In an area that doesn't get much rain, the only real effecient way to water a steep hillside landscape like this is with individual drip to each plant. Watering with a hose, bubblers, or sprinklers would create4 drainage issues and send most of the water down the slope with very little of it soaking deep into the root zone.
Shredded mulches such as cypress mulch are some of the best ground covers to use on a steep slope like this. Since it is shredded and has a lot of stringy fibers, it matts down and sticks together. Bark chunks and smaller gravel will eventually and gradually slide down the hill leaving bare spots.
Larger gravel such as 3" plus would hold onto this hill. The steeper the slope, the larger the gravel or landscape rock needs to be.
While planting sparse and few plants like this may appear to do very little for errosion control on the surface, they actually do more under the surface. The roots from these shrubs will spread out several feet. Even still, the surface needs to be protected. Once the topsoil erodes from over the roots, it's only a matter of time.
Something that most folks don't consider as part of the plan for a hillside front yard with drainage problems is the gutters on the home. Without them channeling the water away, the water would rush down off the roof and pound this front yard landscaping out of place.