Karalarda Su Kütlesi Değişimlerinin Uydu Gravimetrisi İle İzlenmesi

Abstract

Yeryuvarı gravite alanının belirlenmesi ve modellenmesi jeodezinin en önemli çalışma konularından biri olmuştur. Gravite alanına ilişkin bu modellere, jeodezik çalışmaların yanısıra jeofizik, oşinografi ve uzay araştırmalarına ilişkin çalışmalarda da ihtiyaç duyulmuştur. Yeryuvarının dörtte üçünü oluşturan okyanus kitlesi üzerinde herhangi bir gravite bilgisi bulunmadığından, daha önce kullanılan yersel gravimetriyle elde edilen gravite alanı modellerinin güvenilirliği düşüktü. Öte yandan, homojen nitelikte olmayan ölçülerden global bir modelin parametrelerinin kestiriminde de sorun yaşanmaktaydı. Uydu gravimetrisinin gelişimiyle birlikte global gravite modellerinin belirlenmesi çalışmaları hız kazanmıştır. Böylece, Yer’in gravite alanında zamana bağlı değişimlere neden olan ve modellenebilen etkilerin arındırılması ile birlikte zamana bağlı değişime katkısı belirgin olan su kütlesi değişimlerinin izlenmesi mümkün olabilmiştir. Küresel iklim değişimi orta enlemlerde ve orta ve düşük yükseklikteki bölgelerde kuraklığı artırarak milyonlarca insanı su kıtlığı gibi büyük bir problemle karşı karşıya getirmektedir. Bu bölgedeki Orta Doğu ülkeleri gibi ülkemizi de ilgilendiren bu sorunun çözümü için, su havzalarındaki su seviyesi değişimlerinin sık aralıklarla gözlemlenmesi önemli bir bilgi sağlamaktadır. Böyle bir izlemenin yersel ölçülerle yapılması hem yüksek maliyetlidir hem de havzanın tümünü temsil etmez. Diğer taraftan, klasik uzaktan algılama uyduları gerek düşey çözünürlükleri bakımından gerekse yeraltına ilişkin bilgi sağlayamadıklarından kara hidrolojisi çalışmalarında yetersiz kalmaktadır. Klasik uzaktan algılama tekniğinin bu eksikliğini gidermek, yine yapay uydulardan, başka bir yöntem olan uydu gravimetri tekniği ile mümkündür. Bu çalışmada, 30-48 Kuzey Enlemleri ve 20-47 Doğu Boylamları arasında kalan, ülkemizi ve komşu ülkelerin bir bölümünü kapsayan bölgede statik bir gravite alanı modeline (GGM03C) göre olan su kütlesi değişimleri, GRACE (Gravity Recovery And climate Experiment) uydu gravimetri verilerinden hesaplanmış ve bu kütle değişimleri farklı çözünürlükte eşdeğer su kalınlığı (EWT) haritaları olarak hazırlanmıştır. Söz konusu EWT haritaları aylık zamansal ve 300 km ve 600 km gibi iki farklı mekansal çözünürlükle, GRACE uydu misyonunun aylık olarak hesaplanan global küresel harmonik gravite alanı modellerinden (GRACE seviye-2 ya da L2) ve aylık ve yarı aylık zamansal ve 165 km gibi yüksek mekansal çözünürlükle GRACE yörünge üzeri KBR (uydular arası uzaklık değişimleri) ve ivmeölçer verilerinden (seviye-1B ya da L1B) belirlenmiştir.

