On Google Maps, lots of the fields have these concentric patterns, which appear to be tracing out contours. Link.
They almost look like the patterns you get in terraced fields in steep countryside, but the land here is pretty flat.
Looking on street view, the patterns are raised ridges, not ditches, so they don’t seem to be any kind of drainage thing. Clearly people have gone to a lot of effort to make them, though, as they are all over the place in this area.
What are they?
You have circled all around it. It is farming on the contour. It is anti-drainage - when you get not enough rain you want to save all you can; and prevent good topsoil from running off when the rain is excessive.
I tried to look it up for a cite, but I would say that they are as SVD says above. Maybe government sponsored after the 1920s dustbowl disaster, where all the topsoil got blown away after farmers ploughed off the hardy, deep rooted grasses.
The british did something similar trying to cultivate groundnuts in the British trusteeship of Tanganyika, now mainland Tanzania. They had to cope with lions, elephants and angry bees as well. It was a total disaster and the land was ruined.
Farmers here aren’t looking for water drainage, they’re looking for water containment. The ridges are meant to hold water on the field, they’re the opposite of ditches used to drain water away.
Ah, just like the spirals in that south african dust bowl lake bed… (right near the track used by someone famous in a land speed record attempt )…
( A major segway but … Of course the land speed record attempts are now moot, as the rocket vehicles are being programmed to touch down to land to the minimum amount neccessary, its more like a touch and go rocket… )
Experienced son-in-law from Oz here. Wife was a farmer’s daughter from a very dry part of the country. The problem is, water that hits the ground (and doesn’t soak in) will naturally drain away via the most optimal path. Over time, this optimal path tends to become more defined - hence streams, rivers etc. Eventually you wind up with gullies, natural drains and ditches, naturally eroded channels all over the place, which is very efficient at removing water from the land.
Now, while this is a natural process, it is not ideal for farming large fields in a predominately dry environment. So drainage banks are designed to make the water run across the field in multiple small channels - giving the water the maximum time to soak into the ground, without having it all soak into the one small stream bed. If there is excess water from a single large rainfall event - too much to soak into the ground - the banks can eventually direct the water to a holding dam for later use (eg, stock watering).
I learnt all about this over many dinner-table talks, and even drove the tractor with the blade attached to build a few banks. Naturally, I was poor at this and had to receive much instruction on what I had done wrong.