A. M. Ukkola

Land surface models systematically overestimate the intensity, duration and magnitude of seasonal-scale evaporative droughts

Land surfacemodels (LSMs)must accurately simulate observed energy andwater fluxes during droughts in order to provide reliable estimates of futurewater resources.We evaluated 8 different LSMs (14model versions) for simulating evapotranspiration (ET) during periods of evaporative drought (Edrought) across sixflux tower sites. Using an empirically defined Edrought threshold (a decline in ET below the observed 15th percentile), we show that LSMs simulated 58 Edrought days per year, on average, across the six sites,∼3 times asmany as the observed 20 d. The simulated Edrought magnitudewas∼8 times greater than observed and twice as intense. Ourfindings point to systematic biases across LSMswhen simulating water and energy fluxes underwater-stressed conditions. The overestimation of key Edrought characteristics undermines our confidence in themodels’ capability in simulating realistic drought responses to climate change and haswider implications for phenomena sensitive to soilmoisture, including heat waves.

Vegetation plays an important role in mediating future water resources

Future environmental change is expected to modify the global hydrological cycle, with consequences for the regional distribution of freshwater supplies. Regional precipitation projections, however, differ largely between models, making future water resource projections highly uncertain. Using two representative concentration pathways and nine climate models, we estimate 21st century water resources across Australia, employing both a process-based dynamic vegetation model and a simple hydrological framework commonly used in water resource studies to separate the effects of climate and vegetation on water resources. We show surprisingly robust, pathway-independent regional patterns of change in water resources despite large uncertainties in precipitation projections. Increasing plant water use efficiency (due to the changing atmospheric CO2) and reduced green vegetation cover (due to the changing climate) relieve pressure on water resources for the highly populated, humid coastal regions of eastern Australia. By contrast, in semi-arid regions across Australia, runoff declines are amplified by CO2-induced greening, which leads to increased vegetation water use. These findings highlight the importance of including vegetation dynamics in future water resource projections.

Evaluating CMIP5 Model Agreement for Multiple Drought Metrics

AbstractGlobal climate models play an important role in quantifying past and projecting future changes in drought. Previous studies have pointed to shortcomings in these models for simulating droug...