If that's the case, that most desert days you're near a town or kibbutz, couldn't that significantly lessen the number of water caches you have to make?

Excellent!

Generally, I average about 125 miles a week. Admittedly this isn't usually under desert conditions but still, 15-20 miles a day is fairly easy for me. (I've done the AT, PCT and most of the CDT.) I don't want to be burying so much water that it forces me to  stop too early in the day.

On a related topic, has anyone ever thought of creating some sort of "lock box" where an animal-proof large metal container that's anchored down could be placed at strategic locations along the desert route? A combination lock could be used and every year the locks changed. For a fee (to cover the cost of installing) the combination could be "rented" out to interested hikers. I don't know anyone who's done this but it makes sense to me! That way the whole system could be simplified. A hiker could consult a map before the start of the trip, rent a car and take his gallon jugs of water to each of these lockers/dumpsters and drop them off.

Or... he could hire someone to do just that. Call in advance, and arrange for the water to be dropped off. Then he could start right away instead of spending 2 days digging holes, drawing up maps and hoping for the best.

Yankale wrote   The distance in the desert is not very flexible. You start and end the day where there is water. Let's say that you have a day of 15 miles and you complete it by 2 p.m. If you don't get before sunset to the next water cache/small village you just stop and wait until the next day. The alternative is carrying more water 10-12 liters with you which is enough for two days, but then it's very heavy during the first day...

This doesn't make sense to me.  Why if you are stopping at water sources more frequently than once a day would you need to carry two days worth of water?

Lets say you are starting at point A.  15 miles later you have water cache B.  15 miles after that water cache C. 15 mles after that your end point D.

According to you what you are saying is you must:

Day 1 -  hike 15 miles, camp at cache B.

Day 2 -  hike 15 miles camp at cache C.

Day 3 - hike 15 miles end at end point D. 

Or you must carry two days worth of water.

Assuming you need about 5 liters of drinking water a day....

Couldn't you do the following:

Day 1 - start with 4-5 liters of water.  Hike to water cache B arrive early afternoon.  Stop for an hour cook a meal, wash dishes, spending about an hour relaxing and drinking water.  Leave cache B with about 5-6 liters of water.  Hike another 8 miles.  Set up camp eat a dinner that doesn't require cooking or washing dishes (sandwich).   (total milage 23 miles)

Day 2 -  either eat a cold breakfast or don't wash dishes when done eating.  Hike 7 miles to water cache B, arriving mid morning.  Wash breakfast dishes.  Cook a meal.  Bathe.  Wash dishes.  Spending about an hour relaxing and drining water.  Leave Cache C with 4-5 liters of water.  Walk another 15 miles to point D. (total miles 22)

Two days to go the 45 miles instead of three.  Never needing to carry two days worth of water. 

Not saying I would do it this way.  I am a slow hiker I would rather average 15 mile days instead of 22.5 mile per day.  I just don't think your claim that if you don't sleep next to the water caches, then you need to carry two days worth of water on your back.  And thus making going 22.5 miles per day impossible. 

 In which two places has the wooden box approach been implemented?