Min-Hui Lo

Satellites measure recent rates of groundwater depletion in California's Central Valley

In highly-productive agricultural areas such as Californias Central Valley, where groundwater often supplies the bulk of the water required for irrigation, quantifying rates of groundwater depletion remains a challenge owing to a lack of monitoring infrastructure and the absence of water use reporting requirements. Here we use 78 months (October, 2003–March, 2010) of data from the Gravity Recovery and Climate Experiment satellite mission to estimate water storage changes in Californias Sacramento and San Joaquin River Basins. We find that the basins are losing water at a rate of 31.0 ± 2.7 mm yr−1 equivalent water height, equal to a volume of 30.9 km3 for the study period, or nearly the capacity of Lake Mead, the largest reservoir in the United States. We use additional observations and hydrological model information to determine that the majority of these losses are due to groundwater depletion in the Central Valley. Our results show that the Central Valley lost 20.4 ± 3.9 mm yr−1 of groundwater during the 78-month period, or 20.3 km3 in volume. Continued groundwater depletion at this rate may well be unsustainable, with potentially dire consequences for the economic and food security of the United States.

Groundwater depletion in the Middle East from GRACE with implications for transboundary water management in the Tigris-Euphrates-Western Iran region

In this study, we use observations from the Gravity Recovery and Climate Experiment (GRACE) satellite mission to evaluate freshwater storage trends in the north-central Middle East, including portions of the Tigris and Euphrates River Basins and western Iran, from January 2003 to December 2009. GRACE data show an alarming rate of decrease in total water storage of approximately −27.2±0.6 mm yr−1 equivalent water height, equal to a volume of 143.6 km3 during the course of the study period. Additional remote-sensing information and output from land surface models were used to identify that groundwater losses are the major source of this trend. The approach used in this study provides an example of “best current capabilities” in regions like the Middle East, where data access can be severely limited. Results indicate that the region lost 17.3±2.1 mm yr−1 equivalent water height of groundwater during the study period, or 91.3±10.9 km3 in volume. Furthermore, results raise important issues regarding water use in transboundary river basins and aquifers, including the necessity of international water use treaties and resolving discrepancies in international water law, while amplifying the need for increased monitoring for core components of the water budget.

Quantifying renewable groundwater stress with GRACE

Abstract Groundwater is an increasingly important water supply source globally. Understanding the amount of groundwater used versus the volume available is crucial to evaluate future water availability. We present a groundwater stress assessment to quantify the relationship between groundwater use and availability in the worlds 37 largest aquifer systems. We quantify stress according to a ratio of groundwater use to availability, which we call the Renewable Groundwater Stress ratio. The impact of quantifying groundwater use based on nationally reported groundwater withdrawal statistics is compared to a novel approach to quantify use based on remote sensing observations from the Gravity Recovery and Climate Experiment (GRACE) satellite mission. Four characteristic stress regimes are defined: Overstressed, Variable Stress, Human‐dominated Stress, and Unstressed. The regimes are a function of the sign of use (positive or negative) and the sign of groundwater availability, defined as mean annual recharge. The ability to mitigate and adapt to stressed conditions, where use exceeds sustainable water availability, is a function of economic capacity and land use patterns. Therefore, we qualitatively explore the relationship between stress and anthropogenic biomes. We find that estimates of groundwater stress based on withdrawal statistics are unable to capture the range of characteristic stress regimes, especially in regions dominated by sparsely populated biome types with limited cropland. GRACE‐based estimates of use and stress can holistically quantify the impact of groundwater use on stress, resulting in both greater magnitudes of stress and more variability of stress between regions.

