Since many irrigation projects are financed by outside donors and lenders, specific system configurations may be precluded because of international policies and attitudes. The preceding discussion of factors affecting the choice of irrigation systems at the farm level is not meant to be exhaustive. The designer, evaluator, or manager of irrigation systems should be aware of the broader setting in which irrigated agriculture functions. Ignorance has led to many more failures or inadequacies than has poor judgement or poor training. As the remainder of this guide deals with specific surface irrigation issues, one needs to be reminded that much of the engineering practice is art rather than science.
Experience is often a more valuable resource than computational skill, but both are needed.
It is a poor engineering practice that leaves perfectly feasible alternatives just beyond one's perspective. The term 'surface irrigation' refers to a broad class of irrigation methods in which water is distributed over the field by overland flow. A flow is introduced at one edge of the field and covers the field gradually. Secondary factors include field slope, surface roughness, and the geometry or shape of the flow cross-section. The practice of surface irrigation is thousands of years old.