Understanding trends in hydrologic extremes across Australia

Highlights

• Australian Landscape Water Balance model is used to evaluate hydrologic trends.

• Trends in modelled streamflow are evaluated against observed streamflow.

• Rainfall, soil moisture, and streamflow trends are consistent with tropical expansion.

• Changes in flood and drought are linked to mean rainfall changes at continental scale.

• Trends are more likely to be driven by local factors at the catchment scale.

Abstract

Changes in the hydrologic cycle have far reaching impacts on agricultural productivity, water resources availability, riverine ecosystems, and our ability to manage environmental assets, bushfire risk, and flood hazard. For example, declining rainfall in the southeast of Australia has led to a prolonged period of drought, with serious impacts on agriculture, the environment, and water supply to urban and rural towns. Here, using the continental wide Australian Water Resources Assessment Landscape model (AWRA-L), we evaluate historical trends from 1960 to 2017 in rainfall, soil moisture, evapotranspiration, and runoff to explain changing drought and flooding. Northern parts of Australia have experienced increasing annual rainfall totals, resulting in increased water availability in the tropics with increased soil moisture, evapotranspiration, and runoff, particularly during the hot, wet monsoon season. In contrast, the southwest and southeast coast of Australia have experienced declines in rainfall, particularly in the colder months, corresponding with decreasing evapotranspiration, soil moisture, and runoff. Trends in flooding are aligned with runoff trends, and closely follow trends in rainfall, with changes in soil moisture of secondary influence. Streamflow droughts, measured by the standardised runoff index, are increasing across large parts of Australia, with these increases more widespread than changes in rainfall alone. Increases in rainfall in the tropics of northern Australia appear to be related to decreasing drought occurrence and extent, but this trend is not universal, suggesting changes in rainfall alone are not an indicator of changing drought conditions.

Introduction

One of the most important questions in hydrology is how the hydrologic cycle is being impacted by climate change (Huntington, 2006, Koutsoyiannis, 2020). Water in the landscape and rivers is essential for: agriculture, town and industrial water supply, the environment, cultural values, recreation, hydropower and more; hence changes to when and where water is available has far reaching impacts. In terms of hydrologic extremes, flood and droughts are among the costliest natural disasters in the world, causing risk to life, food security and, in the case of drought, increased bushfire risk.

Globally, precipitation has increased as per energy constraints (Allan et al., 2020, Allen and Ingram, 2002) at approximately 2.4 mm/decade (Becker et al., 2013, Hartmann et al., 2013) in line with climate model projections of a 2% increase in precipitation per degree increase in global mean temperature (Kharin et al., 2013). Most of the increases in precipitation have occurred over tropical areas with decreases elsewhere (Beck et al., 2019), while precipitation extremes such as annual maxima of daily rainfall have increased universally at a rate close to 7% per degree increase in global mean temperature (Sun et al., 2020, Westra et al., 2013). There is evidence that, consistent with changes in rainfall, soil moisture has experienced wetting trends in the tropics and drying trends in the extra tropics (Feng and Zhang, 2015, Liu et al., 2019) but trends differ depending on the data set used (Albergel et al., 2013).

Compared to changes in annual average conditions, changes in hydrological extremes are more challenging to quantify and understand. Increasing temperatures have increased the likelihood of drought conditions in many regions of the world (Hartmann et al., 2013, Liu et al., 2019), though again, results differ depending on the index used (Asadi Zarch et al., 2015). Despite many studies pointing to increases in extreme precipitation (Donat et al., 2013, Groisman et al., 2005, Martinez-Villalobos and Neelin, 2018, Sun et al., 2020, Westra et al., 2013), there is a very mixed trend in flood response (Sharma et al., 2018) with more sites globally showing decreases in flooding than increases (Do et al., 2017). Despite this variability, trends in high flows follow average annual flow trends both globally (Gudmundsson et al., 2019) and across Australia (Zhang et al., 2016). In regions free of snow-melt, it has been postulated that changes in flooding are due to changing antecedent soil moisture conditions (Ivancic and Shaw, 2015, Sharma et al., 2018, Wasko et al., 2019). However, few studies have studied trends in antecedent conditions explicitly (Tramblay et al., 2019, Wasko and Nathan, 2019a).

Australia forms a valuable test case for investigating the impacts of climate change on the hydrologic cycle. It straddles the tropics and extratropics, experiencing a wide range of climates, and is largely free of snowmelt. In addition, trends in Australian precipitation and temperature reflect global patterns (Alexander et al., 2007), although streamflow exhibits extremely high variability compared to the rest of the world (Chiew and Mcmahon, 2002, McMahon et al., 2007, Peel et al., 2004).

Mean annual rainfall has increased in the tropical north of Australia and decreased in the south (Dey et al., 2019), particularly along the south-east and south-west coasts due to changed cutoffs and frontal systems (Risbey et al., 2013) consistent with a tropical expansion (Grise et al., 2018, Staten et al., 2018). For example, south-western Australia has undergone decreased cool season rainfall in April to October, with sharp decreases in May-July rainfall since 1970. There has been a similar decline of rainfall over south-eastern Australia in April to October rainfall since the late 1990s. Meanwhile, in northern Australia, there has been an observed rainfall increase since the 1970s, especially in the northwest (CSIRO & BOM, 2018).

Since the 1950s increasing streamflow in the northern tropics and decreasing streamflow over southern Australia has been observed (Zhang et al., 2016). Across Australia studies of trends in evapotranspiration show mixed trends temporally and spatially. Prior to 1999, decreasing trends in observed pan evaporation across Australia were linked to declining wind speeds (Johnson and Sharma, 2010), but since then, many sites have shown increased pan evaporation, possibly due to an increased vapor pressure deficit (Stephens et al., 2018). Despite increases in global estimates of terrestrial evapotranspiration (Zeng et al., 2018), evapotranspiration estimates derived from remote sensing and flux towers show a decrease in evapotranspiration between 1998 and 2008 attributed to moisture limitations (Jung et al., 2010).

There is a large discrepancy in trends calculated between different soil moisture products, but in general datasets point to more severe droughts in arid and semi-arid regions (Liu et al., 2019). Using data obtained from the European Space Agency, soil moisture was found to be decreasing across Australia between 1979 and 2013, with the exception of the tropical north (Feng and Zhang, 2015). However, another study for 1988–2010 using multiple reanalysis and satellite derived data sets indicated largely conflicting results between data sets, particularly at the regional scale (Albergel et al., 2013). This may be because reanalysis data sets, particularly ERA-INTERIM, have large rainfall biases (Andersson et al., 2015, Weedon et al., 2014) while remote sensed data sets measure soil moisture at different depths (Holgate et al., 2016).

The magnitude of extreme hourly and daily rainfall, across a range of exceedance thresholds, has increased over the last half a century across Australia (Alexander and Arblaster, 2017, Guerreiro et al., 2018) with shifts to a greater frequency of longer duration events in the tropics and shorter duration events in extra-tropics (Dey et al., 2020). Despite more evidence for increasing rainfall extremes compared to decreases, more streamflow gauging sites show decreases in annual maxima than increases (Ishak et al., 2013, Zhang et al., 2016). Decreased flooding has been attributed to decreased antecedent soil moisture prior to the onset of the storm events across southern parts of Australia (Wasko and Nathan, 2019a). A better understanding of the drivers of changes on streamflow and flooding, that is, rainfall, evapotranspiration and soil moisture, and how they interact, is required to better predict the likely impacts of climate change on the hydrological cycle, and subsequently economic activity, the environment, communities, and ecosystems (Berghuijs et al., 2019).