5.6 Basin irrigation design

Basin irrigation design is somewhat simpler than either furrow or border design. Tailwater is prevented from exiting the field and the slopes are usually very small or zero. Recession and depletion are accomplished at nearly the same time and nearly uniform over the entire basin. However, because slopes are small or zero, the driving force on the flow is solely the hydraulic slope of the water surface, and the uniformity of the field surface topography is critically important.

An effort will not be made to develop a design procedure for irregularly shaped basins or where the advancing front is very irregular. Rather, the water movement over the basin is assumed to occur in a single direction like that in furrows and borders. Three further assumptions will be made specifically for basin irrigation.

5.6.1 An example of basin design

The problem. A comparison of basin irrigation with the furrow and border systems in previous subsections should provide an interesting view of the three systems collectively. To remind the reader, an irrigation project is in the planning stages in which a basic field block of 2 hectares has been chosen for field design. A preliminary survey has revealed that the fields are configured in 100 m widths and 200 m lengths. The typical slopes are .8% in the 100 m dimension and .1% in the other. Soils appear to be relatively non-erosive and have been tested to yield the following infiltration functions:

First Irrigations Z = 0.00484 r ^.388 + 0.00008 r

Later Irrigations Z = 0.0053 r ^.327 + 0.000052 r

Z has units of m³/m of length/m of width, and r has units of minutes. Anticipated application depths per irrigation based on an evaluation of cropping patterns and crop water requirements are 8 cm.

The water supply to the field is set by the project at 1.8 m³/min, available for 36 hours every 10 days. Quality of water supply is good and hopefully these deliveries will be made as expected so far as rate, duration, and frequency are concerned.

5.7 Summary

It is not possible to illustrate effectively the judgement or 'art' required to evaluate and design surface irrigation systems. The previous examples demonstrate the procedures described in this guide and, to a limited extent, alert the reader to factors he or she will need to determine on a case by case basis. There are major influences on the design process one might expect which lie far outside a mathematical treatment. For example, the size and shape of individual land holdings and their future change in response to customs for inheritance, governmental interventions such as land consolidation and resettlement, farmer preference and attitudes, harvesting and cultivating equipment limitations, etc. In short, there is not a universal algorithm for design and evaluation that eliminates the need for good judgement. On the other hand, good judgement is no substitute for the mathematical aids presented herein. One might demonstrate this by comparing the performance of a system properly designed with one where selection of inflow and cutoff time is made arbitrarily.

To be skilled in design is to completely understand the relationships among the selectable and manageable variables governing surface irrigation, particularly the effects of infiltration and stream size on advance. The mathematical treatment, if followed, helps illustrate some of the more important individual processes occurring in the field.

Because the irrigator has the latitude of changing flow rates and cutoff times, the field system may not respond as designed. The problem is unlike sprinkler and trickle irrigation where having selected and installed the system's piping, the hydraulics of the system's operation are defined. Consequently, surface irrigation design cannot provide a guaranteed level of performance but must rely on the farmer to operate and manage it efficiently. It is apparent therefore, that the role of extension and technical assistance to farmers is critical for surface irrigated regimes.

As a final thought in this section, something should be stated regarding costs associated with surface irrigation. It would be most desirable to present a comprehensive review, but such is impractical because surface irrigation systems

themselves are so widely varied. There are a number of irrigation technologies. The units here are $/ha but should be used only to indicate the relative magnitude of various system costs under agricultural conditions typical of the western United States. Other systems enter the picture as one moves from country to country.