Your gardens and landscapes require a lot of water to keep them looking lush and healthy. Unfortunately, here in southern California, we are suffering from rapidly depleting freshwater reserves. So while there are many good reasons to compost, using 100% organic matter to help retain our precious resource, fresh water, it is at the top on our list.
Compost helps bind clusters of soil particles, called aggregates, which provide good soil structure. Such soil is full of tiny air channels & pores that hold air, moisture and nutrients. Compost alters soil structure, making it less likely to erode; it holds nutrients tight enough to prevent them from washing out, but loosely enough so plants can take them up as needed. (http://whatcom.wsu.edu)
Compost may suppress diseases and harmful pests that could overrun poor, lifeless soil. Healthy soil is an important factor in protecting our water supplies. Adding compost increases soils ability to retain water and decrease runoff. Runoff pollutes water by carrying soil, fertilizers and pesticides to nearby watersheds.
A hundred pounds of average soil (a 1×10-foot row tilled six inches deep) with a pound of compost mixed in will hold an additional 33 pounds (4 gallons) of water. Take the organic content to five pounds, and that hundred pounds of soil will hold nearly two hundred pounds (25 gallons) of water! Compost is an excellent way to increase the water holding power of root systems, which is vital to growth. (www.denverwater.com)
In short, adding compost to your gardens and landscapes will help you utilize water more efficiently. You will simply get more value out of rainfall or whatever water you apply through the hose.