Why? Because water is heavy and crops need water nearly continuously throughout their growing cycle.
Worry not about what your CSA box does to your annual carbon budget. Farmer Tanaka employs ultra-efficient drip irrigation, often in conjunction with plastic row covers that minimize evaporative losses from the soil, AND he uses reclaimed water from the Irvine Ranch Water District.
On my farm here in Irvine we use a drip irrigation system on our crops that puts the water right in the root zone so as to use our precious water as efficiently as possible with no runoff. We also use reclaimed water supplied by the IRWD (Irvine Ranch Water District) who produces the cleanest and safest reclaimed water in the country.Using reclaimed water gives the added benefit of not adding salts to his soil. Gardeners in this region may be familiar with the heavy load of salts in our "hard" water. If we irrigate regularly with hard water and the water evaporates or is taken up by the plants, you may notice a white powdery substance left behind. That's the salt building up in the soil. (Even if you are not a gardener, you can see the white calcium salt deposits in your kitchen and bathroom.)
If the winter rains do not come to flush the salts away, gardeners and farmers need to deep water (often repeatedly) the land to flush the salts away artificially. This is very water intensive.
The reclaimed water is so pure, it doesn't have the salts found in freshly-imported river water. Thus, Farmer Tanaka can drip irrigate just the amount needed by his plants, without worrying about poisoning his fields with salt.
What does that mean for your CSA food? It may have a lower carbon and water footprint than even food you grow in your own backyard*.
* Residents of north Redondo Beach get about half our water supply from local aquifers, which are replenished with reclaimed water from West Basin Water District's Edward C. Little Water Recycling Facility in El Segundo, California. Read a report about a visit to the facility. You can also learn more in walking my watershed.
Unfortunately, pumping the water into the ground and then pumping it back up uses energy and loads the water up with salts. I'd much rather use the cleaner reclaimed water directly from the plant, but the general public is still resistant due to the "eww" factor. I hope you will be a vector for getting the truth out.