Terrestrial Water Storage from GRACE and Satellite Altimetry in the Okavango Delta (Botswana)

The HYDRO-GRAV projectOle B. Andersen, P. E. Krogh (DNSC/DTU, Denmark)

P. Bauer-Gottwein, S. Leiriao (DTU, Environment and Ressources) R. Smith, P. Berry, J. Freeman (De Montford, UK)

F. Lemoine (NASA/GSFC, USA)

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Overview

• The Hydrograv Project

• Preliminary results from The Okavango Delta (Botswana)

• NASA/GSFC GRACE Mascon• CNES / BGI Global Spherical harmonic solutions• EAPRS retracked ERS-2/Envisat satellite altimetry• Local river gauge observations.• Hydrological models

– Annual signal – Importance of 3D gravity modelling

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HYDROGRAV

Improving Calibration and Reliability of Large-Scale Hydrological Models with Ground-Based and Space-Borne Time-lapse Gravity Surveys (HYDROGRAV)

Objectives of HYDROGRAV – To understand in detail the relationship between hydrological and

gravity signals over a range of spatial and temporal scales. – To demonstrate the utility of gravity surveys for hydrological

analysis in two target regions in Denmark and Africa. – Improving the GRACE data using groundbased information.– Constrain hydrological parameters using GRACE data

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HYDROGRAV

DTU Danish National Space Center.

DTU Institute of Environment and ressources

COWI – Int. Engineering, environmental science

DHI Water and Ressources

Consortium

Funding Danish Agency for Science, Technology and Innovation3.5 mill kroner = 500.000 Euro

DTU Co-sponsoring two Ph.d students (Pl. see their Poster).

Period

Testregions

2007 – 2010 (3.5 years)

Okavango Delta - Botswana (In situ + Grace) Denmark (pump test)

WWW.HYDROGRAV.DK

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The HYDROGRAV Approach

Numerical hydrological modelImprove parameter estimates

(i.e. Okavango)

Water storage changes Simulated gravity changes

Hydro observationsIn-Situ gravity observationsGrace Gravity Observations

Hydro-Geophysical inverse problem

Hydrological data Conceptual hydro model

Parameter estimates

Geophysical forward problem

TimesteppingHydro model

Hydrology - Gravityinverse constrainGRACE solution

Okavango Gravity survey 2008-2010CG5 Temporal and spatial survey

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Testregions: Okavango , Botswana

Size of Denmark, Essential to a prospority of wildlifeSeasonal Flooding, Water evaporates

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Okavango Delta

Δ S can be obtained from hydrological model (MODFLOW)Δ S can be obtained from GRACE

Δ SSW can be obtained from Retracked altimetry / river gauge

Subsurface water storage: Δ SSM + Δ SGW = ΔS - Δ SSW

ΔS = P – E - RΔS = Δ SSW + Δ SSM + Δ SGW

Hydrological Balance

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Retracking AltimetryEAPRS retracking system

EAPRS expert system identifies eleven waveform shapes and applies tailored retracking algorithm to obtain heights.

EAPRS system was used to analyze ERS-1/2 & ENIVSAT waveform shapes over inland water.

Difficult regions.Retracking varies with timeMost places river dries out.

High slope

Slope

Water

Land/water

Complex

Very wide

SeaIce-Like

Near specular

PatchOcean

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Retracked by R. Smith, EAPRS

Retracked Altimetry10 Years of ERS-2

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ENVISAT

Altimetry range generally larger than model

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NASA/GSFC GRACE MASCONS• Processed GRACE level 1B Data from Jan 2003 – July 2007• Upgraded atmospheric series, improved ocean tide models, improved processing. • Mascons solved on a 4ºx4º grid every ten days, where sufficient data were

available to construct a solution.• GRACE signature from a mascon only manifest itself over the area of overflight.• => 150 10-day solutions (≈ 5 years)

=> Okavango within one MASCON

GRACE

BGI / CNES global GRACE constrained solutions• 5 years of GRACE Level 2 spherical harmonic solutions• Spherical harmonic degree and order 50. • Temporal resolution = 10 days.• 156 10-day solutions ( ≈ 5 years)• Interpolated spatial resolution = 1 degree

0 10 20 30 40 50-40

-30

-20

-10

0

Latit

ude

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GRACE Mascon + Hydrology + Altimetry

GRACE (blue)

GLDAS(green)

ENVISAT(cyan)

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Annual Signal - Okavango

Jan03 Jan04 Jan05 Jan06 Jan07-20

0

20

0

400

800GRACE BGIGRACE masconGLDAS

Discharge[m

3s-1]

Gra

vity

sig

nal

[cm

H20

]

90 = 1 april

138 cm H20

Satellite Altimetry

101 = 10-april86 = 26-march112 = 21-april98 = 07-aprilPhase (days)

9.5 cm H207.2 cm H204.4 cm H20Amplitude

Discharge MohemboGLDASGRACE

masconGRACE BGI

Discharge

Estimated annual + bi-annual signal (6 parameter solution). BGI Bi-annual signal is siqnificantMascon Annual signal is significantly larger than BGI (omission ??)All but Discharge shows trend (íncreased ΔS ?)

Computation: Pernille E. Krogh. Se Poster (Tuesday) by Christiansen and Krogh

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Water Storage Change

Using MODFLOW hydrological model of Okavango Delta.Forward gravity response.

Compute accurate forward gravity response to timevarying hydrological changes. ”Terrain” modeling system from geodesy.Forsberg (exact) close, McMillian(intermediate) and Point mass system outside.

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Preliminary 3D modelling of Okavango

MODFLOW hydrological model Gravity change simulation

Computation S. Leireio

GRACE observations

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The HYDROGRAV Project

Investigate Satellite Altimetry and GRACE at regional scales:

The high resolution GRACE MASCONS shows both annual+interannual signal

GRACE MASCON annual is significantly larger than BGI signal

Altimetric river level observations, gauge and GRACE in agreement on phase

Investigate methods for accurate gravity field response to hydrological change

Will carry out comparison between in-situ and satellite based observations

Use full 3D hydrological model DHI MODFLOW river mask system

Outlook / Summary