Improving parameter estimation and water table depth simulation in a land surface model using GRACE water storage and estimated base flow data

Several previous studies have shown the significance of representing shallow groundwater in land surface model (LSM) simulations. However, optimal methods for parameter estimation in order to realistically simulate water table depth have received little attention. The recent availability of Gravity Recovery and Climate Experiment (GRACE) water storage data provides a unique opportunity to constrain LSM simulations of terrestrial hydrology. In this study, we incorporate both GRACE (storage) and estimated base flow (flux) data in the calibration of LSM parameters, and demonstrate the advantages gained from this approach using a Monte Carlo simulation framework. This approach improves parameter estimation and reduces the uncertainty of water table simulations in the LSM. Using the optimal parameter set identified from the multiobjective calibration, water table simulation can be improved due to close dependence of both base flow and total subsurface water storage on the water table depth. Moreover, it is shown that parameters calibrated from short-term (2003–2005) GRACE and base flow data can be validated using simulations for the periods of 1984–1998 and 2006–2007, which implies that the proposed multiobjective calibration strategy is robust. More important, this study has demonstrated the potential for the joint use of routinely available GRACE water storage data and streamflow records to constrain LSM simulations at the global scale.

Uncertainty in global groundwater storage estimates in a Total Groundwater Stress framework

Abstract Groundwater is a finite resource under continuous external pressures. Current unsustainable groundwater use threatens the resilience of aquifer systems and their ability to provide a long‐term water source. Groundwater storage is considered to be a factor of groundwater resilience, although the extent to which resilience can be maintained has yet to be explored in depth. In this study, we assess the limit of groundwater resilience in the worlds largest groundwater systems with remote sensing observations. The Total Groundwater Stress (TGS) ratio, defined as the ratio of total storage to the groundwater depletion rate, is used to explore the timescales to depletion in the worlds largest aquifer systems and associated groundwater buffer capacity. We find that the current state of knowledge of large‐scale groundwater storage has uncertainty ranges across orders of magnitude that severely limit the characterization of resilience in the study aquifers. Additionally, we show that groundwater availability, traditionally defined as recharge and redefined in this study as total storage, can alter the systems that are considered to be stressed versus unstressed. We find that remote sensing observations from NASAs Gravity Recovery and Climate Experiment can assist in providing such information at the scale of a whole aquifer. For example, we demonstrate that a groundwater depletion rate in the Northwest Sahara Aquifer System of 2.69 ± 0.8 km3/yr would result in the aquifer being depleted to 90% of its total storage in as few as 50 years given an initial storage estimate of 70 km3.

A decade of sea level rise slowed by climate-driven hydrology

By land or by sea How much of an effect does terrestrial groundwater storage have on sea-level rise? Reager et al. used gravity measurements made between 2002 and 2014 by NASAs Gravity Recovery And Climate Experiment (GRACE) satellites to quantify variations in groundwater storage. Combining those data with estimates of mass loss by glaciers revealed groundwaters impact on sea-level change. Net groundwater storage has been increasing, and the greatest regional changes, both positive and negative, are associated with climate-driven variability in precipitation. Thus, groundwater storage has slowed the rate of recent sea-level rise by roughly 15%. Science, this issue p. 699 Recent storage of excess groundwater has measurably slowed sea level rise. Climate-driven changes in land water storage and their contributions to sea level rise have been absent from Intergovernmental Panel on Climate Change sea level budgets owing to observational challenges. Recent advances in satellite measurement of time-variable gravity combined with reconciled global glacier loss estimates enable a disaggregation of continental land mass changes and a quantification of this term. We found that between 2002 and 2014, climate variability resulted in an additional 3200 ± 900 gigatons of water being stored on land. This gain partially offset water losses from ice sheets, glaciers, and groundwater pumping, slowing the rate of sea level rise by 0.71 ± 0.20 millimeters per year. These findings highlight the importance of climate-driven changes in hydrology when assigning attribution to decadal changes in sea level.

