Aggregate Stability and Size Distribution

An aggregate is a group of primary particles that cohere to each other more strongly than to other surrounding soil particles. Most adjacent particles adhere to some degree. Therefore, disintegration of the soil mass into aggregates requires imposition of a disrupting force. Stability of aggregates is a function of whether the cohesive forces between particles withstand the applied disruptive force

Soil Cohesion as Affected by Time and Water Content

Cohesion increased for several months after disruption in moist soils. Rate at which cohesion increased was slower in airdry soil, but continued for years. Moduli of rupture of soils also increased with time. Effects of water content on the rate at which cohesion increases are compatible with an explanation of the bonding mechanism in terms of slightly soluble components diffusing to and cementing points of contact between particles. Cohesional forces due to water are estimated and found to be large enough to provide a major portion of the cohesion measured in soils. These estimates are supported by decreased cohesion of a silty soil when dried. However, cohesion of soils with larger amounts of clay generally increases when they are dried, indicating that other bonding mechanisms predominate

Turbulent Flow Self Cleaning Trash Screens

Trash and weed seeds are a major problem to irrigators, particularly those using gated pipe or siphon tube irrigation systems. Trash in surface irrigation systems often stops or reduces flow through gates or siphons resulting in inadequate irrigation of the furrows served. Constant surveillance to clean trash out of these orifices is impractical. Weed seeds passing through an irrigation system are distributed throughout the field causing extra tillage operations and reducing yields

Cablegation: II. Simulation and Design of the Moving-Plug Gated Pipe Irrigation System

THE hydraulics of a moving-plug gated pipe irrigation system are analyzed. A relationship was developed for predicting orifice discharge coefficients for a range of typical pipe flow velocity and head conditions. A simulation model was developed to predict the time distribution of orifice flows, the distribution of infiltrated water across a field, and runoff rates. The model can be used to design cablegation systems for fields having variable pipe slopes and variable furrow lengths. Orifice sizes are varied along the pipe line and the plug travel speed is varied in order to obtain optimum net water application for all furrows and to keep the furrow stream sizes within acceptable limits

Using straw in steep furrows to reduce soil erosion and increase dry bean yields

Furrow-irrigated fields often have different slopes along a furrow, which tend to cause different water intakes and erosion rates. Irrigated furrows on the steeper slopes develop narrow channels that reduce the wetted perimeter in the furrow. This results in lower infiltration, and crops growing on the steep acres do not receive adequate water for the highest crop yield. Plants growing adjacent to straw-treated furrows received 1.3 to 2.1 times as much irrigation water as plants growing next to untreated furrows. Dry bean yield increases on the straw-treated furrows, compared to the untreated furrows, ranged from 614 kg/ha to 1,306 kg/ha—a 21 % to 62 % increase, respectively. Also, sediment yield reductions in the straw-treated furrows ranged from 69% to 90 % compared to untreated furrows

Cablegation IV: The Bypass Method and Cutoff Outlets to Improve Water Distribution

TWO techniques of improving the water distribution characteristics of cablegation systems are proposed and evaluated. The bypass method, which largely eliminates the problem of end effects, involves starting the plug at the first outlet and initially bypassing most of the flow to the downstream end of the pipeline, which is plugged. As the plug moves down the pipe, the bypass flow gradually decreases to zero. This method nearly equalizes the inflow distribution to all furrows and allows the use of a constant outlet opening size. The bypass can be accomplished by using a parallel bypass pipe and weir, or with a flow-through bypass plug. The bypass plug appears to be the lower cost method and is as effective as the weir in controlling the bypass. The second technique deals with the low outlet flows during the final stages of a "set" which are insufficient to reach the end of the furrows such that excess water is applied to the upper ends of the furrows. Two types of cutoff outlets, a gravity valve and a siphon type outlet were designed to abruptly cut off the flow at about the same time that runoff ceases, thus maximizing the uniformity of infiltration. The cutoff outlets are recommended for soils having relatively high sustained intake rates

