Critical Low Drainage Usangu, Alert 3 observed
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Searching for critical low drainage flow Ruaha
Visual or mathematical fixation of the critical flow at Nyaluhangu where all drainage from the Usangu Plains passes during low flow, is still a challenge.
The drying of the branch channels (Alert 3) proved tricky. The use of NDWI water area threshold like for prediction of Alert 1, does not yet yield a promising Alert 3 threshold.
What does seems to be a trend observed during the zero flow monitor operations by Maji Yetu, is the reduction of the Alert period. Between 2018-2021 the period between the drying of the Ruaha River at the Mtera Reservoir entrance and the drying at the Ruaha Park HQ (Msembe Bridge) took more than 4 months, while since 2021 the “Alert period” is less than 4 months and even just 9 weeks in 2020. Table 2 has the periods from the three alerts to drying of the Ruaha River at the Park Headquarter.
If consistent, this reduce period between observation of the first drying of the Ruaha at Mtera and the drying at the Ruaha Park Headquarters suggests that a more rapid decline in flow in the Mbarali and Ruaha Rivers is developing. An explanation could either be reduced baseflow, less late rains in March – May or increased abstraction and/or late abstraction from both rivers during the dry season.
For the time being the reduction of the water area of the Ruaha River at Nyaluhanga flow measuring station will be monitored until a robust alternative is selected.
Water Area Alert 3 section Ruaha for May 2025 still stable
The water area NDWI (blue px) of a selected section of the Ruaha River at the flow measuring station of Nyaluhanga, from early may to June 1, 2025 is stable at some 0.8ha. This value is already in the range of Alert 3 water area observations.