Hydrological cycle and water resources in a changing world:A review

Water is the fundamental natural resource that supports life,ecosystems and human society.Thus studying the water cycle is important for sustainable development.In the context of global climate change,a better under-standing of the water cycle is needed.This study summarises current research and highlights future directions of water science from four perspectives:(i)the water cycle;(ii)hydrologic processes;(iii)coupled natural-social water systems;and(iv)integrated watershed management.Emphasis should be placed on understanding the joint impacts of climate change and human activities on hydrological processes and water resources across temporal and spatial scales.Understanding the interactions between land and atmosphere are keys to addressing this is-sue.Furthermore systematic approaches should be developed for large basin studies.Areas for focused research include:variations of cryosphere hydrological processes in upper alpine zones;and human activities on the wa-ter cycle and relevant biogeochemical processes in middle-lower reaches.Because the water cycle is naturally coupled with social characteristics across multiple scales,multi-process and multi-scale models are needed.Hy-drological studies should use this new paradigm as part of water-food-energy frontier research.This will help to promote interdisciplinary study across natural and social sciences in accordance with the United Nation’s sustainable development goals.

气候变化下多年冻土融水对青藏高原径流影响评估

青藏高原约40%的面积分布有多年冻土,相比冰川积雪融水得到的广泛关注,冻土融水对径流的影响仍有待评估.本研究基于耦合冰冻圈过程的分布式水文模型GBEHM(Geomorphology-based Ecohydrological Model),在区域尺度量化了多年冻土融水对径流的影响.研究结果表明:(1)过去40年间,青藏高原多年冻土面积下降13.9%,地下冰储量减少约401.1 Gt,约为同期冰川储量减少量的2倍;(2)多年冻土融水对全青藏高原总径流贡献相对较小(约0.5%),但在某些区域(如黄河上游、长江上游)和高程范围(如河西走廊4000m高程附近),地下冰融水对径流的贡献超过冰川融水贡献,其影响不可忽略;(3)气候变化背景下,地下冰融水径流在未来不可持续,澜沧江、怒江等区域历史阶段已经达到峰值,而对全青藏高原,融水径流拐点预计将在21世纪20年代(SSP1-2.6)、50年代(SSP2-4.5)或90年代(SSP5-8.5)达到.

Response of water-use efficiency to phenology in the natural forest and grassland of the Loess Plateau in China

Ecosystem water use efficiency(WUE)is an integrated physiological metric for the coupling cycle between terrestrial carbon,water,and energy.How WUE responds to vegetation phenology(e.g.,SOS,EOS-start,end of growing season,and GSL-growing season length)shifting in temperate semi-arid regions is a hot spot in relative research fields.Based on remotesensing products and in-situ measured climate data,this study discussed how gross primary productivity(GPP),evapotranspiration(ET),and WUE(quantified by GPP/ET)would change with the altering vegetation phenology and climate in the untouched semi-arid forests and grasslands of the Chinese Loess Plateau during 2001–2020.Our results show that vegetation tended to green-up earlier and brown-down later from 2001 to 2020,causing an extended GSL.The forests had an earlier SOS,later EOS,and longer GSL than the grasslands,but the latter had a bigger variation amplitude.The WUE in the study area decreased significantly during spring and summer,while the grassland WUE increased in autumn;the annual mean reduction rate in grassland WUE was approximately twice that of woodland.Earlier SOS could increase forest WUE but reduce grassland WUE in spring,mainly because leaf unfolding has a more pronounced limitation on soil evaporation beneath the forest canopy.EOS had less impact on WUE,and no apparent difference existed between these two ecosystems.Climate change could affect WUE directly by changing GPP and ET and indirectly by regulating vegetation phenology.Warming can increase GPP and ET,causing an earlier SOS,further promoting GPP and ET(except forest ET).Precipitation significantly affected forest GPP and ET in spring,grassland GPP and ET in summer,and grassland ET in autumn;precipitation affects spring grassland WUE mainly via regulating SOS.Enhanced solar radiation could suppress grassland GPP in spring,promote forest ET in autumn,and regulate grassland WUE by affecting phenology.This study is meaningful for improving the process-based vegetation model and studying arid and semi-arid ecosystems’responses to a changing climate.

Estimating the annual runoff frequency distribution based on climatic conditions and catchment characteristics:A case study across China

Estimation of the annual runoff frequency distribution is an essential basis for water resource management.This study proposes a framework for estimating the annual runoff frequency distribution across252 catchments in China based on climatic conditions and catchment characteristics from 1956 to 2000.The Budyko land-specific parameter n,which intergrates influences other than the mean climate conditions,is firstly estimated based on easily ascertainable catchment characteristics without the requirementsof having long-term runoff observations.Second,the annual runoff statistical parameters,namely,the mean value and standard deviation(STD),are derived based on the Budyko rainfall-runoffmodel with the central moment method.Finally,the annual runoff on any recurrence interval is obtainedby the Pearson-Ill frequency function.Results show that the parameter n can be estimated fromthe catchment average slope,longitude,and climatic seasonality index.The estimated statistical parametersof annual runoff have acceptable agreement with observed values(mean value:R^(2)-0.94,STD:R^(2)-0.91,and both relative errors<10%).In addition,estimated annual runoff at each catchment fortypical wet and dry years(25%and 75%ranked percentiles)coincides well with observed values,with R^(2) of 0.92-0.93 and relative erors less than 10%.This result indicates the robustness of this framework forestimating the annual runoff frequency distribution,which provides a simple and effective tool forungauged orpoorlygauged catchments.

Analysis of catchment evapotranspiration at different scales using bottom-up and top-down approaches

Physically-based hydrological models are used to predict catchment water balance through detailed simulation of hydrological processes at small temporal and spatial scales.However,annual catchment water balance can also be easily and simply predicted using lumped conceptual model.Comparison between physically-based hydrological models and lumped conceptual models can help us understand the dominant factors on catchment water balance at different scales.In this paper,a distributed physically-based hydrological model(i.e.,bottom-up approach)and a simple water-energy balance model(i.e.,top-down approach)are used to predict actual evapotranspiration in nine sub-catchments,and the whole basin of the Luan River in northern China.Both simulations give very close values of annual evapotranspiration and show the same complementary relationship between actual and potential evapotranspiration at annual time scale.From the analysis at different time scales through comparison of the top-down and the bottom-up methods,it is shown that the annual catchment evapotranspiration is controlled mainly by annual precipitation and potential evapotranspiration,and the variability of soil water and vegetation becomes more important at a smaller time scale in the study areas.It is also known that the relationship between potential and actual evapotranspiration shows a highly nonlinear relationship at the annual and catchment scale but can be simplified to a linear relationship at hourly temporal and hillslope scales,which is commonly used in the physicallybased hydrological models.