Antonis D. Koussis

Groundwater and climate in Africa—a review

RICHARD G. TAYLOR, ANTONIS D. KOUSSIS & CALLIST TINDIMUGAYA 1 Department of Geography, University College London, Gower Street, London WC1E 6BT, UK r.taylor@geog,.ucl.ac.uk 2 Institute for Environmental Research, National Observatory of Athens, I. Metaxa & Vassileos Pavlou, GR-152 36 Palaia Penteli, Athens, Greece akoussis@meteo.noa.gr 3 Directorate of Water Resources Management, Ministry of Water & Environment, PO Box 19, Entebbe, Uganda callist.tindimugaya@mwe.go.ug

Tipping points for seawater intrusion in coastal aquifers under rising sea level

This study considers different projections of climate-driven sea-level rise and uses a recently developed, generalized analytical model to investigate the responses of sea intrusion in unconfined sloping coastal aquifers to climate-driven sea-level rise. The results show high nonlinearity in these responses, implying important thresholds, or tipping points, beyond which the responses of seawater intrusion to sea-level rise shift abruptly from a stable state of mild change responses to a new stable state of large responses to small changes that can rapidly lead to full seawater intrusion into a coastal aquifer. The identified tipping points are of three types: (a) spatial, for the particular aquifers (sections) along a coastline with depths that imply critical risk of full sea intrusion in response to even small sea-level rise; (b) temporal, for the critical sea-level rise and its timing, beyond which the change responses and the risk of complete sea intrusion in an aquifer shift abruptly from low to very high; and (c) managerial, for the critical minimum values of groundwater discharge and hydraulic head that inland water management must maintain in an aquifer in order to avoid rapid loss of control and complete sea intrusion in response to even small sea-level rise. The existence of a tipping point depends on highly variable aquifer properties and groundwater conditions, in combination with more homogeneous sea conditions. The generalized analytical model used in this study facilitates parsimonious quantification and screening of sea-intrusion risks and tipping points under such spatio-temporally different condition combinations along extended coastlines.

Groundwater drainage flow in a soil layer resting on an inclined leaky bed

We study subsurface storm flow from a planar hill slope, a problem that is similar hydraulically to lateral flow toward drains in landfills. Our analysis is based on the linearized one-dimensional Boussinesq equation (Dupuit-Forchheimer approximation), which is extended to allow for leakage through the underlying barrier. This linear advection-diffusion equation has a greater range of validity than the kinematic wave equation. Stating it in terms of the discharge, the variable of primary hydrologic interest, we integrate it numerically, using an adaptation of the Muskingum-Cunge routing scheme. A single-step computation of the outflow hydrograph, which combines the convenience of an analytical solution formula with acceptable accuracy, is proposed as a design tool and as a means of parameterization of drainage from hill slopes. Depth profiles are determined afterwards by a simple integration of Darcys law. Examples of the buildup and recession phases, with and without leakage, demonstrate the application of the computational method.

Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: application to the Akrotiri basin and aquifer, Cyprus

Abstract We investigate the general methodology for an intensive development of coastal aquifers, described in a companion paper, through its application to the management of the Akrotiri aquifer, Cyprus. The Zakaki area of that aquifer, adjacent to Lemessos City, is managed such that it permits a fixed annual agricultural water demand to be met, as well as and a fraction of the water demand of Lemessos, which varies according to available surface water. Effluents of the Lemessos wastewater treatment plant are injected into the aquifer to counteract the seawater intrusion resulting from the increased pumping. The locations of pumping and injection wells are optimized based on least-cost, subject to meeting the demand. This strategy controls sea intrusion so effectively that desalting of only small volumes of slightly brackish groundwater is required over short times, while ∼2.3 m3 of groundwater is produced for each 1 m3 of injected treated wastewater. The cost over the 20-year period 2000–2020 of operation is ∼40 M€ and the unit production cost of potable water is under 0.2 €/m3. The comparison between the deterministic and stochastic analyses of the groundwater dynamics indicates the former as conservative, i.e. yielding higher groundwater salinity at the well. The Akrotiri case study shows that the proposed aquifer management scheme yields solutions that are preferable to the widely promoted seawater desalination, also considering the revenues from using the treated wastewater for irrigation. Citation Koussis, A. D., Georgopoulou, E., Kotronarou, A., Mazi, K., Restrepo, P., Destouni, G., Prieto, C., Rodriguez, J. J., Rodriguez-Mirasol, J., Cordero, T., Ioannou, C., Georgiou, A., Schwartz, J. & Zacharias, I. (2010) Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: application to the Akrotiri basin and aquifer, Cyprus. Hydrol. Sci. J. 55(7), 1234–1245.

Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: general framework

Abstract Semi-arid coastal zones often suffer water-stress, as water demand is high and markedly seasonal, due to agriculture and tourism. Driven by scarcity of surface water, the communities in semi-arid coastal regions turn to aquifers as prime water source; but intensive exploitation of coastal aquifers causes seawater intrusion, which degrades the quality of groundwater. The cost-efficient and sustainable development of coastal aquifers can be achieved through a holistic management scheme which combines two non-traditional water sources: (a) saltwater, to be treated to the desired quality, and (b) wastewater, to be re-claimed to augment aquifer recharge for control of seawater intrusion, and also to meet certain demands. This management scheme is based on the idea that it is cost-advantageous to: (i) desalt brackish groundwater, instead of seawater, as the former requires far less energy, and (ii) to re‐use wastewater at only the differential cost to any treatment already practiced. In this paper, we present the general framework of the proposed management scheme, and a decision aid tool (DAT) which has been developed to assist decision makers to explore the schemes decision space. The DAT uses cost as optimization criterion to screen various management scenarios, via modelling of the dynamic natural-engineered system behaviour, and identifies those cost-efficient ones that meet the water demand and achieve aquifer protection. Citation Koussis, A. D., Georgopoulou, E., Kotronarou, A., Lalas, D. P., Restrepo, P., Destouni, G., Prieto, C., Rodriguez, J. J., Rodriguez-Mirasol, J., Cordero, T. & Gomez-Gotor, A. (2010) Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: general framework. Hydrol. Sci. J. 55(7),1217–1233.

On the mathematics of storage routing

Abstract Some mathematical aspects of storage routing are investigated analytically and through numerical simulation. The minimum size of the time interval required to eliminate computed values at the beginning of the outflow hydrograph falling below the initial steady flow rate is given for two routing schemes. The stability behavior of a refined Muskingum scheme is explored; it is found that values of the weighting coefficient up to 1.0 may be used with appropriately specified time intervals. The concept of “relative” storage is analyzed and it is shown that it is unnecessary when the correct initial condition of the storage is considered. The classical best straight-line fit procedure between storage and weighted discharge is investigated. In an example case, it is demonstrated that this method of system identification is not reliable. The possibility of using discharge-dependent parameters is discussed.

Two-dimensional modeling of advection-dominated solute transport in groundwater by the matched artificial dispersivity method

A methodology for modeling, in two dimensions, the transport of dissolved constituents in steady groundwater flow is presented. The model aims at the efficient confrontation of the difficulties associated with advection-dominated conditions (oscillation-free resolution of sharp fronts). Employing a spatial splitting algorithm within the framework of the (curvilinear) principal directions of transport formulation, it computes transport along streamlines by a variant of the pseudoviscosity method. The algorithm is subject to mild grid design constraints that afford efficiency in terms of CPU time and storage; its accuracy is tested against analytical solutions. The model is also applied to the simulation of the chloride plume observed under the Borden landfill in Ontario, Canada. Model calibration issues are discussed.

Analytical solution of transient flow in a sloping soil layer with recharge

Abstract An analytical solution of planar flow in a sloping soil layer described by the linearized extended Boussinesq equation is presented. The solution consists of the sum of steady-state and transient-series solutions, the latter in a separation-of-variables form, and can satisfy an arbitrary initial condition via collocation; this feature reduces the number of series terms, making the solution efficient. Key parameter is the dimensionless linearization depth η o (R), R being the dimensionless recharge. The variable η o (R), not the slope, characterizes the flow as kinematic or diffusive, and R ≈ 0.2 demarcates the two regimes. The transient series converges rapidly for large η o (large R, near-diffusive flow) and slowly as η o → 0 (kinematic flow). The quasi-steady (QS) state method of Verhoest & Troch is also analysed and it is shown that the QS depth profiles approximate the transient ones well, only if Δt exceeds a system-dependent transition time between flow states (possibly >>1 day). In an application example for a 30-day recharge series, the QS solution differs from the transient one by as much as 20% (RMSE = 15%), does not track recharge changes as well and fails to conserve mass.

Reply to the Discussion of “Assessment and review of the hydraulics of storage flood routing 70 years after the presentation of the Muskingum method” by M. Perumal

Perumal (2010) (henceforth, also the Discusser) criticizes the author, in essence, for not having presented the Discusser’s work adequately in the (already 19-pages long) review (Koussis, 2009). This could be, perhaps, partly true because one has to be selective in the choice of references, noting that apparently some findings have been made, seemingly independently, by various authors. Therefore, giving appropriate credits to the earliest sources is difficult. However, it will be demonstrated that the Discusser fails to give credit to the work of others). Frankly however, given the volume of publications on the topic of the Muskingum and related methods, I am surprised that other hydrologists, who have contributed in this field, have not voiced more similar critique. Then, Perumal claims to be the first to have brought out the interpretation of the Muskingum method according to Apollov et al. (1969). I respond to this claim in detail in Section 3 herein, yet, what matters is the original work of Apollov et al. itself; hence my reference that “Apollov et al. (1969) give an elegant derivation of equation (31), already presented in 1960.” 2 ON “TRADITIONAL PRESENTATION OF THE MUSKINGUM STORAGE EQUATION”

CAD comparisons for wastewater treatment facilities

Abstract A comparison was made between two computer aided design (CAD) models for wastewater treatment facilities, CAPDET and EXEC/OP, which are widely used in the U.S.A. The purpose of both programs is to optimize preliminary designs and to determine costs for municipal wastewater treatment plants, however, their size, capabilities, and computational procedures are vastly different. CAPDET is a much larger program than EXEC/OP that provides a larger selection of unit operations and has a more refined costing scheme. CAPDET was found to be user‐friendlier, once installed on a main frame computer system. Testing of their operational and design capabilities against a consultants designs has shown that, despite their flaws, both programs can fulfill their purpose satisfactorily.