Pat YEH

Recent studies have extended the applicability of the Budyko framework from the long-term mean to annual or shorter time scales. However, the effects of water storage change ΔS on the overall water balance estimated from the Budyko models (BM) at annual-to-monthly time scales were less investigated, particularly at the continental or global scales, due to the lack of large-scale ΔS data. Here, based on a 25-yr (1984–2008) global gridded terrestrial water budget dataset and by using an analytical error-decomposition framework, we analyzed the effects of ΔS in evapotranspiration (ET) predicted from BM at both grid and basin scales under diverse climates for the annual, wet-seasonal, dry-seasonal, and monthly time scales. Results indicated that the BM underperforms in the short dry (wet) seasons of predominantly humid (dry) basins, with lower accuracy under more humid climates (at annual, dry-seasonal, and monthly scales) and under more arid climates (at wet-seasonal scale). When the e...

Soil salinization is a major environmental issue in arid and semi-arid regions, and has been accelerated in some areas by removal of native vegetation cover. Partial afforestation can be a practical mitigation strategy if efficiently integrated with farms and pastures. Using an integrated surface-subsurface hydrological model, this study evaluates the water and salt dynamics and soil salinization conditions of a rural intermittent catchment in the semi-arid climate of southeast Australia subjected to four different partial afforestation configurations under different climate change scenarios, as predicted by several general circulation models. The results show that the locations of afforested areas can induce a retarding effect in the outflow of groundwater salt, with tree planting at lower elevations showing the steadier salt depletion rates. Moreover, except for the configuration with trees planted near the outlet of the catchment, the streamflow is maintained under all other conf...

Over the last decades, the global population has been rapidly increasing and human activities have altered terrestrial water fluxes at an unprecedented scale. The phenomenal growth of the human footprint has significantly modified hydrological processes in various ways (e.g., irrigation, artificial dams, and water diversion) and at various scales (from a watershed to the globe). During the early 1990s, awareness of the potential water scarcity led to the first detailed global water resource assessments. Shortly thereafter, in order to analyse the human perturbation on terrestrial water resources, the first generation of large-scale hydrological models (LHMs) was produced. However, at this early stage few models considered the interaction between terrestrial water fluxes and human activities, including water use and reservoir regulation, and even fewer models distinguished water use from surface water and groundwater resources. Since the early 2000s, a growing number of LHMs are inco...

Global-scale land surface models (LSMs) often parameterize the exchange of energy at land surface on a physical basis. On the other hand, ad hoc assumptions are made to conceptualize the runoff generation mechanism and other relevant hydrological processes. One of them is the groundwater process, which has traditionally been completely neglected or treated implicitly at best. Groundwater is the source

Groundwater is the most important freshwater resource and its relevance can be viewed in two aspects; a pervasive and seemingly abundant storage of freshwater and a consistent source of surface water in dry season in form of base runoff, which is nothing but a groundwater reservoir discharging into rivers. For proper estimation of groundwater resources in current and future climate

Received 18 October 2006; revised 18 October 2006; accepted 25 October 2006; published 29 December 2006. [1] Regional groundwater storage changes in Illinois are estimated from monthly GRACE total water storage change (TWSC) data and in situ measurements of ...

ABSTRACT This study explores the capability of GRACE to detect heterogeneous groundwater storage (GWS) variations in two sub-regions of the North China Plain (NCP): the Piedmont Plain (PP, ~54,000 km2, mainly exploiting shallow groundwater) and East-Central Plain (ECP, ~86,000 km2, mainly exploiting deep groundwater). Results show that the GWS anomalies estimated from GRACE data (2003-2013) agree well with those estimated from in situ observations (2005-2010) for both PP (R2 = 0.91) and ECP (R2 = 0.75). The shallow GWS (2003-2013) in PP declines faster (-46.5±6.8 mm/yr) than the deep GWS in ECP (-16.9±1.9 mm/yr). However, the shallow GWS in PP recovered more quickly especially during the 2008-2011 drought period. Despite its lower magnitude, the GRACE-derived GWS depletion in ECP reveals the over-exploitation of deep GWS. This study demonstrated that the heterogeneous GWS variations can potentially be detected by GRACE at the sub-regional scale smaller than the typical GRACE footprint (200,000 km2).

