Potential evapotranspiration and continental drying

Abstract. Hydrologic projections are of vital socioeconomic importance. However, they are also prone to uncertainty. In order to establish a meaningful range of storylines to support water managers in decision making, we need to reveal the relevant sources of uncertainty. Here, we systematically and extensively investigate uncertainty in hydrologic projections for 605 basins throughout the contiguous US. We show that in the majority of the basins, the sign of change in average annual runoff and discharge timing for the period 2070-2100 compared to 1985-2008 differs among combinations of climate models, hydrologic models, and parameters. Mapping the results revealed that different sources of uncertainty dominate in different regions. Hydrologic model induced uncertainty in the sign of change in mean runoff was related to snow processes and aridity, whereas uncertainty in both mean runoff and discharge timing induced by the climate models was related to disagreement among the models regarding the change in precipitation. Overall, disagreement on the sign of change was more widespread for the mean runoff than for the discharge timing. The results demonstrate the need to define a wide range of quantitative hydrologic storylines, including parameter, hydrologic model, and climate model forcing uncertainty, to support water resource planning.

Section: Discussionmentioning

confidence: 98%

Mapping (dis)agreement in hydrologic projections

Melsen

Addor

Mizukami

et al. 2018

“…Keenan et al, 2013). Not accounting for this effect in offline E pot estimations has been shown to lead to an overestimation of continental drying (Milly and Dunne, 2016), which is particularly relevant for drought analyses. Therefore, along with E pot estimates for future time slices using the same r s value as in the baseline (variable pepm), an additional variable (pepm_adjrs) is introduced, which accounts for the impact of CO 2 on stomatal resistance and, therefore, on E pot .…”

Section: Potential Evaporation Estimatesmentioning

confidence: 99%

A large set of potential past, present and future hydro-meteorological time series for the UK

Guillod

Jones

Dadson

et al. 2018

Abstract. Hydro-meteorological extremes such as drought and heavy precipitation can have large impacts on society and the economy. With potentially increasing risks associated with such events due to climate change, properly assessing the associated impacts and uncertainties is critical for adequate adaptation. However, the application of riskbased approaches often requires large sets of extreme events, which are not commonly available. Here, we present such a large set of hydro-meteorological time series for recent past and future conditions for the United Kingdom based on weather@home 2, a modelling framework consisting of a global climate model (GCM) driven by observed or projected sea surface temperature (SST) and sea ice which is downscaled to 25 km over the European domain by a regional climate model (RCM). Sets of 100 time series are generated for each of (i) a historical baseline , (ii) five nearfuture scenarios (2020-2049) and (iii) five far-future scenarios . The five scenarios in each future time slice all follow the Representative Concentration Pathway 8.5 (RCP8.5) and sample the range of sea surface temperature and sea ice changes from CMIP5 (Coupled Model Intercomparison Project Phase 5) models. Validation of the historical baseline highlights good performance for temperature and potential evaporation, but substantial seasonal biases in mean precipitation, which are corrected using a linear approach. For extremes in low precipitation over a long accumulation period (> 3 months) and shorter-duration high precipitation (1-30 days), the time series generally represents past statistics well. Future projections show small precipitation increases in winter but large decreases in summer on average, leading to an overall drying, consistently with the most recent UK Climate Projections (UKCP09) but larger in magnitude than the latter. Both drought and high-precipitation events are projected to increase in frequency and intensity in most regions, highlighting the need for appropriate adaptation measures. Overall, the presented dataset is a useful tool for assessing the risk associated with drought and more generally with hydro-meteorological extremes in the UK.

“…In contrast, Betts et al (2007) argue for a dominant global decrease of evapotranspiration induced by a water-saving response to increasing carbon dioxide (CO 2 ) concentrations. Recent modelling studies have found that ignoring the existence of this water-saving response may overpredict future terrestrial evapotranspiration and drought (Milly and Dunne, 2016;Prudhomme et al, 2014;Swann et al, 2016). Piao et al (2007) argue instead that this physiological effect is cancelled by simultaneous CO 2 -induced increases in plant growth and total canopy leaf area, and that changes in climate and land use are the dominant drivers of evapotranspiration changes.…”

Section: F Jaramillo Et Al: Dominant Effect Of Increasing Forest Bimentioning

confidence: 99%

Dominant effect of increasing forest biomass on evapotranspiration: interpretations of movement in Budyko space

Jaramillo

Cory

Arheimer

et al. 2018

Abstract. During the last 6 decades, forest biomass has increased in Sweden mainly due to forest management, with a possible increasing effect on evapotranspiration. However, increasing global CO 2 concentrations may also trigger physiological water-saving responses in broadleaf tree species, and to a lesser degree in some needleleaf conifer species, inducing an opposite effect. Additionally, changes in other forest attributes may also affect evapotranspiration. In this study, we aimed to detect the dominating effect(s) of forest change on evapotranspiration by studying changes in the ratio of actual evapotranspiration to precipitation, known as the evaporative ratio, during the period 1961-2012. We first used the Budyko framework of water and energy availability at the basin scale to study the hydroclimatic movements in Budyko space of 65 temperate and boreal basins during this period. We found that movements in Budyko space could not be explained by climatic changes in precipitation and potential evapotranspiration in 60 % of these basins, suggesting the existence of other dominant drivers of hydroclimatic change. In both the temperate and boreal basin groups studied, a negative climatic effect on the evaporative ratio was counteracted by a positive residual effect. The positive residual effect occurred along with increasing standing forest biomass in the temperate and boreal basin groups, increasing forest cover in the temperate basin group and no apparent changes in forest species composition in any group. From the three forest attributes, standing forest biomass was the one that could explain most of the variance of the residual effect in both basin groups. These results further suggest that the water-saving response to increasing CO 2 in these forests is either negligible or overridden by the opposite effect of the increasing forest biomass. Thus, we conclude that increasing standing forest biomass is the dominant driver of long-term and large-scale evapotranspiration changes in Swedish forests.