Doesn't matter how fertile the land is if there is no water available.
High yield farms require around 8,000 cubic meters of water per HA on average (Rice is at 10-15k).
Read a few papers on our surface water capacity and I usually find a run off rate of 5 billion cubic meters in Juba and depending on the catchment area something similar in Shabelle.
Source: W-13 Hydraulic Behaviour of the Juba and Shabelle Rivers.pdf
Page 23-24
If deduct for evaporation, leaks etc you would be lucky to have enough water to irrigate 800k hectares. Somalia needs 10-20 million HA
Also 250k tonnes won't feed half the population and probably not even 10% if it's still 12 million.
Since these are the brutal facts we need to instead focus on how to get that foreign currency to import food.
It is possible that you are underestimating Somalia's agricultural prospects. Badhadhe and Afmadow districts in Lower Juba seem to get more than enough rainfall to grow maize and sorghum. It would mean cutting down a large amount of forest, but it is possible. There is next to no farming going on in Badhadhe and Afmadow, so you would need to convince herders to turn their pastures into fields.