Effect of water table dynamics on land surface hydrologic memory

The representation of groundwater dynamics in land surface models has received considerable attention in recent years. Most studies have found that soil moisture increases after adding a groundwater component because of the additional supply of water to the root zone. However, the effect of groundwater on land surface hydrologic memory (persistence) has not been explored thoroughly. In this study we investigate the effect of water table dynamics on National Center for Atmospheric Research Community Land Model hydrologic simulations in terms of land surface hydrologic memory. Unlike soil water or evapotranspiration, results show that land surface hydrologic memory does not always increase after adding a groundwater component. In regions where the water table level is intermediate, land surface hydrologic memory can even decrease, which occurs when soil moisture and capillary rise from groundwater are not in phase with each other. Further, we explore the hypothesis that in addition to atmospheric forcing, groundwater variations may also play an important role in affecting land surface hydrologic memory. Analyses show that feedbacks of groundwater on land surface hydrologic memory can be positive, negative, or neutral, depending on water table dynamics. In regions where the water table is shallow, the damping process of soil moisture variations by groundwater is not significant, and soil moisture variations are mostly controlled by random noise from atmospheric forcing. In contrast, in regions where the water table is very deep, capillary fluxes from groundwater are small, having limited potential to affect soil moisture variations. Therefore, a positive feedback of groundwater to land surface hydrologic memory is observed in a transition zone between deep and shallow water tables, where capillary fluxes act as a buffer by reducing high-frequency soil moisture variations resulting in longer land surface hydrologic memory.

Precipitation response to land subsurface hydrologic processes in atmospheric general circulation model simulations

Several studies have established that soil moisture increases after adding a groundwater component in land surface models, owing to the additional supply of subsurface water. However, the impact of groundwater on the spatial-temporal variability of precipitation has received little attention. This study explores how a groundwater representation in land surface models alters precipitation distributions through coupled groundwater-land-atmosphere simulations. Results indicate that the addition of groundwater yields a global increase in soil water content and evapotranspiration, a decrease in surface air temperature, and an increase in cloud cover fraction. These result in globally inhomogeneous changes in precipitation. In the boreal summer, tropical land regions show a positive anomaly in the Northern Hemisphere and a negative anomaly in the Southern Hemisphere. As a result, an asymmetric dipole is found over tropical land regions along the equator. Furthermore, in the transition climatic zone where the land and atmosphere are strongly coupled, precipitation also increases. Two main mechanisms are suggested for the two different regions with increased precipitation. The “rich-get-richer” mechanism is responsible for the positive precipitation anomalies over the tropical land regions, while a positive feedback of land-atmosphere interaction is the major contributor to increased precipitation over central North America. This study highlights the importance of land subsurface hydrologic processes in the climate system and has further implications for global water cycle dynamics.

Asymmetric Responses of Tropical Precipitation during ENSO

Abstract In response to the zonally symmetric El Nino–Southern Oscillation forcing, hemispherically asymmetric tropical precipitation anomalies associated with the Hadley circulation are found. In boreal spring after an El Nino peak phase, positive tropical precipitation anomalies occur in the Southern Hemisphere, while negative precipitation anomalies are found in the Northern Hemisphere. This zonal asymmetry is more apparent in the El Nino decaying phase than in the El Nino growing phase. The maximum amplitude of this zonal asymmetry lags one season behind the maximum SST anomalies over the tropical eastern Pacific. This lagged response of the asymmetry is mainly because of the tropical precipitation outside the tropical eastern Pacific, which is associated with the SST and tropospheric temperature anomalies outside the tropical eastern Pacific. A combination of the effect associated with the anomalous gross moist stability and the effect of the horizontal moist static energy (MSE) transport is responsi...

Assessing surface water consumption using remotely‐sensed groundwater, evapotranspiration, and precipitation

Estimates of consumptive use of surface water by agriculture are vital for assessing food security, managing water rights, and evaluating anthropogenic impacts on regional hydrology. However, reliable, current, and public data on consumptive use can be difficult to obtain, particularly in international and less developed basins. We combine remotely-sensed precipitation and satellite observations of evapotranspiration and groundwater depletion to estimate surface water consumption by irrigated agriculture in Californias Central Valley for the 2004–09 water years. We validated our technique against measured consumption data determined from streamflow observations and water export data in the Central Valley. Mean satellite-derived surface water consumption was 291.0 ± 32.4 mm/year while measured surface water consumption was 308.1 ± 6.5 mm/year. The results show the potential for remotely-sensed hydrologic data to independently observe irrigated agricultures surface water consumption in contested or unmonitored basins. Improvements in the precision and spatial resolution of satellite precipitation, evapotranspiration and gravimetric groundwater observations are needed to reduce the uncertainty in this method and to allow its use on smaller basins and at shorter time scales.