During the day, the shallower regions of a reservoir sidearm absorb more heat per unit volu1ne than the deeper parts, leading to a horizontal pressure gradient that drives a circulation in the sideann. At night, the shallow regions cool more rapidly, leading to a circulation in the opposite direction. Since the spin-up ti1ne of a typical sideann is at least of the sa1ne order as a day, the flmv -vvithin a diurnally forced sideann is principally an inertia-buoyancy balance. In this paper, a diurnally forced sideann is 1nodellecl by periodically forced natural convection in a triangular cavity. The periodic forcing enters the 1nodel via an internal heating/ cooling tenn in the tmnperature equation. Reservoir sideanns typically have small bottmn slopes and this fact can be exploited to obtain asy1nptotic solutions of the resulting equations. These solutions clearly dmnonstrate the transition frmn the viscous-dmninated flmv in the shallows to the inertia-dmninated flow in the deeper parts. In the inertiadmninated region, the flow response significantly lags the forcing. Nu1nerical solutions of the full nonlinear problmn are consistent with the asymptotic solution~.

Abstract. Assessing the potential for transfer of pollutants and nutrients across catchments is of primary importance under changing land use and climate. Over the past decade the connectivity/disconnectivity dynamic of a catchment has been related to its potential to export material; however, we continue to use multiple definitions of connectivity, and most have focused strongly on physical (hydrological or hydraulic) connectivity. In contrast, this paper constantly focuses on the dynamic balance between transport and material transformation, and defines material connectivity as the effective transfer of material between elements of the hydrological cycle. The concept of exposure timescales is developed and used to define three distinct regimes: (i) which is hydrologically connected and transport is dominated by advection; (ii) which is hydrologically connected and transport is dominated by diffusion; and (iii) which is materially isolated. The ratio of exposure timescales to material processing timescales is presented as the non-dimensional number, N E , where N E is reaction-specific and allows estimation of relevant spatial scales over which the reactions of interest take place. Case studies within each regime provide examples of how N E can be used to characterise systems according to their sensitivity to shifts in hydrology and gain insight into the biogeochemical processes that are signficant under the specified conditions. Finally, we explore the implications of the N E framework for improved water management, and for our understanding of biodiversity, resilience and chemical competitiveness under specified conditions.

Asymptotic and numerical methods are used to analyse periodically forced natural convection over slowly varying topography. This models the diurnal heating/cooling cycle in lakes and reservoirs. The asymptotic solution includes the effects of advection on the temperature. The asymptotic results are confirmed by the numerical results. The numerical results are also used to examine flow regimes where the asymptotic results break down. In particular, the presence of a vertical boundary leads to a permanent stratification in the deeper regions due to a nonlinear pumping process in the shallows. Heat transfer calculations and two limiting cases are also presented.

This paper reports the results of several direct numerical simulations of the withdrawal of a two-layer fluid with a finite-thickness interface through a slot in the base of a finite rectangular cavity. Particular attention is paid to the role of long (basin scale) interfacial waves on the processes leading to drawdown of the interface into the slot. It is shown that these waves play an important role and can either delay or accelerate drawdown. This means that drawdown can occur over a range of Froude numbers. The results are compared with previous work for ideal flow and experimental results.