May Gabat

Main tributary is Kalamino River which drains a large part of Mekelle; it joins May Gabat downstream from the Gereb Segen reservoir.

The runoff footprint or annual total runoff volume is 58 million m³. Peak discharges up to 190 m³ per second occur in the second part of the rainy season (month of August) when there are strong rains and the soils are saturated with water in many places. The percentage of total rainfall that directly leaves the catchment as storm runoff (also called runoff coefficient) is 9%. As limestone is present in 28% of the catchment this runoff coefficient is less than that of adjacent rivers.[1]

Location of previous May Gabat measuring station

The total amount of sediment that is transported by this river amounts to 373,000 tonnes per year. Median sediment concentration in the river water is 1.8 grammes per litre, but may go up to 64 g/L. The highest sediment concentrations occur at the beginning of the rainy season, when loose soil and dust is washed away by overland flow and ends up in the river.[5] As such water contains many nutrients (locally it is called “aygi”), farmers estimate that it strengthens their cattle, which they will bring to the river.[4] All in all, average sediment yield is 752 tonnes per km² and per year. All measurements were done at a purposively installed station near the mouth of the river, in the years 2006–2007.[5] It is anticipated that the mentioned values have strongly decreased after construction of the Gereb Segen reservoir, because it intercepts water and sediment.

Runoff mostly occurs in the form of high runoff discharge events that occur in a very short period (called flash floods). These are related to the steep topography, often little vegetation cover and intense convective rainfall. The peaks of such flash floods have often a 50 to 100 times larger discharge than the preceding baseflow. These flash floods mostly occur during the evening or night, because the convective rain showers occur in the afternoon.[4]

May Gabat downstream from Gereb Segenreservoir – the river has become regularised thanks to seepage from the reservoir

Evidence given by Italian aerial photographs of the catchment, taken in the 1930s show that 48% of the catchment was covered with woody vegetation (against 33% in 2014). This vegetation could better slow down runoff in earlier times. On the other hand, five reservoirs intercepted already the floods in the headwaters and the runoff coefficient was smaller in 2014 (12% in 1935 against 9% in 2014).[6] Up to the 1980s, there was strong pressure on the environment, and much vegetation disappeared.[7] This river had its greatest discharges and width in that period. The magnitude of floods in this river has also been decreased in recent years due to interventions in the catchment. On steep slopes, exclosures have been established; the dense vegetation largely contributes to enhanced infiltration, less flooding and better baseflow.[8] Physical conservation structures such as stone bunds[9][10] and check dams also intercept runoff.[11][12]