Mostafa,
If you are using drip irrigation and there is no rain, what is the pampas grass using for water? Do you have a high water table?
I live in a semi-arid zone (not as dry as where you are) and it takes a very long time (often, years) for organic material (grass clippings, wood chips, logs, etc.) to break down if it is not in direct contact with moist soil. Mulch (organic or plastic) is good in conjunction with drip irrigation, in that it slows down or eliminates evaporation from the soil surface which also significantly reduces soil salinization.
I would have to adapt the
lasagna method, or hugelkultur, to my
local climate. Water is so expensive here that, for now, I am limiting myself to container gardening.