Water harvesting 15 kms west of the Cairo Nile

My one acre land is 70m above sea level, on the desert edges before the drop into the Nile valley. We rely on groundwater which is highly saline, as is the soil. The most common weed on my land is salt cedar.

The few surrounding lots who have moved in all rely heavily on water filtration systems for irrigation, most have their own wells, and plant mostly grass and ornamentals, so I worry about the water levels over time as the quality is already bad and I'm considering building a house here.

I wanted to begin with a swale system to harvest rainwater but the gardeners and neighbors all insist it doesn't rain here. My question is: am I supposed to design for a rain that might never come? My theoretical permaculture background says yes, but those who have been in the area long before me insist that this would be a huge waste of energy. I was told the most rain seen here in the last 5 years was a 20-minute shower once. My land is 60m2, so I estimated that two swales across would store up to 75,000 liters each if they did happen to fill, otherwise I don't know how else to flush the salt out of my land without a filtration system such as my neighbors'.

How can I heal my land and create a home for myself here in a sustainable, ethical way?