Advances in Water Resources 88 (2016) 21–31 Contents lists available at ScienceDirect Advances in Water Resources journal homepage: www.elsevier.com/locate/advwatres Impact of aquifer desaturation on steady-state river seepage Hubert J. Morel-Seytoux a,∗ , Cinzia Miracapillo b , Steffen Mehl c a Hydroprose International Consulting, 328 Beech Avenue, Santa Rosa, CA 95409, United States b University of Applied Sciences Northwestern Switzerland, School of Architecture, Civil Engineering and Geomatics, Institute of Civil Engineering, Gründenstrasse 40, 4132 Muttenz, Switzerland c Department of Civil Engineering, CSU Chico, Chico, CA 95929-0930, United States a r t i c l e i n f o Article history: Received 21 February 2015 Revised 17 September 2015 Accepted 18 September 2015 Available online 12 November 2015 Keywords: Stream–aquifer interaction Saturated exchange Incipient desaturation Unsaturated seepage a b s t r a c t Flow exchange between surface and ground water is of great importance be it for beneficial allocation and use of the water resources or for the proper exercise of water rights. That exchange can take place under a saturated or unsaturated flow regime. Which regimes occur depend on conditions in the vicinity of the interactive area. Withdrawals partially sustained by seepage may not bring about desaturation but greater amounts eventually will. The problem considered in this paper deals only with the steady-state case. It is meant as a first step toward a simple, yet accurate and physically based treatment of the transient situation. The primary purpose of the article is to provide simple criteria for determination of the initiation of desat- uration in an aquifer originally in saturated hydraulic connection with a river or a recharge area. The extent of the unsaturated zone in the aquifer will increase with increasing withdrawals while at the same time the seepage rate from the river increases. However the seepage increase will stop once infiltration takes place strictly by gravity in the aquifer and is no longer opposed by the capillary rise from the water table below the riverbed. Following desaturation simple criteria are derived and simple analytical formulae provided to estimate the river seepage based on the position of the water table mound below the clogging layer and at some distance away from the river bank. They fully account for the unsaturated flow phenomena, including the existence of a drainage entry pressure. Two secondary objectives were to verify that (1) the assumption of uniform vertical flow through a clogging layer and that (2) the approximation of the water table mound below the seepage area as a flat surface were both reasonably legitimate. This approach will be especially ad- vantageous for the implementation of the methodology in large-scale applications of integrated hydrologic models used for management. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The importance of the flow exchange between stream and aquifer is well recognized. It is important for general water resources man- agement and matters of water rights (e.g. [7,13,14,17,35]). The ability to predict accurately the flow exchange and the parameters and vari- ables that determine it (e.g. [6,16,20,33,41]) is a necessary condition for successful planning, operations and abidance with law. That need has been recognized for a long time (e.g. [18]) and many scientists, some previously cited, have addressed this problem. One issue of concern, particularly in semi-arid areas, is the ques- tion of when will the saturated hydraulic connection cease as a result of water table drawdowns in the vicinity of the losing river. What will be the effect of this loss of saturated hydraulic connection on the magnitude of the seepage from the stream? In this article we address ∗ Corresponding author. Tel.: +1 7079783260. E-mail address: hydroprose@sonic.net (H.J. Morel-Seytoux). the case of a river (or a recharge area; for example an unlined retired irrigation canal which is used for the purpose of aquifer recharge). Typically the riverbed tends to develop at its bottom a clogging layer. A significant contribution from recent works [8–10] was (a) to define through numerical examples, the conditions that must exist for (saturated hydraulic) connection to cease and (b) to emphasize the role of the transition stage between saturated connection and the occurrence, under unsaturated flow conditions, of a maximum seep- age rate (i.e. one that cannot be increased by a further drop of the water table elevation at some distance away from the river bank). Though this fact was generally known it had not been addressed in the literature systematically. Our approach differs from all previous studies in that we are interested in developing a more analytical and simpler method to answer similar questions. This approach is par- ticularly important for the purpose of integrating a more efficient methodology into large-scale regional management models where combined analytical and numerical techniques can be used. As stated by Osman and Bruen ([40, p. 74]): “Currently used simple models of stream aquifer interaction are not completely satisfactory. In theory, http://dx.doi.org/10.1016/j.advwatres.2015.09.012 0309-1708/© 2015 Elsevier Ltd. All rights reserved.