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					Monitoring Lake Victoria water balance from space

  Sean Swenson (NCAR) and John Wahr (U of Colorado)




The goal: to understand and monitor the effects of the Owen
Falls dams (there are two of them) on the level of Lake Victoria.
Lake Victoria
Two Dams.
Built in 1959
and 1999.
  Uganda-Egypt treaty (1954):
  Dam discharge should mimic the natural discharge, by following
  an “Agreed Curve” between discharge and lake level.




            From Sutcliffe and Petersen (2007)

But dam discharge data are not normally released to the public.
Has the discharge been consistent with the “Agreed Curve”?
        Lake levels from radar altimetry
heavy precip (1998)    2’nd dam completed (1999)




                                                   Two meters of
                                                   lake level
                                                   decline




                               drought
The drought, and subsequent recovery, is evident in GRACE temporal gravity data.


                                                 GRACE time series, averaged over 500 km
                                                 disc centered on Lake Victoria
Lake levels from radar altimetry, and mass variability from GRACE
        heavy precip (1998)   2’nd dam completed (1999)




                                      drought
Previous studies suggest that about half the 2000-2006 lake
level decline was due to natural variability, and half was due
to discharge in excess of the “Agreed Curve” (Kull, 2006;
Sutcliffe and Peterson, 2007; Awange et al, 2007).


Recently confirmed when in situ discharge data through
2005 became available.


Can we find a method of monitoring future discharge from
space, that does not require the availability of in situ
discharge data?
Several methods used:
(1) Compare catchment water storage trends (from GRACE)
   and lake level trends (from altimetry) for Lake Victoria, with
   those for other large lakes in the region (Lake Tanganyika,
   Lake Malawi).
(2) Use Lake Victoria water balance equation:
     dS/dt = P – E + Qin – Qout
 (S = lake height, P = lake precip, E = lake evap, Qin = inflow,
  Qout= outflow) to solve for Qout .
 Use GRACE, TRMM, radar altimetry lake levels, atmospheric
  and lake surface properties from QuikScat, MODIS, NOAA
  & DMSP satellites.

(3) Use lake levels from radar altimetry and
  ICESat, for downstream lakes.
Monitor Lake
Kyoga lake
level with
altimetry




  Two Dams.
  Built in 1959
  and 1999.
ICESat tracks over Lake Kyoga. Editted for clouds and large saturation values.
Heavy precip. Kyoga
and Victoria both rise.


                                                      Kyoga drops, Victoria rises.
                                                      Implication: reduced Victoria outflow.




                Victoria drops, Kyoga remains high.
                Implication: excessive Victoria outflow.
Estimating Victoria outflow from Kyoga lake level measurements

Use Lake Kyoga water balance equation:
     dS/dt = P – E + Qvin + Qcin – Qout
S = Kyoga lake height, P = lake precip, E = lake evap, Qvin = inflow from Victoria
  Qvin = inflow from Kyoga catchment, Qout= outflow.


Solve for
       Qvin = dS/dt + Qout - P + E - Qcin


Use altimeter lake levels for S and Qout (assume Qout is proportional to S).
(- P + E - Qcin ) is small; use Victoria values, scaled down to the area of Kyoga.
                           Results




 Grey line: Victoria discharge inferred from Kyoga lake level.
 Black line: observed Victoria discharge.



Agreement is good. The Lake Kyoga method seems to work.
                    Conclusions
Satellite altimeter measurements of Lake Kyoga lake elevations
  can be used to monitor Lake Victoria discharge rates.

Could be useful in the future for determining whether there is
  compliance with the 1954 Uganda-Egypt treaty.

				
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posted:5/25/2013
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