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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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