Journal of Irrigation and Drainage Engineering

This paper describes the crucial importance of irrigated agriculture to developing countries and to the world as a whole. This is followed by a summary of key problems that restrict and diminish the benefits of irrigated agriculture. Solution of these problems will above all require a holistic approach and will require changes in the institutions involved. These institutions include those of the developing countries, of the developed countries that provide aid, and of the international financing and technical-assistance agencies. Engineers in the developed countries have had, and will continue to have, a leading role but will need some redirection. ASCE could provide valuable assistance in this redirection.

Chemicals applied to agricultural lands inevitably move below the plant root zone and may contaminate the underlying ground-water reservoirs. Laboratory open system soil column experiments were conducted to evaluate electromigration as a process for concentrating and retaining the nitrates close to the anode in soil subjected to hydraulic flow. Electromigration was found to be an effective means for concentrating and retaining nitrate close to the anode in saturated sandy soil at low flow rates. However, for a given electrical input, the effect was reduced as the hydraulic flow rate increased, being indiscernible for higher flow rates.

Simulation of sloping furrows with free outflow was accomplished to assess the effect of inflow pattern on maximum application efficiency. For five inflow hydrograph patterns, the maximum application efficiency was predicted utilizing a zero inertia model, which describes the movement of water in the furrow with infiltration. The irrigation parameters considered were four infiltration families, three slopes, three roughness coefficients, two field lengths, three volumes, and one furrow shape and size. The maximum application efficiencies averaged over all combinations for the five inflow patterns were constant rate, 58%; cutback, 64%; cablegation, 51%; modified cutback, 64%; and modified cablegation, 62%. Efficiencies ranged from zero for each inflow shape to a magnitude that differs from one to another, with the low values occurring for high infiltration rates and roughness, small slopes, long fields, and low volumes of application. While the constant rate was found to be least sensitive to changes in the input parameters, cutback and modified cutback were most sensitive. Cablegation and modified cablegation were moderately sensitive to changes in the input parameters.

The behavior of the dimensionless advance curves obtained numerically for various values of infiltration parameter is systematically analyzed. It was found that when the advance problem is expressed in terms of new variables conveniently selected, the various advance curves can be described by a single curve independent of infiltration parameter. A simple, explicit, time of advance formula that fits the single curve previously reported is derived. Comparison tests indicate that when an adjusted shape factor for the surface profile is used, the predictions of the proposed formula are in agreement with accurate kinematic wave numerical solutions. An optimal design procedure for furrow irrigation using the above formula is presented.

Adjustment coefficients for air temperature and dew point temperature measurements at dry weather sites are determined for predicting reference evapotranspirations (ET₀) for irrigated conditions. Seasonal and regional variation in changes in temperature parameters versus the ratio of effective precipitation to ET₀ are analyzed. The determined adjustment coefficients are tested for validity using weather data from reference weather stations, and paired reference and dry stations. The effects of adjusting air temperature and dew point temperature for site aridity are analyzed using the FAO Penman-Monteith and the Hargreaves equations. Maximum reductions in ET₀ caused by adjustments of temperature data for aridity effects was as high as 25%.

A factor G for pipelines with equally spaced multiple outlets and outflow at the downstream end is derived. The proposed factor is a function of the number of outlets along the pipeline and also a function of the friction formula used. Factor G allows head loss in such pipelines to be computed directly provided the first outlet is one outlet spacing distance from the pipeline inlet. Under conditions of zero outflow at the downstream end of the pipeline, factor G reduces to the well-known Christiansen's factor F. Factor G allows the design of segments of pipelines with multiple outlets. It may find application with irrigation engineers in designing sprinkler and trickle irrigation laterals and manifolds with multiple-diameter sizes. It also may be used in trickle line hydraulics in flushing mode.

Despite the many theoretical and experimental studies carried out in channels having different cross-section shape, slope, and roughness, the problem of the free overfall as a flow measuring device has not been completely solved, and other experiments are needed. A theoretically based end-depth-discharge relationship is obtained for both subcritical and supercritical flow in rectangular, triangular, and trapezoidal channels. For each cross-section shape, discharges predicted using the theoretical relationship are compared with the available experimental data.