Journal of Irrigation and Drainage Engineering

A mathematical model based on kinematic overland flow and Green‐Ampt infiltration is developed to calculate the time of concentration of overland flow on a rectangular plane surface. The governing equations of the rainfallinfiltration‐overland flow process are written in terms or various dimensionless parameters. The relative time of concentration, defined as the ratio of the time of concentration of an infiltrating plane to that of a geometrically identical impervious surface, can be expressed in terms of only two other dimensionless parameters. These parameters are physically based and they account for the subsurface soil properties and the antecedent moisture content. The results, presented in chart form, can be used for predicting the time of concentration of a rectangular plot under constant‐intensity rainfall.

The clogging of strainers is a major problem in drip irrigation systems. The specific purpose of this work is to study clogging mechanisms of filter screens caused by water containing mostly organic particulates (algae). Different filter screens, filtration velocities, and algal concentrations are used as variables in the investigation. The clogging rate is dependent on these parameters, but does not conform to classical filtration formulas. Filtration index and filter cake specific resistance are calculated to evaluate filter behavior. The phenomenon of larger particles leaving the screen demonstrates a possible detachment mechanism that may result in further clogging of drippers.

A linear‐programming (LP) framework with associated design and cost‐estimating procedures is developed to evaluate the economics of deficit irrigation in system design and to optimize the sizing and operation of irrigation system components. The framework includes linear evapotranspiration crop production functions and simulates crop evapotranspiration and crop yields according to irrigation application levels and soil moisture availability. The LP framework considers and accounts for costs for capital, labor, energy, and water, and provides for hydraulic and economic continuity throughout the system. Sprinkler, pipe, pump, canal, and soil systems are simulated, and total system productivity and profitability are determined.

With the increasing population of the world, the area of irrigated agriculture will have to be increased in order to provide food and work for the people. Data presented at the World Food Conference held in Rome in 1974 indicated that new irrigation programs should provide water for additional 23,000,000 hectares in 10 years. Many new irrigation projects will have to be undertaken and run‐down systems will have to be rehabilitated and properly maintained, primarily in the developing countries to meet the needs of additional irrigated agriculture. Degradation of land by waterlogging and salinity is a common by‐produtft of irrigation. The extension of irrigation can also carry with it the increased threat of health hazards particularly due to the diseases of malaria and schistosomiasis. This paper reviews the environmental impacts of irrigation projects with particular reference to developing countries. It also suggests guidelines at the formulation stage of irrigation projects for minimizing their adverse effects on the environment. Some relevant policy issues for irrigation development have been identified.

Providing water to users on demand usually requires an increase in canal capacity over the same volume of water delivered at a uniform rate. A simulation model was used to develop demand patterns for hypothetical surface irrigation conditions. These results were used to determine the canal capacity required to meet various levels of demand. A modification to these results was hypothesized to be appropriate for delivery schedules where delivery is arranged. These results are expressed in simple, nondimensional terms and compared to capacities for continuous flow and rotation systems and to capacities from Clement's demand formulas. It is shown that Clement's formulas which were developed for sprinkler irrigation are inappropriate for surface irrigation demand since they do not account for the wide variations in possible conditions. Results are particularly inappropriate for small relative service areas. Simple canal capacity equations were developed for demand and arranged surface irrigation distribution systems for a 90% level of service (i.e., water is available 90% of the time when demanded).

Ten forms of the Penman combination evapotranspiration equation are reviewed and compared with lysimeter estimates at three locations. Aerodynamic and canopy resistance forms by Monteith and Thom and Oliver and an empirical form by Wright best predict daily lysimeter measurements at Kimberly, Idaho, and Coshocton, Ohio. Canopy resistances of 40–80 s/m and momentum roughness heights of 15 mm for clipped grass and 45–70 mm for tall grass and alfalfa result in best estimates by the Monteith and Thom‐Oliver methods. The original Penman and Priestley‐Taylor versions underestimated evapotranspiration in the arid Kimberly environment. Average standard errors of estimate of the best equations average about 0.8 mm/day over growing seasons.