Answer: Groundcovers that will survive between walkways are difficult to find in the West. There isn't alot that can take our sun and aridity. Most of those types of plants prefer a more moist and "woodland" environment. I know a nursery wholesaler who is trying to develop/find a surefire plant for this use.
Sagina subulata, commonly called Irish Moss, or Scotch Moss, is a possibility. It takes full sun and regular water, but although it's called moss, it doesn't require the woodsy environment associated with a moss. It needs good drainage and can take some foot traffic.
Arenaria or sandwort can be 2-4 inches high, spreads invasively if given enough water. It prefers ample water.
Two species of Trifolium are typically used in place of lawn. T. fragiferum (strawberry clover) and T. repens (white clover). They full sun or partial shade and don't need as much water as the other two once established. I don't have experience with them but my reference shows they tolerate heat, aridity and moderate salinity, all conditions in the West. I hope this info helps!
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