The analysis of groundwater recharge in Mongolia using vadose zone modeling

Knowledge of groundwater recharge (GR) is vital for optimal water resources management under an arid continental climate. However, in vast territories such as Mongolia, direct measurements of GR are unfeasible because they mandate excessive costs, stemming from time-consuming and labor-demanding efforts. A valid alternative to direct measurements is numerical models based on the monitoring of precipitation (P) and evapotranspiration (ET) for simulating GR. While direct measurements of ET are logistically problematic and unpractical for large-scale applications, a reliable prediction may be derived from crop reference evapotranspiration (ET0) which is calculable from limited data and will feed numerical models to evaluate a (pseudo) realistic GR as output. The crop reference evapotranspiration (ET0) was calculated employing the Hargreaves (Har) temperature-based ET0 method that closely simulated the internationally recognized standard FAO Penman-Monteith (FAO-56 PM) method (calculated with available data at limited locations). The set of weather data required for FAO-56 PM is still mostly unavailable or not easily accessible in data-limited countries such as Mongolia. The Har temperature-based method showed good potential to replace FAO-56 PM in the region according to our analysis. A time-variable and spatially-variable crop coefficient (Kc) was used to convert Har ET0 into a biome-specific potential evapotranspiration (ETp) for 41 study locations. However, there were no readily available estimates of Kc in natural vegetation specific to Mongolia. A dynamic (time-variable) radiation-dependent (in Gobi Desert) or LAI-dependent (in steppe) Kc was adopted from the literature and used for the first time in Mongolia. The LAI dependent Kc was also adjusted due to the climate features of the region. The developed Kc values are important to convert ET0 to ETp with consideration of region’s climate and any factors affecting the vegetation. The mean annual ET0 ranged from 685 mm to 1129 mm, while the ETp ranged from 147 mm to 695 mm. The GR rates were calculated using the estimated ETp as input in the HYDRUS-1D numerical vadose zone model for 41 study locations across Mongolia. The mean annual GR rates were smaller than 12 mm in study locations and the GR tends to decrease when vegetation cover increases. Advisors: Vitaly A.Zlotnik and Erin Haacke

Analysis of Groundwater Recharge in Mongolian Drylands Using Composite Vadose Zone Modeling

Knowledge of groundwater recharge (GR) is important for the effective management of water resources under semi-arid continental climates. Unfortunately, studies and data in Mongolia are limited due to the constraints in funding and lack of research infrastructures. Currently, the wide accessibility of freely available global-scale digital datasets of physical and chemical soil properties, weather data, vegetation characteristics, and depths to the water table offers new tools and basic information that can support low-cost physically based and process-oriented models. Estimates of GR over 41 study sites in Mongolia were obtained using HYDRUS-1D in a 2-m-thick soil profile with root depths of either 0.30 or 0.97m by exploiting the daily precipitation and biome-specific potential evapotranspiration values. The GR simulated by HYDRUS-1D arrives at the water table and becomes the actual GR with a lag time that has been calculated using a simplified form of the Richards equation and a traveling wave model. The mean annual precipitation ranges from 57 to 316mm year−1, and on average about 95% of it is lost by mean annual actual evapotranspiration. In the steppe region, the vegetation cover induces higher-than-normal actual transpiration losses and consequently lower GR. The mean annual GR rates span between 0.3 and 12.0mm year−1, while travel times range between 4 and 558 years. Model prediction uncertainty was quantified by comparing actual evapotranspiration and GR with available maps and by a sensitivity assessment of lag time to the soil moisture in the deep vadose zone. The partial least squares regression (PLSR) was used to evaluate the impact of available environmental properties in explaining the 47.1 and 59.1%variability of the spatially averaged mean annual GR and travel time, respectively. The most relevant contributors are clay content, aridity index, and leaf area index for GR, and depth to the water table and silt content for the lag time. In data-poor, arid, and semi-arid regions such as Mongolia, where the mean annual GR rates are low and poorly correlated to precipitation, the ever-increasing availability of world databases and remote sensing products offers promise in estimating GR

Prediction of Biome-Specific Potential Evapotranspiration in Mongolia under a Scarcity of Weather Data

