The irrigation investigations conducted by the Utah Experiment Station, some of the results of which form this *report, were undertaken for the purpose of adding to our knowledge of the natural laws upon which the art of irrigation may be safely built. The work has had for its dirrect object the study of the mutual relations of plants, soils and water, as these relations may indicate the most economic use of water for plant production. In pursuit of the investigations it became necessary not only to follow the movement of water in soils under irrigation conditions, but to determine also the relative amounts of water evaporated directly from the soil and taken from the soil by plants. Of equal importance became also, the determination of the optimum and minimum amounts of water for the profitable production of vegetable organic matter. Considering the needs of the practical farmer, three great questions continually presented themselves to the investigators: (1) To what extent is it possible to regulate the amount of water that evaporates directly from the soil? (2) Is it possible to regulate the amount of water taken from the soil by plants? and (3) ls it possible to prevent loss of water by seepage? To answer these questions a host of secondary problems arose, such as the effect of cultivation on soil water evaporation, the methods of irrigation for the most effective use of water, the effect of available plant foods on transpiration, the effect of shade on the direct evaporation from the soil, and so on.
Widtsoe, John A., "Bulletin No. 105 - Factors Influencing Evaporation and Transpiration" (1909). UAES Bulletins. Paper 56.