reasonable job of estimating the net irradiance, it was able to pinpoint a problem area: the partitioning between latent and sensible heat fluxes. This is central to LSM performance and the team’s identification of this problem represents a new level of model evaluation formalism within the climate land science community. A related study led by ARCCSS postdoctoral scientist, Dr Anna Ukkola, evaluated the ability of eight different LSMs to simulate seasonal droughts at six different worldwide sites (Ukkola et al 2016a) (see Figure 2). Given the findings of the previously noted PLUMBER study, it was no real surprise to find that model performance was perhaps not as good as it needs to be for many applications. To put that result in perspective, many previous climate (and LSM) model evaluations have tended to focus on the annual averages of relevant quantities because that was considered the key factor for understanding the (30-year average) climate. The results of our new study demonstrate that the ability to simulate the annual average is no guarantee that we can simulate the seasonal cycle or the climate extremes. In a closely related companion study also led by Dr Ukkola, we evaluated a new version of the Australian LSM-Community Atmosphere Biosphere Land Exchange (CABLE) – that included a new hydrologic scheme developed by Dr Mark Decker — against the older version of CABLE (Ukkola et al 2016b). Over annual time steps there was little difference between the old and new versions but at seasonal time scales there were important differences. The results showed that the new hydrology incorporated into CABLE has value but there is still work to be done. Finally, ARCCSS scientists contributed to a Terrestrial Ecosystem Research Network (TERN) led study involving a consortium of Australian plant physiologists and ecologists who evaluated the ability of eight different LSMs to simulate gross primary productivity and the latent heat flux at five flux sites in the top end of the Northern Territory (Whiteley et al 2016). Careful analysis showed that specifying the dynamic nature of vegetation proved critical to model performance. This is a reminder that many (if not most) LSM schemes used in current climate models include seasonally varying vegetation but the vegetation seasonal cycle repeats from one year to the next. In short, the vegetation is unable to adapt to the prevailing conditions and this represents a shortcoming of current LSMs. It is not surprising that one of the long-term grand challenges of the Land program is to incorporate dynamic vegetation into CABLE and other LSMs.
Land Processes and Feedbacks As noted at the start of the report, pan evaporation is the only measure we currently have of the atmospheric evaporative demand. It is a measureable quantity and closely related to a quantity called potential evaporation that has been widely used in offline hydrologic impact models. This has led to many problems because projected increases in potential evaporation have led many to interpret the future for many regions as being one of increased aridity. Recently, following research reported last year that was led by Professor Roderick, there has been a growing realisation that one of the problems in the current use of potential evaporation is that it is an ill-defined quantity. Conceptually, potential evaporation represents the rate of evaporation when water is not limiting. However, in hydrologic practice it has become common practice to measure the radiation, temperature, humidity and wind and then calculate potential evaporation assuming water is freely available. There is a conceptual problem with this approach in dry environments, like those that cover most of Australia. The problem is
that if water were freely available for evaporation then the radiation, temperature, humidity and wind would likely be very different because the surface and near-surface atmospheres are tightly coupled. In a long running Centresupported collaboration with scientists from ETH-Zurich, we were able to develop an entirely new theoretical framework that can quantitatively account for changes in surface coupling when a landscape dries (or wets) (Aminzadeh et al 2016). This is an exciting development and we look forward to applying the new theory when interpreting both observed and projected changes in potential evaporation. While the topic of evaporation changes under global warming has been controversial, there is no controversy that in many temperate parts of the world, spring has been appearing earlier each year because of ongoing warming. One consequence of an earlier spring is an earlier change in the land surface properties (e.g. albedo decreases as ice melts earlier and vegetation greens up earlier). The change in albedo represents a feedback from the land surface to the atmosphere. While this has long been realised, the climate impact of an earlier European spring was first evaluated by ARCCSS scientists in 2016 in a study led by Dr Shaoxiu Ma and published in Geophysical Research Letters (Ma et al 2016) (Figure 3). The research found that an earlier spring was associated with increased heatwaves within 30 days of the spring green-up. An earlier spring is just one instance of a vegetation-related feedback. A further study on the topic of vegetation, led by Dr Ukkola, noted that changes in vegetation can have both positive and negative effects on water resources changes (Ukkola et al 2016c). One of the key vegetation feedbacks relates to increasing atmospheric CO2 and it is now well established that the water-use efficiency of photosynthesis increases as atmospheric CO2 increases. This effect is widely acknowledged in both the plant physiology and terrestrial ecology research communities but it is still to be fully incorporated into the thinking of the hydrology and climate research communities. There are two main challenges. The first is that an increase in water use efficiency could occur because of an increase in photosynthetic carbon uptake or because of a decrease in transpiration, or some combination thereof (see Figure 4). At the moment there is no constraint on and little detailed knowledge about how the increase in water-use efficiency would occur. A further constraint is that most of the detailed knowledge is available from laboratory experiments: how an increase in leaf-scale wateruse efficiency would impact larger-scale hydrologic (e.g. catchment-scale) or climate (e.g. 1 km grid-box) processes is currently unknown. With that in mind, ARCCSS scientists collaborated with a CSIRO team to undertake the first observation-based assessment of changes in precipitation, runoff and vegetation cover over the global tropics (Yang et al 2016). They found that with increasing CO2, there was as yet no evidence for a change in hydrologic partitioning and no evidence for a change in transpiration. The implication is that with fixed transpiration, there must have been a relatively large increase (i.e., ~10%) in photosynthesis across the global tropics. This observation-based study is a global first and there is an urgent need to conduct similar assessments in other major global biomes. How the land interacts with the atmosphere is measured by land-atmosphere coupling. This has been reported on several times in the past but, in her last publication as a PhD student, Annette Hirsch examined how landatmosphere feedbacks could be evaluated using a resistance pathway framework. Hirsch et al. (2016) suggest that land–atmosphere coupling in the modelling system acts
REPORT 2016 ARC Centre of Excellence for Climate System Science 49 <