A hydrological framework for persistent pools along non-perennial rivers (original) (raw)
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A hydrological framework for persistent river pools
2021
Persistent surface water pools along non-perennial rivers represent an important water resource for plants, animals, and humans. While ecological studies of these features are not uncommon, these are rarely accompanied by a rigorous examination of the hydrological and hydrogeological characteristics that create or support the pools. Here we present an overarching framework for understanding the hydrology of persistent pools. We identified perched water, alluvial through flow and groundwater discharge as mechanisms that control the persistence of pools along river channels. Groundwater discharge is further categorized into that controlled by a geological contact or barrier (not previously described in the literature), and discharge controlled by topography. Emphasis is put on clearly defining through-flow pools and the different drivers of groundwater discharge, as this is lacking in the literature. A suite of diagnostic tools (including geological mapping, hydraulic data and hydrochemical surveys) is generally required to identify the mechanism(s) supporting persistent pools. Water fluxes to pools supported by through-flow alluvial and bedrock aquifers can vary seasonally and resolving these inputs is generally non-trivial. This framework allows the evaluation of the susceptibility of persistent pools along river channels to changes in climate or groundwater withdrawals. Finally, we present three case studies from the Hamersley Basin of northwestern Australia to demonstrate how the available diagnostic tools can be applied within the proposed framework.
A hydrological framework for persistent river pools in semi-arid environments
Persistent surface water pools along non-perennial rivers represent an important water resource for the plants, 10 animals, and humans that inhabit semi-arid regions. While ecological studies of these features are not uncommon, these are rarely accompanied by a rigorous examination of the hydrological and hydrogeological characteristics that create or support the pools. Here we present an overarching framework for understanding the hydrology of persistent pools based on data from 22 pools in the Hamersley Basin in Western Australia. Three dominant mechanisms that control the occurrence of persistent pools have been identified; perched pools, through flow 15 pools and groundwater discharge pools. Groundwater discharge pools are further categorized into those that are present because of a geological contact or barrier, and those that are controlled by topography. A suite of diagnostic tools (including geological mapping, hydraulic data and hydrochemical surveys) is generally required to identify the mechanism supporting persistent pools. Perched pools are sensitive to climate variability but their persistence is largely independent of groundwater withdrawals. Water fluxes to pools from alluvial and bedrock 20 aquifers can vary seasonally and resolving these inputs is generally non-trivial. The susceptibility of through-flow and groundwater discharge pools to climate variations and groundwater withdrawals depends on the mechanism of pool persistence and the spatial distribution of stressors relative to the pool. Although this framework was developed in the context of northwestern Australia, this framework can also be applied to pools and springs found along non-perennial rivers around the world. 25
Hydraulically Disconnected Rivers in the Highlands and Southern Riverine Plain of S.E Australia
Sustainability
The rivers of south-eastern Australia flow within a complex meander tract (Coonambidgal Formation) formed by phases of Quaternary stream activity. Pumping tests, hydrochemistry and groundwater monitoring of the Campaspe, Loddon and Murray River Valleys show that for significant parts of their courses, the rivers and their associated strip aquifers form a single integrated hydraulic unit perched above and disconnected from the regional water table by an underlying aquitard developed at the top of a varyingly thick and temporally dynamic vadose zone. Loss to the regional aquifer is not restricted to the riverbed but covers the entire width of the Coonambidgal Formation aquifer, which is one or two orders of magnitude greater. River-bed flux is not a measure of net river loss. Through diffusion and dispersion from the overlying saturated zone, aquitard enhancement or development is augmented by chemical processes active towards the top of the vadose zone. Unlike river-bed clogging, che...
