Evapotranspiration (ET) is the process by which liquid water becomes water vapor and energetically this accounts for much of incoming solar radiation. If this ET did not occur temperatures would be higher, so understanding ET trends is crucial to predict future temperatures. Recent studies have reported prolonged declines in ET in recent decades, although these declines may relate to climate variability. Here, we used a well-validated diagnostic model to estimate daily ET during 1981–2012, and its three components: transpiration from vegetation (Et), direct evaporation from the soil (Es) and vaporization of intercepted rainfall from vegetation (Ei). During this period, ET over land has increased significantly (p < 0.01), caused by increases in Et and Ei, which are partially counteracted by Es decreasing. These contrasting trends are primarily driven by increases in vegetation leaf area index, dominated by greening. The overall increase in Et over land is about twofold of the decrease in Es. These opposing trends are not simulated by most Coupled Model Intercomparison Project phase 5 (CMIP5) models, and highlight the importance of realistically representing vegetation changes in earth system models for predicting future changes in the energy and water cycle.

[1] The headwater catchments of the Yellow River Basin are of great importance for the whole basin in terms of water resources, and streamflow from these catchments has decreased in the last decades. The concept of climate elasticity was used to assess the impacts of climate and land surface change on the streamflow. Results show that for the period 1960-2000 the elasticity of streamflow in relation to precipitation and potential evapotranspiration are 2.10 and À1.04, respectively, indicating that streamflow is more sensitive to precipitation than to potential evapotranspiration. However, land use change played a more important role than climate in reducing streamflow in the 1990s. It is estimated that land use change is responsible for more than 70% of the streamflow reduction in the 1990s, while climate change contributed to less than 30% of the reduction.The precipitation elasticity appears to have an inverse relationship with the runoff coefficient but a positive relationship with the aridity index, showing that the drier the catchment, the more sensitive the streamflow with respect to precipitation change.

Heisenberg interactions are ubiquitous in magnetic materials and have been prevailing in modeling and designing quantum magnets. Bonddirectional interactions 1-3 offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model 4 , which has a quantum spin liquid (QSL) as its exact ground state. Honeycomb iridates, A 2 IrO 3 (A=Na,Li), offer potential realizations of the Kitaev model, and their reported magnetic behaviors may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bonddirectional interactions remains indirect or conjectural. Here, we present direct evidence for dominant bond-directional interactions in antiferromagnetic Na 2 IrO 3 and show that they lead to strong magnetic frustration. Diffuse magnetic xray scattering reveals broken spin-rotational symmetry even above T N , with the three spin components exhibiting nano-scale correlations along distinct crystallographic directions. This spinspace and real-space entanglement directly manifests the bond-directional interactions, provides the missing link to Kitaev physics in honeycomb iridates, and establishes a new design strategy toward frustrated magnetism.Iridium (IV) ions with pseudospin-1/2 moments form in Na 2 IrO 3 a quasi-two-dimensional (2D) honeycomb network, which is sandwiched between two layers of oxygen ions that frame edge-shared octahedra around the magnetic ions and mediate superexchange interactions between neighboring pseudospins (Fig. 1a). Owing to the particular spin-orbital structure of the pseudospin 5,6 , the isotropic part of the magnetic interaction is strongly suppressed in the 90• bonding geometry of the edgeshared octahedra 2,3 , thereby allowing otherwise subdominant bond-dependent anisotropic interactions to play the main role and manifest themselves at the forefront of magnetism. This bonding geometry, common to many transition-metal oxides, in combination with the pseudospin that arises from strong spin-orbit coupling gives rise to an entirely new class of magnetism beyond the traditional paradigm of Heisenberg magnets. On a honeycomb lattice, for instance, the leading anisotropic interactions take the form of the Kitaev model 3 , which is a rare example of exactly solvable models with nontrivial properties such as Majorana fermions and non-abelian statistics, and with potential links to quantum computing 4 . Realization of the Kitaev model is now being intensively sought out in a growing number of materials 7-13 , including 3D extensions of the honeycomb Li 2 IrO 3 , dubbed "hyper-honeycomb" 7 and "harmonichoneycomb" 8 , and 4d transition-metal analogs such as RuCl 3 12 and Li 2 RhO 3 13 . Although most of these are known to magnetically order at low temperature, they exhibit a rich array of magnetic structures including zigzag 14-16 , spiral 17 , and other more complex non-coplanar structures 18,19 that are predicted to occur in the vicinity of the Kitaev QSL phase [20][21][22][23] , which hosts man...

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Abstract:The headwater catchments of the Yellow River basin generate over 35% of the basin's total stream flow and play a vital role in meeting downstream water resources requirements. In recent years the Yellow River has experienced significant changes in its hydrological regime, including an increased number of zero-flow days. These changes have serious implications for water security and basin management. We investigated changes in stream flow regime of four headwater catchments since the 1950s. The rank-based non-parametric Mann-Kendall test was used to detect trends in annual stream flow. The results showed no significant trend for the period 1956 to 2000. However, change-point analysis showed that a significant change in annual stream flow occurred around 1990, and hence the stream-flow data can be divided into two periods: 1956-1990 and 1991-2000. There was a considerable difference in average annual stream flow between the two periods, with a maximum reduction of 51%. Wet-season rainfall appears to be the main factor responsible for the decreasing trend in annual stream flow. Reductions in annual stream flow were associated with decreased interannual variability in stream flow. Seasonal stream flow distribution changed from bimodal to unimodal between the two periods, with winter stream flow showing a greater reduction than other seasons. Daily stream flow regime represented by flow duration curves showed that all percentile flows were decreased in the second period. The high flow index (Q 5 /Q 50 ) reduced by up to 28%, whereas the reduction in the low flow index (Q 95 /Q 50 ) is more dramatic, with up to 100% reduction.

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Despite the observed increase in global temperature, observed pan evaporation in many regions has been decreasing over the past 50 years, which is known as the "pan evaporation paradox". The "pan evaporation paradox" also exists in the Tibetan Plateau, where pan evaporation has decreased by 3.06 mm a −2 (millimeter per annum). It is necessary to explain the mechanisms behind the observed decline in pan evaporation because the Tibetan Plateau strongly influences climatic and environmental changes in China, Asia and even in the Northern Hemisphere. In this paper, a derivation based approach has been used to quantitatively assess the contribution rate of climate factors to the observed pan evaporation trend across the Tibetan Plateau. The results showed that, provided the other factors remain constant, the increasing temperature should have led to a 2.73 mm a −2 increase in pan evaporation annually, while change in wind speed, vapor pressure and solar radiation should have led to a decrease in pan evaporation by 2.81 mm a −2 , 1.96 mm a −2 and 1.11 mm a −2 respectively from 1970 to 2005. The combined effects of the four climate variables have resulted in a 3.15 mm a −2 decrease in pan evaporation, which is close to the observed pan evaporation trend with a relative error of 2.94%. A decrease in wind speed was the dominant factor for the decreasing pan evaporation, followed by an increasing vapor pressure and decreasing solar radiation, all of which offset the effect of increasing temperature across the Tibetan Plateau.