We propose practical guidelines to predict biome-specific potential evapotranspiration (ETp) from the knowledge of grass-reference evapotranspiration (ET0) and a crop coefficient (Kc) in Mongolia. A paucity of land-based weather data hampers use of the Penman–Monteith equation (FAO-56 PM) based on the Food and Agriculture Organization (FAO) guidelines to predict daily ET0. We found that the application of the Hargreaves equation provides ET0 estimates very similar to those from the FAO-56 PM approach. The Kc value is tabulated only for crops in the FAO-56 guidelines but is unavailable for steppe grasslands. Therefore, we proposed a new crop coefficient, Kc adj defined by (a) net solar radiation in the Gobi Desert (Kc adjD) or (b) leaf area index in the steppe region (Kc adjS) in Mongolia. The mean annual ETp obtained using our approach was compared to that obtained by FAO-56 guidelines for forages (not steppe) based on tabulated Kc values in 41 locations in Mongolia. We found the differences are acceptable (RMSE of 0.40 mm d-1) in northern Mongolia under high vegetation cover but rather high (RMSE of 1.69 and 2.65 mm d-1) in central and southern Mongolia. The FAO aridity index (AI) is empirically related to the ETp/ET0 ratio. Approximately 80% and 54% reduction of ET0 was reported in the Gobi Desert and in the steppe locations, respectively. Our proposed Kc adj can be further improved by considering local weather data and plant phenological characteristics

The MOCAST+ Study on a Quantum Gradiometry Satellite Mission with Atomic Clocks

In the past twenty years, satellite gravimetry missions have successfully provided data for the determination of the Earth static gravity field (GOCE) and its temporal variations (GRACE and GRACE-FO). In particular, the possibility to study the evolution in time of Earth masses allows us to monitor global parameters underlying climate changes, water resources, flooding, melting of ice masses and the corresponding global sea level rise, all of which are of paramount importance, providing basic data on, e.g. geodynamics, earthquakes, hydrology or ice sheets changes. Recently, a large interest has developed in novel technologies and quantum sensing, which promise higher sensitivity, drift-free measurements, and higher absolute accuracy for both terrestrial surveys and space missions, giving direct access to more precise long-term measurements. Looking at a time frame beyond the present decade, in the MOCAST+ study (MOnitoring mass variations by Cold Atom Sensors and Time measures) a satellite mission based on an “enhanced” quantum payload is proposed, with cold atom interferometers acting as gravity gradiometers, and atomic clocks for optical frequency measurements, providing observations of differences of the gravitational potential. The main outcomes are the definition of the accuracy level to be expected from this payload and the accuracy level needed to detect and monitor phenomena identified in the Scientific Challenges of the ESA Living Planet Program, in particular Cryosphere, Ocean and Solid Earth. In this paper, the proposed payload, mission profile and preliminary platform design are presented, with end-to-end simulation results and assessment of the impact on geophysical applications

Prediction of Biome-Specific Potential Evapotranspiration in Mongolia under a Scarcity of Weather Data

We propose practical guidelines to predict biome-specific potential evapotranspiration (ETp) from the knowledge of grass-reference evapotranspiration (ET0) and a crop coefficient (Kc) in Mongolia. A paucity of land-based weather data hampers use of the Penman–Monteith equation (FAO-56 PM) based on the Food and Agriculture Organization (FAO) guidelines to predict daily ET0. We found that the application of the Hargreaves equation provides ET0 estimates very similar to those from the FAO-56 PM approach. The Kc value is tabulated only for crops in the FAO-56 guidelines but is unavailable for steppe grasslands. Therefore, we proposed a new crop coefficient, Kc adj defined by (a) net solar radiation in the Gobi Desert (Kc adjD) or (b) leaf area index in the steppe region (Kc adjS) in Mongolia. The mean annual ETp obtained using our approach was compared to that obtained by FAO-56 guidelines for forages (not steppe) based on tabulated Kc values in 41 locations in Mongolia. We found the differences are acceptable (RMSE of 0.40 mm d−1) in northern Mongolia under high vegetation cover but rather high (RMSE of 1.69 and 2.65 mm d−1) in central and southern Mongolia. The FAO aridity index (AI) is empirically related to the ETp/ET0 ratio. Approximately 80% and 54% reduction of ET0 was reported in the Gobi Desert and in the steppe locations, respectively. Our proposed Kc adj can be further improved by considering local weather data and plant phenological characteristics

Gravity field recovery and error analysis for the MOCASS mission proposal based on cold atom interferometry

Satellite missions providing data for a continuous monitoring of the Earth gravity field and its changes are funda-mental to study climate changes, hydrology, sea level changes, and solid Earth phenomena. GRACE-FO (Gravity Recovery and Cli-mate Experiment Follow-On) mission was launched in 2018 and NGGM (Next Generation Gravity Mission) studies are ongoing for the long-term monitoring of the time-variable gravity field. In recent years, an innovative mission concept for gravity measure-ments has also emerged, exploiting a spaceborne gravity gradio-meter based on cold atom interferometers. In particular, a team of researchers from Italian universities and research institutions has proposed a mission concept called MOCASS (Mass Observation with Cold Atom Sensors in Space) and conducted the study to investigate the performance of a cold atom gradiometer on board a low Earth orbiter and its impact on the modeling of different geophysical phenomena. This paper presents the analysis of the gravity gradient data attainable by such a mission. Firstly, the mathematical model for the MOCASS data processing will be described. Then numerical simulations will be presented, consid-ering different satellite orbital altitudes, pointing modes and instrument configurations (single-arm and double-arm); overall, data were simulated for twenty different observation scenarios. Finally, the simulation results will be illustrated, showing the applicability of the proposed concept and the improvement in modeling the static gravity field with respect to GOCE (Gravity Field and Steady-State Ocean Circulation Explorer)