Factors Which Affect Furrow Intake Rates

To apply irrigation water efficiently, the water must be absorbed evenly across the field. In surface irrigation systems, this requires either that the water be spread quickly across the soil surface so that each portion of the field has a nearly equal time to absorb water, and that all portions of the field absorb water at the same rate; or that water intake rate varies across the field to compensate for differences in intake opportunity time. Water distribution in surface irrigation systems is determined by: 1) the water application system capabilities and management, and 2) the infiltration characteristics of the field soils. Improved application systems and design procedures for surface irrigation are being developed. But unless soil infiltration rates can be managed to achieve uniform water intake at desireable rates, high surface irrigation application efficiencies cannot be achieved. Although the problem of nonuniform soil water intake can be solved by applying the water through sprinkler or trickle systems at rates lower than the lowest intake rates, with the present high energy costs, this option is often not economical. The objective of this study is to evaluate several farmer manageable factors which can affect water intake rates into irrigated furrows. The long term research goal is to quantify the effects of farmer practices which decrease intake uniformity, practices he can apply to improve uniformity, and practices which can change intake rates. Intake rate modification can be useful to accelerate advance (thus decreasing variations in intake opportunity times), counteract the effects of variations in intake opportunity times, or better - adapt a field to a fixed or desireable water application system or schedule. The manageable factors which will be discussed are: 1) wheel compaction of furrows 2) surface soil water content 3) flow rates, and 4) intermittent application, such as "surge" irrigation

Soil cohesion as affected by freezing, water content, time and tillage

This study was developed to determine whether there are substantial annual changes in soil cohesion and to identify major factors causing those changes. Aggregate stability was measured throughout the year on soils in Utah and Idaho using wet sieving techniques. Stability generally increased during spring and summer months. Major decreases of cohesion, found when minimum daily air temperatures fell to or below 0 °C during winter and early spring months, were attributed to pressures and associated shearing forces caused by freezing at high water contents. Equivalent disruption occurred when confined soils were frozen in controlled laboratory studies. Disruption also increased as water content at the time of freezing increased for all soils studied. Disruption of soil by rototilling and compaction significantly decreased soil cohesion

Furrow Intake Rates and Water Management

FURROW intake rates and their effects on the uniformity of water distribution throughout the length of run are examined. Yields are substantially reduced by nonuniformities. Several treatments are discussed which can increase or decrease infiltration rates to improve uniformity. Furrow compaction is suggested as a method of compensating for intake opportunity time differences resulting from advance time requirements. Computations show that differentially compacting the furrow along the length of run could provide more uniform application and increase the length of run without increasing erosion. This reduces the farmer's investment in the irrigation system and the time and energy required for planting, cultivating and irrigating

Surface Films Affecting Velocity Profiles of Slowly Moving Water in Open Channels

Rates of movement of surface films and underlying water were measured using observable solid and liquid tracers. Water flow rate, surface width, water depth, and channel length were varied in flumes in which the water was allowed to flow over a broad weir crest at the tail end. In some runs the surface was blocked 5 cm in front of the weir. When this was done, velocity of the surface film immediately upstream from the block decreased to a small fraction of its previous value and this area of slow surface velocity built upstream with time, extending to as far as several meters under steady state conditions. In this reach, the film at the air-water interface of the water causes drag on the moving water similar to that at a solid-water interface. Water surface in channels one cm wide and 100 cm long stopped moving when the shear force caused by water flowing beneath it dropped to less than 0.0013 dynes/cm². This indicates structure in the surface which does not deform or shear at rates proportional to the force applied. Average water flow velocities from 0.3 to 1 cm per second provided shear stresses in excess of 0.002 dynes/cm² and moved these surface films. However, the velocity distribution across the surface of the channel was not parabolic, and indicated that most of the shear in the film was taking place near the edges of the channel. Extrapolation of these observations to water film dimensions present in unsaturated soils indicates that air-water interfaces in unsaturated soils are usually static