Summary. It is not possible to illustrate effectively the judgement or 'art' required to evaluate and design surface irrigation systems

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.