ABSTRACT In this study, the uncertainties of five (three ground measurements and two satellite hybrid products) global observed precipitation datasets and its translation into evapotranspiration and runoff through ensemble hydrological simulations are estimated. A dimensionless index Omega, is used to quantify the 'similarity' among the simulations of ensemble members which is assumed as a surrogate of 'uncertainty' within them. The uncertainty in precipitation is in general amplified in simulated runoff and damped in evapotranspiration, and global average of OmegaP, OmegaET and OmegaR are 0.84, 0.89 and 0.65, respectively. A relatively low OmegaP is found in certain mountainous areas, deserts, and Central Africa around the equator and 20° N. Zonal summation of gauge station numbers and zonal mean of OmegaP exhibit a similar pattern. Globally, the spatial distribution of OmegaET shows a similar pattern of OmegaP but generally with higher values, which indicates more similar temporal variability among ensemble members of simulated evapotranspiration. However, some considerable differences can also be observed in certain regions. Based on OmegaP and OmegaET, the patterns of uncertainty propagation from precipitation to evapotranspiration can be classified into the following four groups: (1) High OmegaP (> 0.8) and higher OmegaET (> 0.9), (2) Low OmegaP and high OmegaET (OmegaET - OmegaP > 0.3), (3) High OmegaP and low OmegaET (OmegaET - OmegaP) < -0.3, and (4) OmegaP (< 0.5) and Low OmegaET (< 0.5). Most regions in the northern Hemisphere, except for arid regions, are included in Group 1, some tropical regions (e.g., archipelago of South Pacific Ocean) are classified as Group 2, large regions of Central Africa are shown as Group 3, and Group 4 includes mostly arid regions and regions where observational networks are sparse. The uncertainty of simulated runoff (OmegaR), in turn, globally marks lower value than OmegaP. It means that the uncertainty in precipitation translates into the amplified uncertainty in runoff simulated, and, therefore, OmegaR - OmegaP shows negative value globally. This amplified uncertainty propagation in runoff simulation is mainly due to the weak uncertainty propagation in evapotranspiration simulation. In terms of water balance, precipitation is partitioned into evapotranspiration and runoff through water storage components (here, snow pack and soil moisture), and the uncertainty also should be allocated through the same mechanism. In global scale, evapotranspiration is rather insensitive to the precipitation uncertainty, and it even dampens the uncertainty since plant growth in many of terrestrial regions is constrained by energy (i.e., radiation and temperature) rather than water availability. It results in much part of precipitation uncertainty translates into an uncertainty in runoff, and the greater size of precipitation (about 2 fold) leads to bigger uncertainty in the standardized index Omega. As a result, global average of OmegaR (0.65) marks a much smaller value than OmegaET (0.89) and OmegaP (0.84). In regional scale, it is found that a number of relatively small OmegaR - OmegaP values are located in transitional regions (e.g., around 10° N and 20° S - 40° S) between water-limited region (e.g., near 20° N) and water-abundant region (e.g., tropical region), as similar to the spatial pattern of OmegaET - OmegaP, and a high peak is observed near the equator.

Global-scale land surface models (LSMs) often parameterize the exchange of energy at land surface on a physical basis. On the other hand, ad hoc assumptions are made to conceptualize the runoff generation mechanism and other relevant hydrological processes. One of them is the groundwater process, which has traditionally been completely neglected or treated implicitly at best. Groundwater is the source

ABSTRACT In this study, we utilize 31-year (1979-2009) hydrometeorological data from multiple sources over 20 world large river basins to detect long-term hydrological cycle change due to changed environmental conditions. The data used include observed-, hydrological modelling-, reanalysis- and remote sensing-based data for various atmospheric and terrestrial hydrology variables. A novel approach, based on the conjunctive use of GRACE water storage and atmospheric reanalysis data, is proposed to more accurately close water budget and characterize the interaction between atmospheric and land branch of hydrological cycle. The results are validated against in-situ hydrological observations in Illinois and Mississippi River basin. The advantage of this approach on the prediction of the hydrological response to future climate change will be outlined.

