Climate change impacts on the hydrology of south-western Australia

<p>Climate change has profoundly affected the hydrology of south-western Australia since at least 1975. It took over a decade before the signal could be detected from annual variability. The impacts of rainfall reductions were exacerbated by higher temperatures and a decrease in wet periods when most recharge and runoff occurred. As a rule-of-thumb, runoff and recharge reduced by 3 percent for each percent reduction in rainfall.</p><p>Reductions in runoff were driven by falling groundwater levels. Stream- and dryland-salinity required levels be monitored, otherwise this driver would have gone unnoticed.</p><p>Runoff into reservoirs has almost ceased as processes irreversibly changed. Using historical records to estimate future runoff had limited application because of non-stationary processes.</p><p>While water resources have diminished, the threats posed by dryland salinity, stream salinity, flooding and waterlogging have decreased. While winter flood risks have dramatically reduced, summer flood risks appear to have increased.   </p><p>Almost all GCMs project an even drier and warmer future. Perth (population 2m) has avoided a ‘Day Zero’ by the rapid expansion of shallow- and deep-groundwater extraction, and seawater desalination. Highly treated wastewater has started to be added to augment drinking water aquifers.</p><p>Recharge under tree canopies have been most reduced. This is due to greater interception losses because showers have largely replaced heavy rain, and trees using a higher proportion of rainfall. Rainfall intensities, at least for long durations, have decreased despite the fear that higher sea surface temperatures (SST) and a warmer atmosphere will result in more intense rainfall. While SSTs have started to rise, there are complications related to El Niño– Southern Oscillation, the Indian Ocean Dipole and the warm Leeuwin Current that flows down the coast of Western Australia. This current results in much higher rainfall than would be expected and may weaken if El Niño becomes stronger and/or more frequent.  </p><p>As well as impacting water resources and rates of land degradation, climate change has affected ecosystems and industries. Abnormally hot and dry years have resulted in the deaths of trees able to withstand harsh Mediterranean summers. Wetlands have dried and groundwater-dependent ecosystems have been lost. Cereal crops are now grown in regions that used to be severely affected by soil waterlogging.  Tree plantations have become unviable due to slow wood growth and deaths.</p><p>Water restriction may have exacerbated urban heat islands as outdoor areas are irrigated less often, losing evaporative cooling. Fortunately, there are opportunities for diverting stormwater and treated wastewater to urban aquifers that provide a non-potable source of water for self-supply.</p><p>Government regulations and planning that have been set during the pre-1975 climate are struggling to keep pace with changes in understanding and future predictions. Restrictions tackling old problems are not being replaced with those needed for new issues. It is difficult to allocate water on a fixed volumetric basis when runoff and recharge are highly impacted. Society is also having to accept water reuse more quickly than is ideal.   </p><p>Lessons learned in SW Australia may be applicable to other Mediterranean climate zones.</p>

Surface water as a cause of land degradation from dryland salinity

Secondary dryland salinity is a global land degradation issue. Drylands are often less developed, less well instrumented and less well understood, requiring us to adapt and impose understanding from different hydro-geomorphological settings that are better instrumented and understood. Conceptual models of secondary dryland salinity, from wet and more hydrologically connected landscapes imposed with adjustments for rainfall and streamflow, have led to the pervasive understanding that land clearing alters water balance in favour of increased infiltration and rising groundwater that bring salts to the surface. This paper presents data from an intra-catchment surface flow gauging network run for 6 years and a surface-water–groundwater (SW–GW) 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 and 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 that 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 show 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 suggest catchment internal redistribution of relatively fresh runoff onto the valley floor is a major contributor to the 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 only acting in isolation to cause secondary salinity but are also interacting 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 a 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.

Surface water as a cause of land degradation from dryland salinity

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.