Unavailable soil water Portion of water in a soil held so tightly by adhesion and other soil forces that it cannot be absorbed by plants rapidly enough to sustain growth.

Uniformity coefficient (Christiansen's) Measure of the uniformity of irrigation water application.  The average depth of irrigation water infiltrated minus the average absolute deviation from this depth, all divided by the average depth infiltrated.

Union  Pipe fitting used to connect two lengths of pipe to facilitate adding a tee.

Unsaturated zone Part of the soil profile in which the voids (pore spaces) are not completely filled with water.

Usable precipitation Portion of total precipitation which becomes available for plant growth.

• consumptive Total amount of water taken up by vegetation for transpiration or building of plant tissue, plus the unavoidable evaporation of soil moisture, snow, and intercepted precipitation associated with vegetal growth.
• nonconsumptive Water that leaves the selected region and not considered consumptive. Examples are runoff, deep percolation, and canal spills.
• beneficial Beneficial use of water supports the production of crops: food, fiber, oil, landscape, turf, ornamentals, or forage. ... 
• nonbeneficial Water utilized in plant growth which can not be attributed as beneficial.
• reasonable In the context of irrigation performance, all beneficial uses are considered to be reasonable uses.  Non-beneficial uses are considered to be reasonable if they are justified under the particular conditions at a particular time and place.
• unreasonable Unreasonable uses are non-beneficial uses that, furthermore, are not reasonable; that is, they are without economic, practical, or other justification.