Comparative analysis among Asia-Pacific geoid models stored at the ISG repository

Geoid models have important applications in geosciences as well as engineering, for example, for the conversion from ellipsoidal heights observed by GNSS techniques to orthometric heights. To meet the user’s demands, the International Service for the Geoid (ISG, https:// www. isgeo id. polimi. it/) provides access to a repository of local, regional, and continental geoid models through its website. Among hundreds of worldwide models, there are many covering countries in the Asia-Pacific area. The focus of this study is about this region, performing a series of analyses to assess the geoid models stored in the ISG repository through some relative comparisons. In particular, three kinds of analyses are performed with the purpose of: (a) investigating the evolution in time of a geoid series referring to the same country, (b) comparing the information provided by local nd regional geoid models on overlapped areas, and (c) assessing the agreement between local and global models. These analyses are firstly performed on sample models, providing a detailed description, and then applied to all Asia-Pacific geoid models currently stored in the ISG repository, providing summary statistics

Open access to regional geoid models: the International Service for the Geoid

The International Service for the Geoid (ISG, https://www.isgeoid.polimi.it/, last access: 31 March 2021) provides free access to a dedicated and comprehensive repository of geoid models through its website. In the archive, both the latest releases of the most important and well-known geoid models, as well as less recent or less known ones, are freely available, giving to the users a wide range of possible applications to perform analyses on the evolution of the geoid computation research field. The ISG is an official service of the International Association of Geodesy (IAG), under the umbrella of the International Gravity Field Service (IGFS). Its main tasks are collecting, analysing, and redistributing local, regional, and continental geoid models and providing technical support to people involved in geoid-related topics for both educational and research purposes. In the framework of its activities, the ISG performs research taking advantage of its archive and organizes seminars and specific training courses on geoid determination, supporting students and researchers in geodesy as well as distributing training material on the use of the most common algorithms for geoid estimation. This paper aims at describing the data and services, includin0g the newly implemented DOI Service for geoid models (https://dataservices.gfz-potsdam.de/portal/?fq=subject:isg, last access: 31 March 2021), and showing the added value of the ISG archive of geoid models for the scientific community and technicians, like engineers and surveyors (https://www.isgeoid.polimi.it/Geoid/reg_list.html, last access: 31 March 2021)

Gravity from space by Cold Atom Interferometry: the MOCASS study and preliminary results

MOCASS (Mass Observation with Cold Atom Sensors in Space) is an on-going study project funded by the Italian Space Agency in the framework of preparatory activities for future missions and payloads of Earth Observation. The object of the proposal is an innovative satellite gravity mission based on advanced cold atom interferometry (CAI) accelerometers, with the aim of modelling the static and time-variable gravity field of the Earth with high accuracy and resolution and of monitoring mass variations that occur on and below the Earth surface. The basic idea is a GOCE mission follow-on, launching a unique spacecraft with an on-board instrument capable of measuring some functionals of the Earth gravitational potential. The improvement with respect to the GOCE mission concept can only be achieved by going beyond the technology of electrostatic gradiometers, taking advantage of a new generation of sensors, such as cold atom interferometers. In the framework of the MOCASS study, the instrument characteristics are defined in terms of long-term stability, accuracy, and spectral responses. Then simulations on gravity field recovery based on the space-wise approach already used for the GOCE data processing are implemented. Finally an analysis on the geophysical signals that can be detected given the simulated mission performance are performed, with particular attention to hydrologic and tectonic modelling of changing masses. First simulations have already been performed by considering the GOCE orbit parameters but assuming that a CAI gradiometer is on board the spacecraft. This allows direct comparisons between GOCE and MOCASS performances. Instrument error spectra have been defined depending on the orbit and CAI configurations, all of them characterized by a flat error spectrum in the low frequencies, differently from the one of the GOCE electrostatic accelerometers. Given the error spectrum and the interferometer integration spectral response, simulated observations have been produced and processed by the space-wise approach, which basically consists in a sequential application of a Wiener filter, a local collocation gridding and a spherical harmonic analysis. From sample statistics, the accuracy of the recoverable gravity field model can then be evaluated and compared with the expected gravity signal from selected geophysical phenomena, e.g. orogen geodynamics and glacier melting. Although the study is at this time not complete, these preliminary investigations show promising results