IWRA Proceedings

AbstractThe 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. The objective of the present work is to propose an alternative method for the estimation of agricultural return flow in hydrological basins, based on the detection and correction of the anomaly generated by return flow in the annual hydrogram of monthly mean natural flows. The method consists of determining the annual hydrogram of monthly mean natural flow from the available historical hydrometric records and the mass conservation equation, with no initial consideration of agricultural return flows. If increments in the natural flows are observed during the dry season, they can only be attributed to return flows. This constitutes an anomaly with respect to the theoretical hydrogram, and occurs during the dry months with the greater demand of irrigation water (from March to June in the Rio Florido basin). The anomaly can be corrected by introducing irrigation return flow values (as a percentage of the volume used for irrigation), until the deformation of the hydrogram disappears. With this procedure it is possible to determine a return flow ratio to a higher degree of certainity. In order to exemplify the method here presented, the middle basin of the Florido river (Río Florido) in the northern Mexican state of Chihuahua was used. In this basin is located the Irrigation District 103, and has hydraulic infrastructure for the use of surface water in agricultural production. The results of the developed example case show that the proposed method is consistent with the values reported in literature, and reduces uncertainty in the estimation of natural flows.