The Influence of Groundwater Discharge in the Mallee Landscape, Victoria, Australia
ABSTRACT: In northern Victoria the regional groundwater flow passes northwestwards towards discharge zones in the Mallee. Discharge landforms occur as small individual salinas and gypsum flats scattered throughout the Maliee landscape, and as large discharge complexes - here termed "boinkas" mainly confined to the Sunset Country of far NW Victoria. The interaction between the groundwater systems and the discharge salinas is reflected in the chemistry of both the salina and the underlying aquifer. The landscape within the boinka is strikingly dissimilar to, the surrounding aeolian Malice landscape, and contains its own distinct suite of landforms. While most boinkas are seen as having been initiated by temporally distant tectonic influences their ongoing deepening and expansion is essentially linked to groundwater discharge processes varyingly active during the Pleistocene. During their development the discharge zones have modified the Mallee landscape by truncating, and at times engulfing and isolating remnants of the linear dunefield on their windward side, and, by blocking further additions from upwind, have disrupted the dunefield downwind in a shadow zone to the lee of the discharge zone. Although regional groundwater discharge is still very active today, the overall distributions of discharge landforms reflects periods of significantly higher water tables than at present.
River–aquifer interactions in a semi-arid environment stressed by groundwater abstraction
2012
A critical hydrological process is the interaction between rivers and aquifers. However, accurately determining this interaction from one method alone is difficult. At a point, the water exchange in the riverbed can be determined using temperature variations over depth. Over the river reach, differential gauging can be used to determine averaged losses or gains. This study combines these two methods and applies them to a 34 km reach of a semiarid river in eastern Australia under highly transient conditions. It is found that high and low river flows translate into high and low riverbed Darcy fluxes, and that these are strongly losing during high flows, and only slightly losing or gaining for low flows. The spatial variability in riverbed Darcy fluxes may be explained by riverbed heterogeneity, with higher variability at greater spatial scales. Although the river-aquifer gradient is the main driver of riverbed Darcy flux at high flows, considerable uncertainty in both the flux magnitude and direction estimates were found during low flows. The reachscale results demonstrate that high-flow events account for 64% of the reach loss (or 43% if overbank events are excluded) despite occurring only 11% of the time. By examining the relationship between total flow volume, river stage and duration for in-channel flows, we find the loss ratio (flow loss/total flow) can be greater for smaller flows than larger flows with similar duration. Implications of the study for the modeling and management of connected water resources are also discussed. PUBLICATIONS measured both the spatial and temporal variation of water fluxes from multiple methods [e.g., Su et al., 2004;.
Hydrology and Earth System Sciences, 2007
This paper reports on a major UK initiative to address deficiencies in understanding the hydro-ecological response of groundwater-dominated lowland catchments. The scope and objectives of this national programme are introduced and focus on one of three sets of research basins the Pang/Lambourn Chalk catchments, tributaries of the river Thames in southern England. The motivation for the research is the need to support integrated management of river systems that have high ecological value and are subject to pressures that include groundwater abstraction for water supply, diffuse pollution, and land use and climate change. An overview of the research programme is provided together with highlights of some current research findings concerning the hydrological functioning of these catchments. Despite the importance of the Chalk as a major UK aquifer, knowledge of the subsurface movement of water and solutes is poor. Solute transport in the dual porosity unsaturated zone depends on fracture/matrix interactions that are difficult to observe; current experimental and modelling research supports the predominance of matrix flow and suggests that slow migration of a time-history of decades of nutrient loading is occurring. Groundwater flows are complex; catchments vary seasonally and are ill-defined and karst features are locally important. Groundwater flow pathways are being investigated using natural and artificial geochemical tracers based on experimental borehole arrays; stream-aquifer interaction research is using a combination of geophysics, borehole array geochemistry and longitudinal profiles of stream flow and solutes. A complex picture of localised subsurface inflows, linked to geological controls and karst features, and significant longitudinal groundwater flow below the river channel is emerging. Management implications are discussed. Strategies to control surface application of nutrients are expected to have little effect on groundwater quality for several decades, and new modelling tools for decision support have been developed to represent these effects. Conventional modelling approaches are limited by the complexities of the subsurface system; catchment areas are difficult to define, hence tracking pollutant pathways to stream receptors is also problematic. Conventional distributed groundwater models have difficulty in capturing key aspects of the groundwater system. This raises important questions concerning the confidence that can be placed in models as routinely used for decision support and the level of knowledge required for catchment management to be placed on a secure scientific foundation.
