Journal of Irrigation and Drainage Engineering

Localized ground‐water mounding beneath larger on‐site sewage disposal fields (leach fields) may be a major problem which previously had not been adequately addressed. Existing methods to predict mounding, mainly developed for ground‐water recharge spreading basins, are reviewed for applicability to sewage disposal fields. A most appropriate and accurate method is selected and considerably simplified to provide a convenient, straightforward prediction procedure for regulatory agencies and designers. The major simplification, to a single equation, is particularly accurate after longer wastewater application times, of say 10 to 20 years, when mounding is of most concern. Sensitivities of the procedure to the various simplifications, site factors, and design and management options are discussed and presented in graphical form where significant. Alternative means to reduce mounding are reviewed. Mounding problems and potential mitigation measures should be made more widely known, and long‐term field verification of the theory is needed.

Hydraulic gradients at exits from porous media are greater than those predicted by laminar flow theory. Exit gradient effects project into the soil away from the exit interface in the manner of a backwater curve. Head losses measured in the soil at an interface may be twice those predicted. Laboratory hydraulic conductivities measured using only inflow and outflow heads may be lower than actual values.

New opportunities for salinity control are under intensive study by the U.S. Bureau of Reclamation. Plans to use saline water for powerplant cooling and coal transport appear promising. Preliminary cost‐effectiveness estimates are reviewed for selected alternatives and potential cost savings are derived. Options for cost‐sharing and multiple use of saline water are also discussed in context of expected energy development in the Colorado River Basin.

A one‐point method for finding out Proctor Standard compacted maximum dry density and optimum moisture content of cohesive soils has been developed. It omits the usual laborious procedure of compaction test and requires only the determination of dry density of soil at 9% moisture content from which the maximum dry density and thereafter optimum moisture content can be found out. The method, therefore, shortens the time for the determination of the variables viz. maximum dry density and optimum moisture content for any given soil. On the basis of experimental data collected at the Irrigation Research Institute, Lahore (Pakistan), some mathematical relationships have been developed to compute the value of maximum dry density and optimum moisture content based on dry density of soil compacted at 9% moisture in a Proctor mold.

The Soil Conservation Service (SCS) dimensionless unit hydrograph model TR‐20 is widely used for hydrologic design. The model uses a dimensionless unit hydrograph, with the drainage area, the depth of the runoff volume, the time‐to‐peak, and the SCS peak rate factor required to dimensionalize the unit hydrograph. A standard value of 484 for the peak rate factor is commonly used. While vague guidelines have been provided for modifying the value of the parameter, almost all designs are made using a value of 484. A method is presented that provides a systematic means of obtaining a value of the peak rate factor that is unique to a watershed. The method only requires a runoff curve number map, which should be developed to use the TR‐20 model anyway. Application of the method to six watersheds, including three coastal basins, indicates that the method increases the accuracy of prediction; the peak rate factors computed using the procedure outlined compared very favorably with the values obtained by optimizing on measured rainfall hyetographs and runoff hydrographs. The results indicate that the value of 484 is too large for coastal watersheds on the Delmarva peninsula.

Soil moisture levels are predicted by a simple, conceptual accounting model run with daily rainfall and evaporation inputs. Model output is used to illustrate practical statistical procedures for drought frequency, duration and severity. Frequency is defined by the probability of a drought starting in a given month, duration is determined by run‐length analysis and severity is given in terms of soil moisture level. Predicted moisture contents for the top 100 mm (4 in.) of soil below ryegrass pasture are statistically similar to actual moisture levels measured in Canterbury, New Zealand. For this location, accurate rainfall forecasts could substantially reduce irrigation water demands.

The estimation, by computers, of correction coefficients for the United Nations Food and Agriculture Organization (FAO) evapotranspiration computation method had required a ``table lookup'' process involving arrays of up to four dimensions. Since such arrays cannot be stored on many computer systems, an alternative approach to estimating the correction coefficients is desirable. This note presents one such approach which utilizes regression of climatic parameters to predict correction coefficients. Predictions obtained by this approach are compared to original coefficient values by use of a normalized error term. While problems have been experienced with certain intermediate values of the independent variables, the results obtained have been generally satisfactory.

Irrigation machines are becoming popular as a means of solving the problem of water and labor shortage in irrigation. Irrigation efficiency is also improved by the implementation of irrigation machines. A universal nomogram is presented for determining the traveling velocity of a movable frontal lateral. The nomogram is based on analytical analysis, and takes into account the outlet spacing, the flow rate, and the required amount of water to be applied.