The total gravity field of the Earth consist of two components: Static gravity field and time-dependent gravity field. Although the static gravity field is changing very long period of time, it is possible assume it stable. Tides caused by the gravitational force of the Sun and the Moon are temporary gravity signals. Hydrology, ocean, cryosphere and atmosphere affect the gravity field of non-tidal and time-varying component. These effects are well known in sizes ranging from minutes to longer temporal scale, generated from measurable (modeled) signals. The basic problem is the information about temporal component of the global gravity field. How much mass is displaced and re-distributed? In the past years, it has been a basic principle to determine the global and regional scale gravity field using the artificial satellites moving on a specific orbit of the Earth. Since the orbital motions of satellites largely determined by the forces of gravity, as a reverse approach, precise orbit solutions can be used to determine the gravity field. Since the time varying signals change fast enough, the source of this change is thought to be occured on the surface of the Earth rather than the depths. This signal is largely caused by the atmosphere, the oceans and above or below water, snow and ice masses of the lands. Except showing the annual cycle, some movements on Earth crust change this signal rapidly. The only exceptionsare tidal effects which can be modelled with a better accuracy than the GRACE gravity measurements and the response of the solid Earth to the loading effect of the surface mass. As soon as the Earth’s gravity field is formed of the mass distirbution of the depths and surface of the Earth, these mass distirbutions are changing permanently. Ocean and solid earth tides cause large mass changes in 12 and 24 hour intervals. Synoptic storms, seasonal changes etc. related to atmospheric disturbances leads to significant changes on the atmosphere, ocean and the water mass distirbution stored on land. Although, mantle convection compared to climate change causes larger amplitude changes throughout the mantle, it follows slower process. Global climate change has been increasing drought in mid-latitude and semi- and low altitudes, and exposing millions of people to water stress. Frequent monitoring of the water level changes in water reservoirs provides valuable information for the solution of this problem which concerns our country as well as the countries in the middle east. Such a monitoring based on terrestrial observations is not economical and does not represent the whole basin. On the other hand, classical remote sensing satellites have shortcomings for land hydrology studies because of their low vertical resolutions as well as their inability of providing information related to the underground. These shortcomings of the classical remote sensing techniques can be met by using another method based on artificial satellites such as satellite gravimetry. It have lead to incorrect and incomplete interpretion about the processes of the Earth system because of the difficulties with monitoring the mass distirbution of the Earth and its movement, and insufficiency of obtaining data from the high orbit satellites. However, this situation has been changed by satellite gravity missions which provide solutions globally to a few hundred kilometers-resolution such as CHAMP (Challenging Minisatellite Payload), GRACE (Gravity Recovery and Climate Experiment) and GOCE (Gravity field and steady-state Ocean Circulation Explorer). Low-orbiting gravity field satellite mission CHAMP was launched in July 2000. Satellite is a nearly circular orbit and very close to the pole with an initial height 454 km. The main aim of prefering to obtain a homogeneous and whole global orbit which is important for high spatial-resolution global gravity field modeling. In order to calculate the parameters of gravity field, CHAMP is equipped with a GPS receiver to obtain low-orbiting satellite data from high-orbiting satellites. Therefore, satellite orbit can be determined with high accuracy. Since low-orbiting Earth satellites (LEOs) are affected from anomalies while passing through the masses, which cause to disturb satellite orbit. Thus, Earth’s gravity field is obtained from satellite orbit by measuring the position of the satellite and gravity dependent acceleration measured by star camera and three-axes accelometer. Since hl-SST (high-low satellite to- satellite tracking) data is especially sensitive to the inaccuracies at medium and long wavelengths, CHAMP mission has considerably developed the accuracy of gravity field models at these wavelengths. However, CHAMP satellite could not achieve a gravity field depending on time as presumed. Therefore, all attention has been directed to another gravity mission, GRACE. On the other hand, another vital gravity mission, GOCE satellite launched in March 2009 has determined average Earth gravity field with few centimeters geoid accuracy that has not been achieved until now. GOCE has been planned to operate for two years in a sun-synchronous orbit that is close to the pole with an inclination of 98.5. Although GOCE continue to operate effectively, it has been reached to expected accuracy with obtained 2 year data. NASA/GFZ (GeoForschungsZentrum Potsdam)’s satellite mission GRACE (Gravity Recovery And Climate Experiment) has been launched on March 2002 with the primary science objective of measuring the static gravity field of the Earth and its time variation and understanding its relation to the climate change. The global gravity field computed from the variations in the accelerations and the positions of the two identical satellites freely moving on the same circular orbit, as well as the high precision (± 10 m) inter-satellite range measurements and their variations in time have improved the accuracy of the best known global gravity fields prior to 2002 (EGM96) by a factor of 50. Since the well known effects such as ocean, solid Earth and pole tides, atmospheric mass redistribution and barotropic ocean response to atmospheric loading were forward-modelled and removed from the GRACE observations prior to the calculation of the spherical harmonic model coefficients using the monthly GRACE observations, the differences between these coefficients of consecutive months are caused primarily by the variations of the continental water storage (river, lake, ground water, soil moisture etc.). Moreover, the orbital dynamics of the GRACE twin satellites are also affected by the variations of the continental water storage. In this study, which is aimed to precisely determine the variations in the water levels in lakes, dam reservoirs and in particular in ground water, the water level variations in the region between 20-47 North Latitudes and 30-48 East Longitudes covering Turkey as well as the northern countries in the middle east were estimated from GRACE in monthly and sub-monthly periods with <200 km spatial resolution from GRACE in-situ KBR ranging and accelerometer data (GRACE L1B). Iterative least-squares estimation was enforced using the regional inversion of GRACE L1B data to determine equivalent water thickness (EWT) within a thin layer of the Earth. Potential diference data which were used to estimate water level changes in the region of each of a 1.5°×1.5 grids are selected on a more narrow area (22 -45 longitude and 32 -46 latitude). However, EWT was estimated for larger region (20 -47 longitude and 30 -48 latitude). Total number of grids or in other words the total number of unknowns is 216, and the mean number of observations are approximately 2000 for monthly solutions. The main objective of this selection is to consider aliasing effect within the boundaries of the study area, which is called edge effect , and to minimize this effect. This is the main reason of obtaining 1-2 cm accuracy for monthly solutions. Thus, water level changes with homogeneous accuracies were obtained for the solutions of each month or higher temporal (10 or 15 days) resolutions for all the grids within study area. The other data derived from GRACE observations ismonthly time series of global geopotential models in terms of spherical harmonic coefficients (SHC) which is named level-2 (L2) data products. As the well-known effects on the orbital perturbations such as the planetary bodies, ocean tides, solid Earth tides and other high-frequency variations in ocean and atmosphere are forward modeled prior to the estimation of monthly SHC, the difference between the SHC mainly represents the changesof climate sensitive signals such as hydrology, ice sheet mass balance and ocean mass change. Although the SHC still include the residual effects of tides and atmosphere due to imperfect models and temporal aliasing, recentstudies have shown that the hydrology signal can be estimated with an accuracy of several cm in equivalent water thickness and a resolution of several hundred km. The raw data should be processed by removing PGR effect, decorrelation filter, Gaussian smoothing function with different radiuses and removing leakage. After these processes EWT maps could be produced with monthly temporal and different spatial resolutions, i.e. 300 km and 600 km, based on monthly mean global spherical harmonic gravity field models of GRACE satellite mission.