The Mbarali New NAFCO Main Canal has a small hydro power plant using 1 cumecs. Immediately downstream of the plant a return drain is constructed to secure return of the 1 cumecs to the Mbarali River after passing through the turbine. The practice of returning the water “borrowed” for power generation is not always honored. Remotely monitoring of the return flow is possible with low flow in the Mbarali River at the New NAFCO headworks. And it is especially during the low flow period very important to maximize drainage from the Usangu Rice Plain including the draiage from the Mbarali River. Monitoring of the return flow by the authorities is therefore an important management tool.

During low flow some 2 cumecs is diverted from the Mbarali River and the excess is passing over the weir or leaking through the sluice doors. On July 6, 2009 some few hundred liters per second are left in the Mbarali River downstream of the weir as shown on the high resolution image below.

High resolution image New NAFCO Headworks on July 6, 2009. With closed sluice doors all river water is diverted into the main canal on the right and flow in excess of ~ 2cumecs flows over the weir at the left. An estimated few hundred liters per second is on this day flowing over the crest of the weir.

The remaining water in the Mbarali River reaches the return drain outlet some 4.6km downstream of the headworks (as the bird flies). On July 6, 2009 the returned flow in the drain seems to be maximal as the water level is close to the allowable 0,6m freeboard at the first lined section at the junction with the main canal. This is concluded from comparing the next two images of which the second is from December 26, 2021 when almost no drainage water is diverted.

High water level in return drain from the Mbarali (New NAFCO) main canal on the top right. Water comes through the main canal after passing the hydro power plant from the New NAFCO Headworks and flows from right to left. The image is from July 6, 2009. (Click to enlarge)
High resolution detail Return Drain canal junction on December 26, 2021. (Click to enlarge)

The Return Flow shown on the July 6, 2009 high resolution image as it joins again the Mbarali River is also detected remotely by the Landsat 5 near infrared image as is demonstrated in the second image below.

With only a trickle keft in the Mbarali River as it reaches the return drain outlet, the flow from the drain coming from the Mew NAFCO main canal is clearly visible on July 6, 2009. Click to enlarge image.
The Landsat 5 image is less sharp as the Sentinel-2 images which were not yet operative in 2009 but the Near Infrared image with dark blue for deeper water and reddish for wet sediments and riverine trees, reflects the gully formed by the return drainage from the main canal clearly in the middle of the image where it flow to the west (left). The image was taken on the same day as the high resolution image above.

As the return flow to Mbarali River can be distinguished with the high resolution images like the one above from July 6, 2009, especially with (near to) zero flow in the Mbarali River at the outlet. It is now a matter to search for “foot prints” of incoming return flow at the outlet in the almost dry riverbed of the upstream Mbarali River.

Using the analyses for the New NAFCO will confirm the low flow status of the Mbarali River by verifying the (near) zero flow over the weir. The exposure of the riverbed sediments between the outlet of the return drain and the New NAFCO Headworks will also confirm the low flow to zero flow status in the Mbarali.

The natural color and near infrared Sentinel 2 images with 10m resolution are with some manipulation suitable to show the gullies with flowing water from the outlet.

Panchromatic images with 15m resolution can also indicate deeper pools of water as is demonstrated for the return flow captured on August 23, 2011. The Panchromatic image from Landsat 7 is taken a few days later (September 6, 2011) and indicates the deeper pools of water in the return flow as it flows into the Mbarali River.

High resolution image of the Return Drain Outlet along the Mbarali River on August 23, 2011.Click image to enlarge.
The same section is captured by Landsat 7 panchromatic image on September 6, 2011. The dark spots are deeper sections of surface water flowing into the Mbarali River which is on that date near to dry with a few hundred liters flowing over the weir and almost all Mbarali River flow diverted into the New NAFCO main canal.