Interpretation of GRACE data of the Nile Basin using a groundwater recharge model

Assessing and quantifying natural water storage is becoming increasingly important as nations develop strategies for economic growth and adaptations measures for climate change. The Gravity Recovery and Climate Experiment (GRACE) data provide a new opportunity to gain a direct and independent measure of water mass variations on a regional scale. Hydrological models are required to interpret these mass variations and partition them between different parts of the hydrological cycle, but groundwater storage has generally been poorly constrained by such models. This study focused on the Nile basin, and used a groundwater recharge model ZOODRM (Zoomable Object Oriented Distributed Recharge Model) to help interpret the seasonal variation in terrestrial water storage indicated by GRACE. The recharge model was constructed using almost entirely remotely sensed input data and calibrated to observed hydrological data from the Nile. GRACE data for the Nile Basin indicates an annual terrestrial water storage of approximately 200 km3: water input is from rainfall, and much of this water is evaporated within the basin since average annual outflow of the Nile is less than 30 km3. Total annual recharge simulated by ZOODRM is 400 km3/yr; 0–50 mm/yr within the semi arid lower catchments, and a mean of 250 mm/yr in the sub-tropical upper catchments. These results are comparable to the few site specific studies of recharge in the basin. Accounting for year-round discharge of groundwater, the seasonal groundwater storage is 100–150 km3/yr and seasonal change in soil moisture, 30 km3/yr. Together, they account for between 50 and 90% of the annual water storage in the catchment. The annual water mass variation (200 km3/yr) is an order of magnitude smaller than the rainfall input into the catchment (2000 km3/yr), which could be consistent with a high degree of moisture recycling within the basin. Future work is required to advance the calibration of the ZOODRM model, particularly improving the timing of runoff routing

A geophysical approach to the granite batholith under the eastern Southern Uplands, Scotland

From our interpretation of the Bouguer gravity and aeromagnetic anomalies in south-east Scotland, we conclude that a massive granite batholith underlies the greater part of the eastern Southern Uplands. The granite model which we computed earlier from gravity anomalies in the Tweeddale area fits the observed magnetic anomalies closely, if a normal magnetization of 0.095 A m-1 is assigned, similar to values found for exposed local granites. Further gravity modelling shows that, apart from the Tweeddale boss, the granite shallows to less than 1 km near Lammer Law in East Lothian and extends north of the Lammermuir Fault. A model for the East Lothian volcanics was computed from their aeromagnetic anomalies, then their gravitational effect was combined with that estimated for the Devonian and Carboniferous sediments and the result stripped off the observed gravity field. The residual gravity anomalies were used to generate a two-dimensional model for the granite north of the Lammermuir Fault. The expected tectonic consequences of a massive granite batholith in the eastern Southern Uplands are compared with the known development of faults and sedimentary basins around its margins. © 1982 Birkhäuser Verlag

A geophysical approach to the granite batholith under the eastern Southern Uplands, Scotland

From our interpretation of the Bouguer gravity and aeromagnetic anomalies in south-east Scotland, we conclude that a massive granite batholith underlies the greater part of the eastern Southern Uplands. The granite model which we computed earlier from gravity anomalies in the Tweeddale area fits the observed magnetic anomalies closely, if a normal magnetization of 0.095 A m-1 is assigned, similar to values found for exposed local granites. Further gravity modelling shows that, apart from the Tweeddale boss, the granite shallows to less than 1 km near Lammer Law in East Lothian and extends north of the Lammermuir Fault. A model for the East Lothian volcanics was computed from their aeromagnetic anomalies, then their gravitational effect was combined with that estimated for the Devonian and Carboniferous sediments and the result stripped off the observed gravity field. The residual gravity anomalies were used to generate a two-dimensional model for the granite north of the Lammermuir Fault. The expected tectonic consequences of a massive granite batholith in the eastern Southern Uplands are compared with the known development of faults and sedimentary basins around its margins. © 1982 Birkhäuser Verlag

Newly compiled gravity and topographic data banks of Greece

A brief description of the gravity and topographic data banks of Greece is given. About 22 000 gravity stations are included in the newly compiled gravity data bank from an initially available data set of 33 000 stations, after the application of specific rejection criteria, and classified in squares of 100 km × 100 km. The data cover an area between 18° and 27°E, and 33° and 42°N (Fig. 1). A matrix of 2 km × 2 km mean elevation values was also estimated for the same area. These data constitute the topographic data bank. The preparation and structure of both data banks are also briefly discussed here

Reference gravity stations. on the IGSN71 standard in Britain and Greece

The International Gravity Standardisation Net 1971 (Morelli et al. 1974) includes no reference sites in Greece. We describe composite observations with LaCoste and Romberg (LCR) gravity meters which tie a newly established site at Athens Airport to IGSN71 sites in Rome and Edinburgh, as well as determining the absolute scale of the Greek National Calibration Line on Mount Parnis. As a by‐product, the manufacturers’ scale of the LCR meter G‐275 is found to require correction by the factor 1.000 622 ± 0.000 027 against IGSN71. Using this factor, we confirm the IGSN71 scale in Britain by deducing a gravity difference of 387.185 ± 0.018 mGal between Edinburgh A and Teddington A, compared with the IGSN71 value of 387.19 ± 0.030 mGal. The same interval for the National Gravity Reference Net 1973 (Masson Smith et al. 1974) is 397.128 ± 0.023 mGal, so that our observations confirm Turnbull's (1986) conclusion that the NGRN73 connection to Edinburgh Turnhouse Airport was faulty. There are now no significant differences in either overall scale or individual station values between our observations on a selection of NGRN73 sites and those obtained under a program of re‐observation by the British Geological Survey. However, we found an error exceeding 0.2 mGal in one of the original NGRN73 values. Copyright © 1988, Wiley Blackwell. All rights reserve

Regional gravity anomalies 1: northern Britain

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