Hydrogeologic controls on underflow in alluvial valleys : implications for Texas water law

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Larkin, Randall G.

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Groundwater flow in alluvial valleys consists of two components, baseflow and underflow. The baseflow component of the Darcy flux flows normal to the river and contributes to the surface flow. The underflow component moves downstream in the same direction as the river but at a much slower rate. Underflow is important in Texas because the conjunctive use of groundwater and surface water is regulated by controlling the diversion of underflow by wells. Land owners in Texas are legally entitled to unrestricted use of the underground water beneath their property. Stream underflow, however, has been expressly excluded from the definition of underground water. The distinction is important because it allows the State to legally restrict the non-domestic pumpage of groundwater (in an "underflow zone") near streams. Regulators are interested in controlling pumpage near rivers in order to prevent streamflow depletion. Historically, the underflow exemption has not been well recognized by the courts. In large measure, this may be due to the fact that our understanding of underflow in alluvial valleys is incomplete. If the underflow rule is to be successfully implemented, a complete understanding of the nature and occurrence of underflow is imperative. This study was initiated to: 1) determine the hydrogeologic factors that control underflow (and baseflow) in alluvial valley aquifers in Texas and the United States; and 2) to examine the suitability of the underflow criterion as a management tool for the prevention of streamflow depletion by wells. To accomplish this, a data base of 23 alluvial river basins was compiled and a 3-dimensional digital model of a hypothetical alluvial valley aquifer was constructed. Examples from the data base indicate that alluvial aquifers can be classified into three types based on the predominant regional groundwater flow direction: baseflow-dominated, underflow-dominated, and mixed flow. Flow patterns can be transient, however, and respond rapidly to changing river stage if the aquifer and the riverbed are highly permeable. Therefore, the distinction must be made between local, transient underflow and baseflow occurring near the river and regional, steady state underflow and baseflow away from the river. Underflow dominated aquifers are found in classic bedload depositional settings which are characterized by high channel gradient, high width to depth ratio, low channel sinuosity, and low river penetration. Linear regressions performed on the parameter values in the data base verify the validity of the data. The degree of correlation provides the basis for a method of estimating the predominant regional groundwater flow direction in an alluvial aquifer based on geomorphologic and morphometric data. The results from the digital model agree with the findings from the data base. Digital simulations indicate that the amount of underflow is directly related to the channel gradient, the amount of recharge, the aquifer hydraulic conductivity, and the streambed hydraulic conductivity. The riverbed hydraulic conductivity is the most critical hydraulic factor controlling the amount of underflow. The output from the model is 100 percent underflow at low values of riverbed permeability. Both the model results and published field data do not support the existence of a significant local "underflow zone" adjacent to rivers in large alluvial systems. Close to the river, the baseflow component may predominate even in regionally underflow-dominated systems due to the influence of high transverse valley gradients. There are many problems associated with the use of underflow as a management tool. The definition is vague and ambiguous. Underflow can be transient and spatially variable. Texas alluvial systems are baseflow dominated and there is probably no significant "underflow zone" near rivers. Lastly, the presence of underflow has been difficult to prove in court. It is the finding of this study that the underflow criterion is insufficient to prevent streamflow depletion by wells. The underflow rule in the Texas Water Code should be reconsidered, or perhaps abandoned, in favor of criteria that are more justifiable.