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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2019-405
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-405
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Aug 2019

Submitted as: research article | 23 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Surface water as a cause of land degradation from dryland salinity

J. Nikolaus Callow1, Matthew R. Hipsey1, and Ryan I. J. Vogwill2 J. Nikolaus Callow et al.
  • 1The University of Western Australia, 35 Stirling Highway, Crawley, Perth, WA 6009, Australia
  • 2Hydro Geo Enviro Pty Ltd

Abstract. Secondary dryland salinity is a global land degradation issue. Because drylands are often less-developed, less-well instrumented and less-well understood, we often adapt and impose an understanding from different hydro-geomorphological settings. Dryland catchments are likely to exhibit some functional qualities of wet and hydrologically-connected landscapes, but also those more typical of flat and arid rangelands, smooth plainlands and deserts, where flow (dis)connectivity is an important feature. The functional hydrological mechanisms used to conceptualise causes of dryland salinity, originate from wet and more hydrologically-connected landscapes. They are then imposed with adjustments for rainfall and streamflow quantity to describe how hillslope-recharge processes interact with groundwater to cause dryland salinity. The pervasive understanding concludes that low flow yield from the end-of-catchment gauging stations indicates that land clearing alters water balance in favour of increased infiltration and rising groundwater that bring salts to the surface, causing land degradation from dryland salinity.

This paper presents data from an intra-catchment surface flow gauging network run for six years and a surface water–groundwater interaction site to assess the adequacy of our conceptual understanding of secondary dryland salinity in environments with low gradients and runoff yield. The aim is to (re)conceptualise pathways of water and salt redistribution in dryland landscapes, to investigate the role that surface water flows and connectivity plays in land degradation from salinity in low-gradient drylands. Based on the long-term end-of-catchment gauge, average annual runoff yield is only 0.14 % of rainfall. The internal gauging network operated from 2007–2012 found pulses of internal water (also mobilising salt) in years when no flow was recorded at the catchment outlet. Data from a surface water–groundwater interaction site shows top-down recharge of surface water early in the water year, that transitions to a bottom-up system of discharge later in the water year. This connection provides a mechanism for the vertical diffusion of salts to the surface waters, followed by evapo-concentration and downstream export when depression storage thresholds are exceeded. Intervention in this landscape by constructing a broad-based channel to address these processes, resulted in a 25 % increase in flow volume and a 20 % reduction in salinity, by allowing the lower catchment to more effectively support bypassing of the storages in the lower landscape that would otherwise retain water and allow salt to accumulate.

Results from this study suggests catchment internal redistribution of relatively fresh runoff onto the valley floor is a major contributor to development of secondary dryland salinity. Seasonally inundated areas are subject to significant transmission losses and drive processes of vertical salt mobility. These surface flow and connectivity processes are not acting in isolation to cause secondary salinity, but are also interact with groundwater systems responding to land clearing and processes recognised in the more conventional understanding of hillslope recharge and groundwater discharge. The study landscape appears to have three functional hydrological components: upland, hillslope flow landscapes that generate fresh runoff; valley floor fill landscapes with high transmission losses and poor flow connectivity controlled by the micro-topography that promotes surface–groundwater connection and salt movement; and the downstream flood landscapes, where flows are recorded only when internal storages (fill landscapes) are exceeded. This work highlights the role of surface water processes as a contributor to land degradation by dryland salinity in low-gradient landscapes.

J. Nikolaus Callow et al.
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Short summary
Secondary dryland salinity is a global land degradation issue, but understanding of processes causing it comes from wet and hydrologically-connected landscapes. The pervasive understanding concludes that low end-of-catchment runoff indicates land clearing alters water balance in favor of increased infiltration and rising groundwater that bring salts to the surface in drylands. Internal gauging and surface to groundwater connection shows surface flows play an important role in dryland salinity.
Secondary dryland salinity is a global land degradation issue, but understanding of processes...
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