The importance of vegetation in understanding terrestrial water storage variations

[摘要] So far, various studies have aimed at decomposing theintegrated terrestrial water storage variations observed by satellitegravimetry (GRACE, GRACE-FO) with the help of large-scale hydrologicalmodels. While the results of the storage decomposition depend on modelstructure, little attention has been given to the impact of the way thatvegetation is represented in these models. Although vegetation structure andactivity represent the crucial link between water, carbon, and energy cycles,their representation in large-scale hydrological models remains a majorsource of uncertainty. At the same time, the increasing availability andquality of Earth-observation-based vegetation data provide valuableinformation with good prospects for improving model simulations and gainingbetter insights into the role of vegetation within the global water cycle. In this study, we use observation-based vegetation information such asvegetation indices and rooting depths for spatializing the parameters of asimple global hydrological model to define infiltration, root water uptake,and transpiration processes. The parameters are further constrained byconsidering observations of terrestrial water storage anomalies (TWS), soilmoisture, evapotranspiration (ET) and gridded runoff ( Q ) estimates in amulti-criteria calibration approach. We assess the implications of includingvarying vegetation characteristics on the simulation results, with aparticular focus on the partitioning between water storage components. Toisolate the effect of vegetation, we compare a model experiment in whichvegetation parameters vary in space and time to a baseline experiment inwhich all parameters are calibrated as static, globally uniform values. Both experiments show good overall performance, but explicitly includingvarying vegetation data leads to even better performance and more physicallyplausible parameter values. The largest improvements regarding TWS and ET areseen in supply-limited (semi-arid) regions and in the tropics, whereas Q simulations improve mainly in northern latitudes. While the total fluxes andstorages are similar, accounting for vegetation substantially changes thecontributions of different soil water storage components to the TWSvariations. This suggests an important role of the representation ofvegetation in hydrological models for interpreting TWS variations. Oursimulations further indicate a major effect of deeper moisture storages andgroundwater–soil moisture–vegetation interactions as a key to understandingTWS variations. We highlight the need for further observations to identifythe adequate model structure rather than only model parameters for areasonable representation and interpretation of vegetation–waterinteractions.

[学科分类] 妇产科学