The correct determination of natural flows in a hydrological basin is of great help in defining control mechanisms that would permit the sustainable use of water resources. The possibility of quantifying the natural flows is conditioned to the existence of historical records of booth gauged flows and the remainder parameters of the mass conservation equation. In most basins water is used for agricultural activity; however, in very few occasions is it possible to have measurements of the water volume that returns to surface flow after its agricultural use. Therefore, agricultural return flow is commonly quantified as a percentage of the total water volume used for this activity. When no previous estimates of the agricultural return flows in the studied basin are available, it is a common practice to use typical values reported in literature, usually taking the return flow to be between 20% to 50%. This introduces severe uncertainty to the determination of natural flows in the basin.