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I I I I I I I I I I I I I I I I I I I , ~t,;~~ 757 operation to capture and reuse these waters. Recoverable water losses to tailwater runoff have been minimized in most High Plains irrigation ope rat ions. Transpiration (T) - that water actually used by plants for growth and cooling purposes, being emitted to the atmosphere as vapor mainly through small pores (stomata) in leaf surfaces. Stage and rate of growth of the plants and soil moisture conditions, in addition to those factors influencing evaporation, are determinants of the rate of transpiration. Transpiration rates typically increase in direct propor- tion to plant growth and size and in later stages of growth typically become the single most significant source of soil moisture loss. Unit Water Demand - The total quantity of applied irrigation water used to irrigate a unit (acre, hectare or other unit of agricultural production) for a cropping season. APPLICATION SYSTEMS Surface/Gravity/Flood Systems - any of a wide variety of irrigation methods which apply streams of water directly to the land surface and utilize the gradient of the land for distributing the water.* 1. Row (furrow)/Alternate row methods - common methods of water di stri buti on on heavi er (cl ay, sil ty cl ay, cl ay 1 oams, etc.) soil s which use the natural gradient or slope of the land to move water across a field by gravity. Water is applied at the upper end of each row (or alternate rows) by gated portable pipe or by siphons from ditches. Rate of application, row length, slope, soil type and other factors are design elements in effective row irrigation systems. Typical water use efficiencies (WUE) are relatively low for row irrigation but good efficiencies and coefficients of * See illustration in Figures 1 and 2, pages 13 and 16. xiii