between the mass or extent of resicrop-residue cover is commonly used to reduce surface sealing. We due cover and the increase in infiltration is an important conducted numerical experiments to quantify effects of the percentage and distribution of residue cover on infiltration, and to provide guide-issue in dealing with the surface-sealing problem. lines for residue management. Residue cover was simulated over the Baumhardt and Lascano (1996) conducted a field expersoil surface in circular patches. Excess surface water from the bare iment near Lubbock, TX. Simulated rain was applied surface-sealed areas was available for infiltration in nonsealed areas. at 65 mm h Ϫ1 for 1 h on a bare and residue-covered Numerical simulations were conducted for combinations of (i) soil Olton clay loam soil. They found that cumulative infiltype, either a clay loam or loamy sand soil; (ii) percentage residue tration was lowest (28.7 mm) on bare soil, and increased cover (P rc); (iii) saturated hydraulic conductivity of the surface seal curvilinearly with increasing residue amounts, leveling (K c) relative to bulk soil (K s); (iv) residue-patch size with a constant off to a limit (49.0 mm). The leveling off (asymptotic P rc ; and (v) rainfall intensity. The K c values had the greatest influence limit) occurred at a residue amount of 2.4 ton ha Ϫ1. on infiltration as a function of P rc. This influence increased with rainfall Increases in infiltration were related to the residue intensity. For a given P rc , smaller patches gave greater relative infiltraamount and not influenced by residue geometry, or their tion due to differences in the lateral redistribution of infiltrated water. The target values of P rc that provided 95% relative infiltration varied location on the bed or furrow. Lang and Mallett (1984) from 40 to 80% for most combinations. Changing the geometry of compared six levels of maize stover, expressed as a perthe residues made no significant difference. We also tested a onecentage ground cover (0, 10, 20, 30, 45, and 75%) under dimensional model with a spatially averaged saturated hydraulic cona rainfall simulator (rainfall intensity of 63.5 mm h Ϫ1) ductivity (K ce) for both covered and surface-sealed areas, and found to assess the effect of surface residues on infiltration that infiltration into a partially residue-covered soil could be estimated and soil loss on a clay loam soil with a 3.5% slope. The by the one-dimensional model for all cases of this study, when K c Ͼ increase of infiltration was curvilinearly related to the 0. Finally, simulated infiltration qualitatively agreed with data sets of ground-cover percentage, and the infiltration was 54% two independent field experiments under similar soil and rainfall congreater with 45% residue cover than without residue ditions. cover. It would be extremely useful to know what level of

The Root Zone Water Quality Model (RZWQM) is a process-based model developed recently by USDA-ARS scientists. The model integrates physical, chemical and biological processes to simulate the fate and movement of water and agrochemicals over and through the root zone at a representative point in a field with various management practices. The model was evaluated using field data for the movement of water and bromide, and the transformation and transport of cyanazine and metribuzin in the soil profile. The model reasonably simulated soil water and bromide movement. Pesticide persistence was predicted reasonably well using a two-site sorption model that assumes a rate-limited (i.e. long-term) adsorption-desorption process with the additional assumption of negligible degradation of inter-aggregate adsorbed pesticides.

Water capture and precipitation use efficiency are of great importance in dryland cropping systems because the world's dependence on food produced in dryland areas continues to increase. Growing season evapotranspiration potential greatly exceeds growing season precipitation rates in dryland areas, creating a water deficit for crops. Management practices that positively impact soil physical properties increase the potential for soils to capture water. One way to assess the ability of soils to capture water is through the measurement of sorptivity. Sorptivity is defined as the cumulative infiltration proportionality constant and is governed by surface soil physical properties such as texture, degree of aggregation and aggregate stability. A study was conducted to determine how crop residue accumulation after 12 years of no-till management affects surface soil sorptivity under semi-arid dryland conditions and how sorptivity is related to surface soil physical properties known to be related to crop residue accumulation. Surface soil sorptivity, bulk density, porosity (total and effective) and aggregation measurements were made across cropping systems and soil positions representing a wide gradient of crop residue accumulation at 3 sites in eastern Colorado. Results show that increasing crop residue accumulation will have the indirect effect of increased sorptivity via improvements in soil aggregation, bulk density, and porosity that are conducive to water infiltration. Management practices that result in greater amounts of crop residue returned to the soil system lead to beneficial soil physical properties that increase water sorptivity, greatly reducing the potential for runoff and erosion, and thereby increase the precipitation use efficiency of the system.

Deep water percolation and chemical leaching is a recognized environmental problem with furrow irrigation. Alternate‐furrow irrigation (AFI) was hypothesized as a method to increase water‐use efficiency and decrease chemical leaching compared with every‐furrow irrigation (EFI). The SWMS_2D finite‐element model was used to investigate water and CaCl2 movement in EFI and AFI with furrow‐placed or ridge‐placed fertilizer bands. Model simulations were conducted for a Crook loamy sand (mixed, mesic, Ustic Torripsamment) and a Nunn clay loam (fine, montmorillonitic, mesic Aridic Argiustoll). Water isolation zones occurred with AFI and EFI that contributed little to overall profile drainage. The soil water contents after infiltration and redistribution were more uniform for EFI than for AFI for both soils. Water distribution was more uniform with AFI in the clay loam than in the loamy sand. Chemical movement was least with AFI and CaCl2 placement under the nonirrigated furrow. The greatest chemical leaching was predicted with furrow placement of CaCl2 and EFI. Results with CaCl2 suggest soluble chemicals or fertilizers placed under the nonirrigated furrow in loamy sand may not be available for plant uptake because the soil did not wet during irrigation. With either form of furrow irrigation, placement of a fertilizer in the ridge rather than in the furrow would decrease leaching of the fertilizer and keep the fertilizer in the root zone.