ABSTRACT Among global water cycle components, Terrestrial Water Storage (TWS) is one of the most difficult to estimate. In this study, basin-scale regional TWS variations simulated by a global-scale land surface model, after validating with GRACE data and observed streamflow, are used to investigate the dominant TWS components as well as the interactions among TWS components over some largest river basins. The analysis is based on an integrated water resources assessment modelling framework developed by incorporating human impact schemes (i.e., reservoir operation, irrigation, withdrawal, groundwater pumping, and environmental flow requirements) into a land surface model – the Minimal Advanced Treatments of Surface Interaction and Runoff (MATSIRO). MATSIRO simulates the majority of land hydrologic processes on a physical basis at the global 1° × 1° resolution. The terrestrial water storage (TWS) simulated consists of soil moisture, groundwater, river water, snow and ice, and the human impact components such as reservoir storage. The effects of irrigation and groundwater pumping on TWS variations are also considered in certain regions where their impacts are known to be significant (e.g. The High Plains Aquifer, US). Moreover, a comparison on the TWS components is made with the MATSIRO simulation without considering human impact. The difference between them is a direct measure on the extent to which human anthropogenic impacts affect regional hydrology.

Groundwater is the most important freshwater resource and its relevance can be viewed in two aspects; a pervasive and seemingly abundant storage of freshwater and a consistent source of surface water in dry season in form of base runoff, which is nothing but a groundwater reservoir discharging into rivers. For proper estimation of groundwater resources in current and future climate

The total terrestrial water storage (TWS) in six major river basins of the world for the period 1986-1995 were estimated from (1) the Phase-2 of the Global Soil Wetness Project (GSWP2) Multi-Model Analysis (MMA), (2) the combined land-atmospheric water balance method using the ERA-40 and JRA-25 global reanalysis data, and (3) the satellite observation by the Gravity Recovery And Climate

ABSTRACT Water table dynamics, a basic hydrologic process, was traditionally not considered in global-scale land surface models (LSMs). In this study, a representation of water table dynamics is integrated into a global LSM to address the shortcomings of existing global-scale modeling studies and the appropriateness of specifying certain parameters as globally constant. Evaluation of model simulation using globally varying parameters against river discharge observations in selected large rivers shows improvements when the water table dynamics is included in the model. The mechanisms by which the water table dynamics affects land surface hydrologic simulation are then investigated by analyzing the sensitivity simulation of groundwater (GW) capillary flux. The result indicates that global mean evapotranspiration (ET) increases by similar to 9% when GW capillary flux is considered. The semiarid regions with marked dry season have the largest increase (similar to 25%), while the humid and high-latitude regions with sufficient moisture but limited radiation energy have the smallest increase. Increase in ET is more pronounced in dry season when GW recharge becomes negative (upward moisture supply from the aquifer), but its magnitude depends on the water table depth (WTD). On the other hand, a deeper WTD caused by the GW capillary flux is found to decrease runoff throughout the year in regions with a large increase in ET in dry season only. Based on our modeling result, about 50% of global land area (especially in humid and high-latitude regions) is simulated to have the mean WTD shallower than 5m, which emphasizes the significance of representing water table dynamics in global-scale LSMs.