Geomorphology, 2002
The allocation of water for environmental purposes is a key management issue in many dryland regions. Many different methods have been developed for determining environmental water requirements but these are not directly applicable to dryland rivers because of inherent flow and habitat variability. An ecosystem approach for determining environmental water allocations in dryland regions is presented in this paper. This four-step process involves (1) a hierarchical characterisation of the river system, to assess what mesohabitats are present and where they are located; (2) the determination of flows that would inundate these habitats and perform other key ecological processes; (3) hydrological analyses in which the key hydrological signatures of the river are identified and the impact of water resource development on these is determined; and (4) the derivation of a water management decision tree that enables managers to allocate water to consumptive users during individual flood pulses (events). It is recommended that the flood pulse should be the focus for environmental flow management in dryland regions. If rivers are indeed nested hierarchies, then a change in hydrological behaviour at the scale of a flood pulse will, with time, extend throughout the hydrological hierarchy. Current environmental flow management strategies in dryland river systems are essentially focused at the flow regime and history scale; this is inappropriate given the inherent flow variability of these systems. The ecosystem approach is outlined for the Condamine-Balonne River, a large dryland system in Australia.
Downstream hydraulic geometry of rivers in Victoria, Australia
Geomorphology, 2008
Downstream variation of hydraulic geometry in rivers, characterized by fine textured banks and low width to depth ratios (7-25), is investigated in Victoria, Australia, with the aim of developing predictive models of channel geometry for large-scale spatial modeling applications. A onedimensional hydraulic model is used to determine the mean bank-full geometry and discharge (Q bf ) for 93 sites which are investigated in relation to discharge of fixed average recurrence interval (ARI). The median bank-full ARI is estimated at 0.8 years with 75% of sites between 0.5 and 2.5 years. Exponents in the downstream hydraulic geometry relations for width, depth and velocity are respectively 0.43, 0.40 and 0.18 (Q = Q bf ) and 0.44, 0.38 and 0.03 (Q = Q 2 , i.e., 2-year ARI), falling near the mode of global values. Q 2 and slope explain 66% of variance in Q bf , while Q 2 explains 73% and 69% of the variance in width and depth relations, respectively: Q 2 provides a reliable substitute for Q bf in spatial modeling applications. Spatial variation in hydraulic geometry relations within and between river basins remains largely unexplained. The W/D ratio characteristically decreases with increasing distance along the lower reaches of most rivers and this has contributed to the lower than expected value for the width exponent.
Water regimes and littoral plants in four weir pools of the River Murray, Australia
Regulated Rivers: Research & Management, 2000
The composition and distribution of littoral vegetation in four weir pools of the lower Murray were surveyed in summer 1994. Between-weir gradients in the amplitude of water level fluctuations were reflected in the typical distributions of plants, with a 4-6 m elevational range in upper-pool sites, where levels fluctuate most, and a 1 -1.5 m band in the lower-pool sites, where levels are more stable. Forty-one of 48 species occurred across much of the longitudinal × elevational site matrix within this cone-shaped distribution, indicating considerable tolerance to flooding and exposure; this was especially apparent for Phragmites australis, Cyperus spp. and Centipeda spp. The 41 species were represented in seven of nine water-regime groups identified by cluster analysis. The remainder, found within 9 1 m of the water surface in lower-pool reaches, were aquatic macrophytes such as Vallisneria americana and Typha spp. and amphibious 'mudmats' such as Glossostigma elatinoides. Water regimes at given sites were measured by the number of days in 2 years flooded to any depth ( \ 0 cm), or to 0 -30 cm, and by days exposed by \ 100 cm. Inter-pool differences in the median number of days flooded to \ 0 cm and 0 -30 cm were 3 -30% and B 8%, respectively, for all species except Typha spp. but an order of magnitude for the number of days exposed by \ 100 cm. However, eight of 14 common or representative species analysed showed significant inter-pool differences in the number of days flooded to \0 cm, indicating that sufficient variation exists to necessitate considerable intra-pool replication to allow for the detection of statistical differences in a multi-pool experiment. The practice of maintaining stable weir pool levels limits vegetation processes, e.g. germination, recruitment, decomposition. An increase in the amplitude of river level fluctuations during low flows, from the current 10 -20 cm range to 20 -50 cm, would reinstate water regimes suitable to the majority of species surveyed.