ABSTRACT We explore the mechanisms whereby groundwater influences terrestrial water storage (TWS) in the Amazon using GRACE observations and two contrasting versions of the LEAF-Hydro-Flood hydrological model: one with and the other without an interactive groundwater. We find that, first, where the water table is shallow as in northwestern Amazonia and floodplains elsewhere, subsurface stores (vadose zone and groundwater) are nearly saturated year-round, hence river and flooding dominate TWS variation; where the water table is deep as in southeastern Amazonia, the large subsurface storage capacity holds the infiltrated water longer before releasing it to streams, hence the subsurface storage dominates TWS variation. Second, over the whole Amazon, the subsurface water contribution far exceeds surface water contribution to total TWS variations. Based on LEAF-Hydro-Flood simulations, 71% of TWS change is from subsurface water, 24% from flood water, and 5% from water in river channels. Third, the subsurface store includes two competing terms, soil water in the vadose zone and groundwater below the water table. As the water table rises, the length of vadose zone is shortened and hence the change in groundwater store is accompanied by an opposite change in soil water store resulting in their opposite phase and contributions to total TWS. We conclude that the inclusion of a prognostic groundwater store and its interactions with the vadose zone, rivers, and floodplains in hydrological simulations enhances seasonal amplitudes and delays seasonal peaks of TWS anomaly, leading to an improved agreement with GRACE observations.

Terrestrial hydrological processes play a significant role in the climate system, primarily through the exchange of water and energy at the land surface, and terrestrial water storage (TWS), i.e., the sum of soil moisture, groundwater, snow and ice, water in biomass, and surface water in lakes, reservoirs, wetlands and river channels, is a fundamental component of it. Among TWS components,

ABSTRACT Global sea level has been rising over the past half century, according to tide-gauge data1, 2. Thermal expansion of oceans, melting of glaciers and loss of the ice masses in Greenland and Antarctica are commonly considered as the largest contributors, but these contributions do not entirely explain the observed sea-level rise1. Changes in terrestrial water storage are also likely to affect sea level3, 4, 5, 6, but comprehensive and reliable estimates of this contribution, particularly through human water use, are scarce1. Here, we estimate sea-level change in response to human impacts on terrestrial water storage by using an integrated model that simulates global terrestrial water stocks and flows (exclusive to Greenland and Antarctica) and especially accounts for human activities such as reservoir operation and irrigation. We find that, together, unsustainable groundwater use, artificial reservoir water impoundment, climate-driven changes in terrestrial water storage and the loss of water from closed basins have contributed a sea-level rise of about 0.77 mm yr−1 between 1961 and 2003, about 42% of the observed sea-level rise. We note that, of these components, the unsustainable use of groundwater represents the largest contribution.

Anthropogenic activities have been significantly perturbing global freshwater flows and groundwater reserves. Despite numerous advances in the development of land surface models (LSMs) and global terrestrial hydrological models (GHMs), relatively few studies have attempted to simulate the impacts of anthropogenic activities on the terrestrial water cycle using the framework of LSMs. From the comparison of simulated terrestrial water storage with the Gravity Recovery and Climate Experiment (GRACE) satellite observations it is found that a process-based LSM, the Minimal Advanced Treatments of Surface Interaction and Runoff (MATSIRO), outperforms the bucket-model-based GHM called H08 in simulating hydrologic variables, particularly in water-limited regions. Therefore, the water regulation modules of H08 are incorporated into MATSIRO. Further, a new irrigation scheme based on the soil moisture deficit is developed. Incorporation of anthropogenic water regulation modules significantly im...

The role of shallow unconfined aquifers in supplying water for evapotranspiration (i.e., groundwater evaporation) is investigated in this paper. Recent results from regional land surface modeling have indicated that in shallow water table areas, a large portion of evapotranspiration comes directly from aquifers. However, little field evidence at the regional scale has been reported to support this finding. Using a comprehensive 19-yr (1984–2002) monthly hydrological dataset on soil moisture, water table depth, and streamflow in Illinois, regional recharge to and evaporation from groundwater are estimated by using soil water balance computation. The 19-yr mean groundwater recharge is estimated to be 244 mm yr−1 (25% of precipitation), with uncertainty ranging from 202 to 278 mm yr−1. During the summer, the upward capillary flux from the shallow aquifer helps to maintain a high rate of evapotranspiration. Groundwater evaporation (negative groundwater recharge) occurs during the period...